Class 8 Science Force and Pressure Exam Notes

This Class 8 Science exam notes page on Force and Pressure covers the definition and types of forces, Newton's three laws of motion with solved numericals, thrust and pressure, pressure in fluids, Pascal's law, atmospheric pressure, buoyancy and Archimedes' principle, and density. It also gives the practice questions of Exercise 1 and Exercise 2 with answers.

FORCE

The external agent which can change the speed and direction of motion of a body, or the shape of a body, is called force.
It is a vector quantity.

Types of Forces

(A) Contact forces
The forces that act on bodies when they are in actual contact are known as contact forces.
Ex. Frictional force, normal reaction force, tension in string, force exerted during collision, force applied as a push or a pull, etc.

(B) Non-contact forces
The forces that act on bodies without being touched are called non-contact forces.
Ex. Gravitational force, electrostatic force, magnetic force, etc.

NEWTON'S LAWS OF MOTION

(A) Newton's First law
A body cannot change its state of motion by itself. If the object is at rest it will remain at rest, and if it is in uniform motion, it continues to be in motion unless some external force is applied on it.
This law is also known as the law of inertia.

(B) Newton's second law
Newton's second law can be written as F = ma = m \( \frac{v - u}{t} \)

Ex.1 Calculate the force required to produce an acceleration of 5 m/s² in a body of mass 2.4 kg.

Answer:
1. Force = mass × acceleration.
2. Force = 2.4 kg × 5 m/s² = 12.0 N.
3. Hence, the force required is 12.0 N.

Teacher's Note:
a) Force is always mass multiplied by acceleration, so check the units first.
b) Use kilograms and metres per second squared to get the answer in newtons.

Ex.2 A force acts for 0.2 s on a body of mass 2.5 kg initially at rest. The force then ceases to act and the body moves through 4 m in the next one second. Calculate the magnitude of force.

Answer:
1. When the force ceases to act, the body moves with a constant velocity.
2. Since it moves a distance of 4 m in 1 s, its uniform velocity = 4 m/s.
3. Initial velocity u = 0, final velocity v = 4 m/s and time interval t = 0.2 s.
4. Acceleration a = \( \frac{v - u}{t} \) = \( \frac{4 - 0}{0.2} \) = 20 m/s².
5. From the relation F = ma, Force F = 2.5 × 20 = 50 N.

Teacher's Note:
a) The body reaches its speed only during the 0.2 s when the force acts.
b) Students often use the distance 4 m in the formula for acceleration; use velocity change instead.
c) The force is found only after the acceleration is known.

Ex.3 A ball of mass 20 gm is initially moving with a velocity of 100 m/s. On applying a constant force on the ball for 0.5 s, it acquires a velocity of 150 m/s. Calculate the following:
(i) Acceleration of the ball
(ii) Magnitude of the force applied

Answer:
1. Given: m = 20 gm = 0.02 kg, initial velocity u = 100 m/s, time interval t = 0.5 s, final velocity v = 150 m/s.
2. (i) Acceleration a = \( \frac{v - u}{t} \) = \( \frac{150 - 100}{0.5} \) = 100 m/s².
3. (ii) Force F = mass × acceleration = 0.02 × 100 = 2.0 N.
4. Hence, the acceleration is 100 m/s² and the force applied is 2.0 N.

Teacher's Note:
a) Convert grams into kilograms before using F = ma.
b) The ball is already moving, so subtract the initial velocity from the final velocity.

Ex.4 A cricket ball of mass 200 gm moving with a speed of 40 m/s is brought to rest by a player in 0.04 s. Calculate the average force applied by the player.

Answer:
1. Mass m = 200 gm = \( \frac{200}{1000} \) kg = 0.2 kg.
2. Initial velocity u = 40 m/s, final velocity v = 0, time t = 0.04 s.
3. Change in momentum = m (v - u) = 0.2 × (0 - 40) = -8.0 kg m/s.
4. Average force = \( \frac{\text{Change in momentum}}{\text{Time}} \) = \( \frac{-8.0}{0.04} \) = -200 N.
5. The negative sign shows that the force is applied in a direction opposite to the direction of motion of the ball.

Teacher's Note:
a) Average force is change in momentum divided by time.
b) The magnitude of the force is 200 N, and the minus sign only gives its direction.

Ex.5 A motorcycle is moving with a velocity of 108 km/hr and it takes 5 s to stop it after the brakes are applied. Calculate the force exerted by the brakes on the motorcycle if its mass along with the rider is 250 kg.

Answer:
1. Initial velocity of the motorcycle = 108 km/hr = 30 m/s.
2. Final velocity = 0 m/s, time taken to stop = 5 s, mass with rider = 250 kg.
3. Change in velocity in 5 s = 0 - 30 = -30 m/s.
4. Acceleration a = \( \frac{-30}{5} \) = -6 m/s², which is a retardation of 6 m/s².
5. Force applied by the brakes = mass × acceleration = 250 kg × 6 m/s² = 1500 N.

Teacher's Note:
a) Convert km/hr to m/s by multiplying by 5/18.
b) The brakes act opposite to the motion, so the magnitude of the force is quoted here.

(C) Newton's third law of motion
Newton's third law of motion states that "if a body A exerts a force on the body B, the body B will also exert an equal and opposite force on A."
Newton's third law is also stated as "to every action there is an equal and opposite reaction."
The force exerted by A on B is called action, while the force exerted by B on A is called the reaction.
Action and reaction always act on different bodies.
Forces always occur in pairs.
Example: by hitting a table with the palm we apply a force. The table also exerts a force on the palm on hitting it.

THRUST AND PRESSURE

(A) Thrust
The force acting normally on a surface is called 'thrust'.
It is a vector quantity.
It is measured in newton (N).

(B) Pressure
The thrust on a unit area of a surface is called 'pressure'.
Pressure = Thrust / Area, or P = \( \frac{F}{A} \)
Unit: The SI unit of pressure is newton per metre square or N/m². Other units of pressure are pascal and bar.
One Pascal: One pascal is defined as the pressure exerted on a surface area of 1 m² by a thrust of 1 newton, i.e. 1 Pascal = 1 N/m².

Some examples based on pressure
Inserting a pointed nail in a wooden block is an easier task than inserting a rod inside a wooden block with the same force, because the nail has a smaller area and thus it experiences more pressure even with the same force.
A sharp knife cuts better than a blunt knife.
While walking, a man exerts more pressure on the ground in comparison to when he is standing.

PRESSURE IN FLUIDS

A substance that can flow is called a 'fluid'. Liquids and gases are considered as fluids.

Laws of pressure
Pressure exerted by a liquid is the same in all directions about a point.
Pressure exerted is the same at all points in a horizontal plane as well as in a stationary liquid.
Pressure at a point inside a liquid increases with depth from the free surface.
The pressure exerted anywhere in a confined liquid is transmitted equally and undiminished in all directions throughout the liquid, which is called 'Pascal's law'.

(A) Hydrostatic Pressure
The normal force (or thrust) exerted by a liquid at rest per unit area of the surface in contact with it is called "pressure of liquid or hydrostatic pressure."

(B) Atmospheric Pressure
The pressure exerted by the atmosphere is called atmospheric pressure.
At sea level, atmospheric pressure is the pressure exerted by 0.76 m of mercury column, i.e. h = 0.76 m.

Ex.6 A force of 150 N is applied on an area of 1.5 m². Calculate the pressure exerted.

Answer:
1. Force F = 150 N and area A = 1.5 m².
2. Pressure = \( \frac{\text{Force}}{\text{Area}} \) = \( \frac{150 \text{ N}}{1.5 \text{ m}^2} \) = 100 N/m².

Teacher's Note:
a) Pressure is force divided by area, never the other way round.
b) Always state the unit of pressure as N/m².

Ex.7 A force of 500 dynes is applied on an area of 20 cm². Calculate the pressure exerted.

Answer:
1. Force F = 500 dynes = 500 × 10⁻⁵ N.
2. Area A = 20 cm² = 20 × 10⁻⁴ m².
3. Pressure P = \( \frac{F}{A} \) = \( \frac{500 \times 10^{-5} \text{ N}}{20 \times 10^{-4} \text{ m}^2} \) = 2.5 N/m².

Teacher's Note:
a) Convert dynes into newtons and cm² into m² before dividing.
b) One newton equals 10⁵ dynes, so 500 dynes is a very small force.

BUOYANCY

When a body is immersed in a liquid, the liquid exerts an upward force on the body called the 'upthrust' or 'buoyant force'.

Factors affecting upthrust
The larger the volume of the body submerged in the liquid, the greater is the upthrust.
The larger the density of the liquid, the greater is the upthrust.

Archimedes' principle
Archimedes' principle states that when a body is immersed in a liquid partially or completely, it experiences an upthrust equal to the weight of the liquid displaced. In other words, the loss in weight of the body (buoyant force) is equal to the weight of the liquid displaced.

Ex.8 A body weighs 300 N in air and 260 N when completely immersed in water. Calculate the following:
(i) Loss in weight of the body
(ii) Upthrust on the body

Answer:
1. Weight of the body in air = 300 N.
2. Weight of the body in water = 260 N.
3. (i) Loss in weight of the body = 300 - 260 = 40 N.
4. (ii) Upthrust on the body = Loss in weight = 40 N.

Teacher's Note:
a) Loss in weight in a liquid always equals the upthrust.
b) Weight in air minus apparent weight in water gives the upthrust directly.

DENSITY

Density = \( \frac{\text{Mass}}{\text{Volume}} \) or d = \( \frac{M}{V} \)
The SI unit of density is kg/m³ and the CGS unit of density is g/cm³.

Water has anomalous expansion. When water is cooled to 4°C, its volume decreases, but on further cooling its volume starts increasing.
So, the density of water is maximum at 4°C.

EXERCISE # 1

A. Very Short Answer Type Questions

FORCE

Q.1 Define force.

Answer: Force is a push or pull on an object.

Teacher's Note:
a) Force is a push or a pull, and it can start, stop or change motion.
b) Give the definition in one clear sentence in the exam.

Q.2 An inflated balloon was pressed against a wall after it has been rubbed with a piece of synthetic cloth. It was found that the balloon sticks to the wall. What force might be responsible for the attraction between the balloon and the wall ?

Answer: Electrostatic force.

Teacher's Note:
a) Rubbing the balloon with synthetic cloth charges it by friction.
b) Electrostatic force is a non-contact force, so the balloon sticks without any pressure.

Q.3 How many objects should be present for a force to come into play ?

Answer: There should be at least two objects for a force to come into play.

Teacher's Note:
a) A force is always an interaction between two objects.

Q.4 Two friends A and B are applying a force of 2 newton and 4 newton on a box in the same direction. What will be the total force applied by them ?

Answer: The total force will be 6 newton, i.e., the sum of their individual forces.

Teacher's Note:
a) Forces in the same direction are added.
b) If the forces were opposite, the smaller would be subtracted from the larger.

Q.5 In a tug of war, side A applies 10 newton force and side B applies 8 newton force. Which side will the rope move ?

Answer: The rope will move towards side A as more force is applied by side A.

Teacher's Note:
a) The net force is 10 N - 8 N = 2 N towards side A.
b) The side with the larger force wins the tug of war.

Q.6 What happens to the speed of a body when a force is applied ?

Answer: The speed of a body can be increased or decreased by applying force.

Teacher's Note:
a) A force along the direction of motion increases speed.
b) A force opposite to the motion decreases speed.

Q.7 Can we change the direction of the moving object by applying a force ?

Answer: Yes, we can change the direction of the moving object by applying a force.

Teacher's Note:
a) A force at an angle to the motion bends the path of a moving object.

Q.8 Is it possible that a force changes the direction of motion but not the speed of an object ?

Answer: Yes, it is possible when a body is moving on a circular path.

Teacher's Note:
a) On a circular path the direction keeps changing while the speed stays the same.
b) The force is always directed towards the centre of the circle.

Q.9 Give an example to show that force can change the shape of an object.

Answer: Pressing a rubber ball with the hand changes its shape.

Teacher's Note:
a) Squeezing clay, dough or a sponge are other examples.
b) The change in shape goes away when the force is removed in elastic objects.

Q.10 What is meant by contact force ?

Answer: A force which is applied only when it is in contact with an object is called a contact force.

Teacher's Note:
a) Friction and the push of a hand are contact forces.
b) Gravity and magnetic force are non-contact forces.

PRESSURE

Q.11 Define the term atmospheric pressure.

Answer: The pressure exerted by air is known as atmospheric pressure.

Teacher's Note:
a) Atmospheric pressure is caused by the weight of the air above us.

Q.12 How would 'thrust' on the bottom of a liquid level change if 'area' is doubled keeping the 'pressure' same ?

Answer: Thrust will also be doubled.

Teacher's Note:
a) Thrust = pressure × area, so with pressure fixed, thrust is proportional to area.
b) Doubling the area doubles the thrust.

Q.13 Two objects of masses M and 2M are lying on an equal area. Determine the ratio of pressure exerted by them on the ground.

Answer:
1. P1 = \( \frac{M}{A} \) and P2 = \( \frac{2M}{A} \).
2. Ratio of pressure P1 : P2 = 1 : 2.

Teacher's Note:
a) The areas are equal, so they cancel in the ratio.
b) The heavier object exerts double the pressure.
c) Keep the order of the ratio the same as the order of the objects in the question.

Q.14 Define pressure.

Answer: Pressure is the force acting per unit area.

Teacher's Note:
a) Pressure = force / area, and its SI unit is N/m² or pascal.

Q.15 Do the gases and liquids exert pressure on the walls of the container ?

Answer: Yes, liquids and gases exert pressure on the walls of the container.

Teacher's Note:
a) Both are fluids, so they push outwards on every wall they touch.
b) Balloons and water bottles show this pressure clearly.

Q.16 Why is it comfortable to lift a school bag with broad straps than thin straps ?

Answer: Pressure is inversely proportional to area. Broad straps have a greater area, therefore the pressure on the shoulder decreases.

Teacher's Note:
a) The same weight spread over a larger area gives less pressure.
b) Broad straps reduce pain on the shoulder.
c) Thin straps concentrate the same force on a small area.

Q.17 Why do mountaineers suffer from nose bleeding at high altitudes ?

Answer: The atmospheric pressure decreases with high altitude. Since the pressure of the blood inside the body is high, the nose starts bleeding.

Teacher's Note:
a) At high altitude the air outside pushes less, so the blood pressure inside the body is relatively greater.
b) The blood vessels in the nose are thin and can burst under this difference.

Q.18 Why is easier to hammer a sharp nail into wood than a blunt one ?

Answer:
1. Pressure = force / area.
2. When we hammer a sharp nail, the force acts on a smaller area.
3. So the sharp nail exerts more pressure on the wood, and it goes in easily.

Teacher's Note:
a) The sharp tip concentrates the same hammer force on a tiny area.
b) Knives, pins and needles work on the same principle.

Q.19 How would pressure change if area is doubled keeping force constant ?

Answer: If area is doubled keeping the force constant, then pressure becomes half.

Teacher's Note:
a) Pressure is inversely proportional to area when force is constant.

Q.20 How would pressure change if force is doubled keeping area constant ?

Answer: If force is doubled keeping area constant, then pressure becomes double.

Teacher's Note:
a) Pressure is directly proportional to force when area is constant.
b) Compare this with Q.19 to see the difference between the two relations.

B. Short Answer Type Questions

FORCE

Q.21 Give two examples each of situations in which you push or pull to change the state of motion of objects.

Answer:
Push: moving a loaded cart, batsman hitting a ball.
Pull: opening a drawer, drawing a bucket of water from a well.

Teacher's Note:
a) A push or pull changes the state of motion when it starts, stops or changes the speed of an object.
b) Give examples from daily life, as the examiner expects.
c) Batting and drawing water are both examples of changing the state of motion.

Q.22 Give two examples each of situations in which applied force causes a change in the shape of an object.

Answer:
(a) Pressing a lump of dough with hand.
(b) Pressing an inflated balloon.

Teacher's Note:
a) Both examples involve squeezing the object with a force.
b) Clay moulding and crushing a paper cup are other valid examples.

Q.23 If the force is applied opposite to the motion, what will happen to the speed of the object?

Answer: When the force is applied opposite to the motion of the object, either the speed decreases or the direction changes.

Teacher's Note:
a) A force opposite to motion slows the object down.
b) If the force is large enough, the object may even stop and move back.

Q.24 State the two factors which describe the state of motion of an object.

Answer: The state of motion is described by its speed and direction of motion.

Teacher's Note:
a) Speed tells how fast the object moves.
b) Direction tells where the object is moving.

Q.25 How do the mud particles fly off the wheel of a vehicle moving on the wet road ?

Answer: The direction of the mud particles changes at every point as the wheel of the vehicle moves.

Teacher's Note:
a) Inertia makes the mud move straight ahead when the wheel turns below it.
b) Mud guards on vehicles stop this mud from splashing on the rider.

PRESSURE

Q.26 Define Pressure. Write the relation between pressure force and area. Name the instrument used to measure atmospheric pressure.

Answer:
1. Pressure is the force acting normally per unit area.
2. Relation: Pressure = \( \frac{\text{Force}}{\text{Area}} \), or P = \( \frac{F}{A} \).
3. Atmospheric pressure is measured by a barometer.

Teacher's Note:
a) The relation is P = F / A, so pressure depends on force and area.
b) A mercury barometer is the common instrument used for atmospheric pressure.

Q.27 Why is it difficult to cut vegetables with a blunt knife ?

Answer: Pressure is inversely proportional to area. The area of a blunt knife is more, and therefore the effect of the force is less. So more force has to be applied.

Teacher's Note:
a) A sharp edge has a very small area of contact.
b) The same hand force produces much more pressure on a sharp edge.

Q.28 Trucks intended to carry heavy loads have eight tyres instead of four tyres. Why ?

Answer: Trucks intended to carry heavy loads have eight tyres, so as to increase the area of contact with the road. Since pressure is inversely proportional to area, less pressure is applied on the road.

Teacher's Note:
a) More tyres mean more contact area for the same load.
b) Less pressure on the road reduces damage to the road surface.

Q.29 Give reasons for the following:
(a) The skiers use flat and broad skis
(b) Deep sea divers wear special suits.

Answer:
(a) The skiers use flat and broad skis to ski on the snow. The larger surface of the skis reduces the pressure on the snow and helps them to slide instead of sinking.
(b) Deep sea divers wear special suits, because the pressure of water increases with depth. The increased pressure may hurt the body of divers.

Teacher's Note:
a) Broad area gives less pressure on the snow.
b) Pressure in a liquid increases with depth, so the suit protects the diver from the greater pressure.

Q.30 How does the medicine enter a dropper ?

Answer: When the dropper is pressed, the air inside the dropper is driven out. The pressure inside the dropper decreases and the medicine rushes inside the dropper.

Teacher's Note:
a) Releasing the rubber bulb lowers the air pressure inside the dropper.
b) The higher atmospheric pressure outside pushes the medicine up the tube.

C. Long Answer Type Questions

FORCE

Q.31 Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Answer:
1. The forces acting on the bucket are its own weight acting downwards and the muscular force of the hand acting upwards.
2. Both the forces are equal and act in opposite directions, so they balance each other.
3. Since the forces are balanced, they do not bring any change in the state of motion of the bucket. The hand feels tired due to the weight of the bucket.

Teacher's Note:
a) Balanced forces do not change motion.
b) Weight acts vertically downwards, while the hand pushes up with muscular force.
c) Feeling tired is due to the muscles working to hold the weight, not due to motion.

Q.32 (a) What is weight ?
(b) What is the unit of weight ?
(c) Name the device used for measuring the weight of an object.
(d) Can weight be taken as a measure of force ?

Answer:
(a) Weight of an object is the force of gravity acting on the object.
(b) The unit of weight is newton or kilogram weight.
(c) A spring balance is used for measuring the weight of an object.
(d) Yes, weight can be taken as a measure of force.

Teacher's Note:
a) Weight is a gravitational force, so it always acts towards the centre of the earth.
b) A spring balance measures weight, while a common balance compares masses.

Q.33 Name the type of force in the following cases.
(a) Raindrops falling on the earth.
(b) Holding a book on your hand.
(c) Running a comb through your dry hair.
(d) A bar magnet suspended freely.
(e) Bullocks ploughing the field.

Answer:
(a) Force of gravity.
(b) Force of gravitation (weight).
(c) Electrostatic force.
(d) Magnetic force.
(e) Muscular force.

Teacher's Note:
a) Gravity, electrostatic and magnetic forces act without contact.
b) Muscular force comes from the animal's muscles.

PRESSURE

Q.34 (a) Define one atmosphere.
(b) Where is the pressure greater, 10 m below the surface of sea or 20 m below ?
(c) Where is pressure greatest and the least inside a bottle filled with water.

Answer:
(a) The pressure which can support 76 cm of mercury in a mercury barometer is called one atmosphere.
(b) 20 m below the surface of sea, the pressure is greater.
(c) The pressure is greatest at the bottom and least at the surface of the water filled in a bottle.

Teacher's Note:
a) One atmosphere is about 1.01 × 10⁵ Pa.
b) Pressure in a liquid increases with depth, so the deeper point always has more pressure.
c) The water at the top has the least weight above it.

Q.35 What happens to the atmospheric pressure if,
(a) the temperature is high.
(b) the humidity in air increases
(c) metrological office predicts fair weather.
(d) there is a storm.
(e) the weather is dry

Answer:
(a) Atmospheric pressure decreases.
(b) Atmospheric pressure decreases.
(c) Atmospheric pressure increases.
(d) Atmospheric pressure decreases.
(e) Atmospheric pressure increases.

Teacher's Note:
a) Warm air is lighter, so it exerts less pressure.
b) Humid air is lighter than dry air, so the pressure falls.
c) High pressure is linked to clear, dry and fair weather.

Q.36 Define force and pressure. What do you do to get maximum pressure with a minimum forces ? Name atleast one appliance based on this principle.

Answer:
1. Force is a push or pull on an object.
2. Pressure is force acting per unit area.
3. As pressure is inversely proportional to area and directly proportional to force, to get maximum pressure with a minimum force we can decrease the area.
4. A sharp knife and pointed nails are based on this principle.

Teacher's Note:
a) Reducing the area is the only way to increase pressure without increasing the force.
b) Needles, chisels and axes are other appliances based on this principle.
c) Keep the words force and pressure separate in the definitions.

EXERCISE # 2

Single correct answer type questions

FORCE

Q.1 Which of the following is the action-at-distance force ?
(A) muscular force
(B) frictional force
(C) magnetic force
(D) mechanical force

Answer: (C) magnetic force

1. A force that acts at a distance without touching the body is called an action-at-distance force.
2. Magnetic force acts at a distance, so the correct option is (C).

Teacher's Note:
a) Gravity and electrostatic forces are also action-at-distance forces.
b) Friction and muscular force need contact.

Q.2 The force exerted by one object on another by virtue of their masses is
(A) magnetic force
(B) electrostatic force
(C) gravitational force
(D) frictional force

Answer: (C) gravitational force

1. Force that depends on the masses of two objects is gravitational force.
2. Hence, the correct option is (C).

Teacher's Note:
a) Gravitational force depends on the masses of the objects and the distance between them.

Q.3 The standard unit of force is
(A) metre/second
(B) newton
(C) metre/second²
(D) gram-weight

Answer: (B) newton

1. The SI unit of force is newton (N).
2. Hence, the correct option is (B).

Teacher's Note:
a) Metre per second is the unit of speed, and metre per second squared is the unit of acceleration.
b) Gram-weight is not an SI unit.

Q.4 A spring balance is used for measuring
(A) weight
(B) speed
(C) acceleration
(D) mass

Answer: (A) weight

1. A spring balance works on the stretching of a spring by the weight of the object.
2. Hence, the correct option is (A).

Teacher's Note:
a) A common physical balance measures mass, not weight.

Q.5 A force applied on a moving body may
(A) bring it to rest
(B) increase its speed
(C) decrease the speed
(D) all of the above

Answer: (D) all of the above

1. A force can stop a moving body, increase its speed or decrease its speed.
2. Hence, the correct option is (D).

Teacher's Note:
a) The effect depends on the direction of the force compared with the motion.
b) Brakes and accelerators are examples of this.
c) All three effects are possible.

Q.6 Earth always pulls everything towards it due to
(A) muscular force
(B) mechanical force
(C) gravitational force
(D) electrostatic force

Answer: (C) gravitational force

1. The pull of the earth on every object is the force of gravity.
2. Hence, the correct option is (C).

Teacher's Note:
a) Gravity acts on all objects towards the centre of the earth.

Q.7 A cart being carried by a horse is an example of
(A) muscular force
(B) mechanical force
(C) gravitational force
(D) electrostatic force

Answer: (A) muscular force

1. The horse pulls the cart with the force of its muscles.
2. Hence, the correct option is (A).

Teacher's Note:
a) Animal muscles produce muscular force.
b) Mechanical force is a force produced by machines.

Q.8 If you press an inflated balloon, it deforms due to a type of
(A) contact force
(B) non-contact force
(C) gravitational force
(D) none of these

Answer: (A) contact force

1. Pressing the balloon requires your hand to touch it.
2. A force acting through touch is a contact force, so the correct option is (A).

Teacher's Note:
a) Deformation by pressing is always caused by contact force.
b) Gravity can also deform objects but does not need touch.

Q.9 Force exerted by the muscles is known as
(A) mechanical force
(B) gravitational force
(C) electrostatic force
(D) muscular force

Answer: (D) muscular force

1. The force produced by muscles of living beings is called muscular force.
2. Hence, the correct option is (D).

Teacher's Note:
a) Pushing, pulling and lifting with the body use muscular force.

Q.10 A hockey player uses his hockey stick -
(A) To push the ball
(B) To pull the ball
(C) To change its direction
(D) All of these

Answer: (D) All of these

1. The stick can push the ball, pull it back and change its direction.
2. Hence, the correct option is (D).

Teacher's Note:
a) A single force can do more than one job on a moving ball.
b) Think about what happens to the ball's motion in each case.

Q.11 A force when applied brings change in -
(A) Direction of motion of the body
(B) Speed of moving body
(C) Shape of the body
(D) Any of these

Answer: (D) Any of these

1. A force can change the direction of motion, the speed or the shape of a body.
2. Hence, the correct option is (D).

Teacher's Note:
a) These are the three effects of a force.
b) Shape changes are seen in squeezing and stretching.

Q.12 The force responsible for the wearing out of the car tyres is -
(A) Frictional force
(B) Gravitational force
(C) Magnetic force
(D) Muscular force

Answer: (A) Frictional force

1. Friction between the tyre and the road rubs the tyre away.
2. Hence, the correct option is (A).

Teacher's Note:
a) Friction produces heat and wear on contact surfaces.
b) Good tyres are designed with grooves to reduce friction and wear.

Q.13 The force you will use to collect the iron nails scattered on a sandy ground is -
(A) Frictional force
(B) Gravitational force
(C) Magnetic force
(D) None of these

Answer: (C) Magnetic force

1. Iron nails are attracted by a magnet.
2. The force of a magnet is magnetic force, so the correct option is (C).

Teacher's Note:
a) A magnet picks up iron nails from sand without touching them directly.
b) Sand is not magnetic, so only the nails are collected.

Q.14 The force you use to stretch a rubber band is -
(A) Frictional force
(B) Gravitational force
(C) Magnetic force
(D) Muscular force

Answer: (D) Muscular force

1. The pull that you apply with your fingers comes from your muscles.
2. Hence, the correct option is (D).

Teacher's Note:
a) Stretching a rubber band changes its shape.
b) Muscles act with the help of hand and finger action.

Q.15 The SI unit of force is
(A) metre
(B) newton
(C) pascal
(D) second

Answer: (B) newton

1. The SI unit of force is newton (N).
2. Hence, the correct option is (B).

Teacher's Note:
a) Pascal is the unit of pressure, not force.

Q.16 A contact force cannot act through
(A) empty space
(B) touching
(C) touching with a metal rod
(D) touching with a wooden rod

Answer: (A) empty space

1. A contact force acts only when the bodies touch each other.
2. Empty space has no body to touch, so the correct option is (A).

Teacher's Note:
a) Options B to D all involve touching, so they are possible.
b) Remember the word contact in the definition.

Q.17 A force that opposes the motion of one surface sliding over another is called
(A) friction
(B) newton
(C) lubrication
(D) ball bearing

Answer: (A) friction

1. The force that opposes sliding of one surface over another is friction.
2. Hence, the correct option is (A).

Teacher's Note:
a) Lubrication and ball bearings reduce friction but are not forces.
b) Newton is the unit of force, not a force.

PRESSURE

Q.18 If a given force is applied on a smaller area of contact the pressure exerted by it
(A) decreases
(B) increases
(C) does not change
(D) none of these

Answer: (B) increases

1. Pressure = force / area.
2. For the same force, a smaller area gives a larger pressure, so the correct option is (B).

Teacher's Note:
a) This is why sharp tools are used for cutting.

Q.19 A camel can walk/run in deserts very easily as compared to horse, donkey etc., because is -
(A) feet are smaller
(B) weight is lesser
(C) feet are broader
(D) heavier body

Answer: (C) feet are broader

1. Broad feet have a large area, so the pressure on the sand is less.
2. Hence, the correct option is (C).

Teacher's Note:
a) Less pressure means the camel does not sink into soft sand.
b) Snow shoes and skis work on the same principle.

Q.20 The SI unit of pressure is
(A) atmosphere
(B) pascal
(C) cm of mercury
(D) mm of mercury

Answer: (B) pascal

1. The SI unit of pressure is pascal (Pa), which is equal to N/m².
2. Hence, the correct option is (B).

Teacher's Note:
a) Atmosphere, cm of mercury and mm of mercury are other units, not SI units.
b) One atmosphere is about 1.01 × 10⁵ Pa.

Q.21 Pressure is defined as
(A) force
(B) force × distance
(C) force per unit area
(D) force × area

Answer: (C) force per unit area

1. Pressure is the force acting normally per unit area.
2. Hence, the correct option is (C).

Teacher's Note:
a) Force × area gives a wrong quantity, so avoid it in the formula.
b) Force × distance is work.

Q.22 Pressure can be measured in the units of .............
(A) Newton/m²
(B) Pascal
(C) Both A & B
(D) none of these

Answer: (C) Both A & B

1. Newton per metre square and pascal are the same unit, since 1 Pa = 1 N/m².
2. Hence, the correct option is (C).

Teacher's Note:
a) Both units give the same value for the same pressure.
b) Pascal is just a short name for N/m².

Q.23 Approximate value of pressure exerted by air is .........
(A) 1 kilo Pascal
(B) 10 kilo Pascal
(C) 100 kilo Pascal
(D) none of these

Answer: (C) 100 kilo Pascal

1. Atmospheric pressure at sea level is about 1 atmosphere = 1.01 × 10⁵ Pa.
2. This is about 100 kilo Pascal, so the correct option is (C).

Teacher's Note:
a) Memorise the value 1 atm ≈ 100 kPa for quick answers.
b) Pressure falls as altitude increases.

Q.24 With the depth of a liquid, exerted pressure......
(A) decreases
(B) ceases
(C) increases
(D) no change

Answer: (C) increases

1. Pressure in a liquid increases with depth from the free surface.
2. Hence, the correct option is (C).

Teacher's Note:
a) Deeper dams are built thicker for this reason.
b) Divers feel more pressure in their ears as they go deeper.

Q.25 At high altitudes the air pressure is ..... than at sea level
(A) less
(B) more
(C) same
(D) can't say

Answer: (A) less

1. Air above a high place is thinner, so less air is above it.
2. Hence, the air pressure is less, and the correct option is (A).

Teacher's Note:
a) The weight of the air column above a place decides atmospheric pressure.

Q.26 The substances that do not have a fixed shape and can flow are commonly called....
(A) Gases
(B) Liquids
(C) both (A) and (B)
(D) none of these

Answer: (C) both (A) and (B)

1. Gases and liquids both flow and do not have a fixed shape.
2. Such substances are called fluids, so the correct option is (C).

Teacher's Note:
a) Solids have a fixed shape, so they are not fluids.
b) Fluids take the shape of their container.

Q.27 The pressure applied on a body depends on
(A) Force
(B) Mass
(C) Both force and mass
(D) Both force and area

Answer: (D) Both force and area

1. Pressure = force / area.
2. So pressure depends on both force and area, and the correct option is (D).

Teacher's Note:
a) Mass alone does not decide pressure.
b) Change either force or area and the pressure changes.

Q.28 You have two nails, one with sharp end and other with blunt end. If you apply equal force on each, the nail that will be hammered first will be
(A) The nail with pointed
(B) The nail with blunt end
(C) Both will be hammered in same time
(D) None of these can be hammered

Answer: (A) The nail with pointed

1. The pointed nail has a smaller area, so the same force gives it more pressure.
2. Hence, the correct option is (A).

Teacher's Note:
a) More pressure makes the wood yield more easily.
b) The blunt nail needs more force for the same effect.

Q.29 The formula for pressure is
(A) force × area
(B) force / area
(C) Area / force
(D) none of these

Answer: (B) force / area

1. Pressure = \( \frac{\text{Force}}{\text{Area}} \).
2. Hence, the correct option is (B).

Teacher's Note:
a) Keep force on the top and area at the bottom of the fraction.

Q.30 The pressure increases with
(A) Decreasing depth
(B) Increasing depth
(C) Depth has no effect on pressure
(D) None of these

Answer: (B) Increasing depth

1. Pressure in a liquid increases with depth.
2. Hence, the correct option is (B).

Teacher's Note:
a) This is the law of pressure in liquids.

Q.31 The instrument used to measure the pressure is
(A) Hydrometer
(B) Manometer
(C) Galvanometer
(D) Anemometer

Answer: (B) Manometer

1. A manometer measures the pressure of a gas or liquid.
2. Hence, the correct option is (B).

Teacher's Note:
a) A hydrometer measures density, a galvanometer measures current and an anemometer measures wind speed.
b) A mercury barometer measures atmospheric pressure.

Q.32 Which of the following is not a unit of pressure?
(A) bar
(B) Newton
(C) atm
(D) Pascal

Answer: (B) Newton

1. Bar, atm and pascal are units of pressure.
2. Newton is the unit of force, so the correct option is (B).

Teacher's Note:
a) Newton per metre square is the unit of pressure, but newton alone is not.

Q.33 Deep-sea diving vessels have to withstand pressure from the crushing effect of sea water acting
(A) upwards
(B) downwards
(C) sideways
(D) in all directions

Answer: (D) in all directions

1. Pressure in a liquid acts equally in all directions at a point.
2. So the sea water crushes the vessel from every side, and the correct option is (D).

Teacher's Note:
a) Pressure at a point in a liquid acts in all directions.
b) Vessels are made round because round shapes handle this pressure well.

Q.34 Which among the following will exert maximum pressure when pushed with the same amount of force ?
(A) An eraser of area 2 cm²
(B) A sharpened pencil tip
(C) The blunt end of a pencil
(D) The rear portion of closed safely pin

Answer: (B) A sharpened pencil tip

1. A sharpened pencil tip has the smallest area of the four options.
2. For the same force, the smallest area gives the maximum pressure, so the correct option is (B).

Teacher's Note:
a) Compare the areas before comparing pressures.
b) The sharp tip is the smallest contact area in this list.

Q.35 How does pressure vary as we move from sea level to the mountain top ?
(A) Decreases
(B) Increases
(C) Increases upto a few kilometre and then decreases
(D) Decreases upto a few kilometres and then increases

Answer: (A) Decreases

1. Atmospheric pressure is caused by the air above, which becomes thinner with height.
2. So the pressure decreases steadily from sea level to the top, and the correct option is (A).

Teacher's Note:
a) Less air above means less weight pressing down.
b) This is why climbers carry oxygen at great heights.

Q.36 At sea level, atmospheric pressure is about
(A) 10³ Pa
(B) 10⁴ Pa
(C) 10⁵ Pa
(D) none of these

Answer: (C) 10⁵ Pa

1. Atmospheric pressure at sea level is about 1.01 × 10⁵ Pa.
2. Hence, the correct option is (C).

Teacher's Note:
a) The value is written as 10⁵ Pa for estimation.
b) 10³ Pa and 10⁴ Pa are much too small for the air pressure.

Q.37 Pascal is used as a unit for
(A) thrust
(B) weight
(C) pressure
(D) work

Answer: (C) pressure

1. Pascal is the SI unit of pressure.
2. Hence, the correct option is (C).

Teacher's Note:
a) Thrust is measured in newton and work in joule.

Q.38 SI unit of thrust is
(A) N
(B) Kgm-3
(C) Nm-2
(D) Joule

Answer: (A) N

1. Thrust is a force, and the SI unit of force is newton (N).
2. Hence, the correct option is (A).

Teacher's Note:
a) Kg m-3 is the unit of density, and N m-2 is the unit of pressure.
b) Joule is the unit of work and energy.

Q.39 The force divided by area on which it acts is called
(A) pressure
(B) weight
(C) thrust
(D) work

Answer: (A) pressure

1. Force divided by the area on which it acts is pressure.
2. Hence, the correct option is (A).

Teacher's Note:
a) Thrust is the force itself, not the force per area.

Q.40 1 Pa equals
(A) 10 Nm-2
(B) 1 Nm-2
(C) 1/10 Nm-2
(D) 10⁵ Nm-2

Answer: (B) 1 Nm-2

1. One pascal is defined as 1 newton per square metre.
2. Hence, 1 Pa = 1 Nm-2, and the correct option is (B).

Teacher's Note:
a) 10⁵ Nm-2 is roughly the pressure of the atmosphere, not one pascal.

Q.41 The unit of pressure used for meterological purpose is called
(A) a bar
(B) pascal
(C) kg wt.
(D) atm

Answer: (A) a bar

1. The bar is the unit of pressure used in weather reports.
2. Hence, the correct option is (A).

Teacher's Note:
a) One bar is equal to 10⁵ Pa.
b) Weather maps give pressure in millibars or hectopascals.

Q.42 A high altitudes the air pressure (as compared to pressure o the surface of the Earth) is
(A) less
(B) more
(C) same
(D) none of these

Answer: (A) less

1. At high altitude the air above is thinner, so the air pressure is less than at the surface of the earth.
2. Hence, the correct option is (A).

Teacher's Note:
a) Less air above gives less pressure.
b) Mountain climbers need to adjust to this drop in pressure.

Q.43 The pressure in a liquid at greater depth is
(A) smaller
(B) greater
(C) same
(D) none of these

Answer: (B) greater

1. Pressure in a liquid increases with depth.
2. Hence, the correct option is (B).

Teacher's Note:
a) The weight of the liquid above adds to the pressure at greater depth.
b) Dams are made thicker at the bottom for this reason.

Q.44 The pressure at any point in a liquid at rest depends only on the depth and on the ...... of the liquid.
(A) density
(B) weight
(C) colour
(D) none of these

Answer: (A) density

1. Pressure at a point in a liquid at rest depends on the depth and the density of the liquid.
2. Hence, the correct option is (A).

Teacher's Note:
a) Colour and weight of the container do not change the pressure at a point.
b) A denser liquid gives more pressure at the same depth.

Area, A = 20 cm² = 20 × 10^–4 m²

♦ Density = Mass/Volume or d =M/ V

Sol.13 P1 =M/A , P2 2M/A ; P1/ P2 = M x A/A x 2M = 1/ 2   ∴ ratio of pressure is 1 : 2

(A) 103 Pa

(B) 104 Pa

(C) 105 Pa