CBSE Class 9 Physics Force And Laws Of Motion Worksheet Set B

Question. A bullet of mass 100 g is fired from a gun of mass 20 kg with a velocity of 100 msl. Calculate the velocity of recoil of the gun.
Answer.
Mass of bullet, m = 100 g – 1000 kg
Velocity of bullet, u = 100 ms^–1
Mass of gun, M = 20 kg
Let recoil velocity of gun = V
Step 1. Before firing, the system (gun + bullet) is at rest, therefore, initial momentum of the system = 0
Final momentum of the system
= momentum of bullet + momentum of gun
= mu + MV = 10
1 × 100 + 20 V
V = 10 + 20 V
Step 2. Apply law of conservation of momentum Final momentum = Initial momentum
i.e. 10 + 20 V = 0
20 V = 10
or V = – 0.5 ms^–1
Negative sign shows that the direction of recoil velocity of the gun is opposite to the direction of the velocity of the bullet.

Question. Define force. What are different types forces?
Answer. Force : It is a push or pull on an object that produces acceleration in the body on which it acts. The S.I. unit of force is Newton.
Types of forces :
Balanced force : When the forces acting on a body from the opposite direction do not change the state of rest or of motion of an object, such forces are called balanced forces.
Unbalanced force : When two opposite forces acting on a body move a body in the direction of the greater force or change the state of rest, such forces are called as unbalanced forces.
Frictional force : Force of friction is the force that always opposes the motion of object.

Question. Why are road accidents at high speeds very much worse than accidents at low speeds?
Answer. The time of impact of vehicles is very small at high speed. So, they exert very large forces on each other.
Hence, road accidents at high speeds are highly fatal.

Question. Name two factors which determine the momentum of a body.
Answer. Two factors on which momentum of a body depend is mass and velocity. Momentum is directly proportional to the mass and velocity of the body.

Question. What decides the rate of change of momentum of an object?
Answer. The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Question. Why do athletes run some distance before jumping?
Answer. Athlete has the inertia of motion and thus continues to move past the line.

Question. Is force a scalar quantity or a vector quantity?
Answer. Force is a vector quantity. It has both magnitude and direction.

Question. Why action and reaction do not cancel each other?
Answer. Action and reaction act simultaneous but on different objects. Hence, they do not cancel each other.

Question. What is inertia? Explain different types of inertia.
Answer. Inertia : The natural tendency of an object to resist change in their state of rest or of motion is called inertia. The mass of an object is a measure of itsinertia. Its S.I. unit is kg.
Types of inertia :
Inertia of rest : The object remain in rest unless acted upon by an external unbalanced force.
Inertia of motion : The object in the state of uniform motion will continue to remain in motion with same speed and direction unless external force is not appliedon it.

Question. What is the acceleration produced by a force of 5 N exerted on an object of mass 10 kg?
Answer.
Here F = 5 N; m = 10 kg; a = ?
Now F = ma or a = F/m
a = 0.5 ms^–2

Question. Which would require greater force : accelerating a 10 g mass at 5 ms^–2 or 20 g mass at 2 ms^–2?
Answer.
In first case m₁ = 10 g = kg = 0.010 kg;
Now al = 5 ms^–2 ; F₁ = ?
F₁ = m₁a₁ = 0.010 × 5
F₁ = 0.050 Newton
In second case, m₂ = 20 g = 0.020 kg
a₂ = 2 ms^–2; F₂ = ?
Now F₂ = m₂a₂ = 0.020 × 2
or F₂ = 0.04 Newton
We find that F₁ > F₂, hence more force is required to accelerate 10 g at 5 ms^–2 than accelerating 20 g at 2 ms^–2.

Question. State Newton’s second law of motion.
Answer. The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

Question. An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.
Answer. Yes, it is possible. An object moving in some direction with constant velocity will continue in its state of motion as long as there are no external unbalanced forces acting on it. In order to change the motion of the object, some external unbalanced force must act upon it.

Question. When a carpet is beaten with a stick, dust comes out of it. Explain.
Answer. When the carpet is beaten with a stick, the stick exerts a force on the carpet which sets it in motion. The inertia of the dust particles residing on the carpet resists the change in the motion of the carpet. Therefore, the forward motion of the carpet exerts a backward force on the dust particles, setting them in motion in the opposite direction. This is why the dust comes out of the carpet when beaten.

Question. Why is it advised to tie any luggage kept on the roof of a bus with a rope?
Answer. When some luggage is placed on the roof of a bus which is initially at rest, the acceleration of the bus in the forward direction will exert a force (in the backward direction) on the luggage. In a similar manner, when a bus which is initially in a state of motion suddenly comes to rest due to the application of brakes, a force (in the forward direction) is exerted on the luggage.
Depending on the mass of the luggage and the magnitude of the force, the luggage may fall off the bus due to inertia. Tying up the luggage will secure its position and prevent it from falling off the bus.

Question. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because (a) the batsman did not hit the ball hard enough. (b) velocity is proportional to the force exerted on the ball. (c) there is a force on the ball opposing the motion. (d) there is no unbalanced force on the ball, so the ball would want to come to rest.
Answer. When the ball rolls on the flat surface of the ground, its motion is opposed by the force of friction (the friction arises between the ground and the ball). This frictional force eventually stops the ball. Therefore, the correct answer is (c).
If the surface of the level ground is lubricated (with oil or some other lubricant), the friction that arises between the ball and the ground will reduce, which will enable the ball to roll for a longer distance.

Question. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if it’s mass is 7 tonnes (Hint: 1 tonne = 1000 kg.)
Answer.
Given, distance covered by the truck (s) = 400 meters
Time taken to cover the distance (t) = 20 seconds
Initial velocity of the truck (u) = 0 (since it starts from a state of rest)

Question. A stone of 1 kg is thrown with a velocity of 20 ms-1 across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice?
Answer.
Given, Mass of the stone (m) = 1kg
Initial velocity (u) = 20m/s
Terminal velocity (v) = 0 m/s (the stone reaches a position of rest)
Distance travelled by the stone (s) = 50 m

Question. An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate: (a) the net accelerating force and (b) the acceleration of the train
Answer.
(a) Given, force exerted by the train (F) = 40,000 N
Force of friction = -5000 N (the negative sign indicates that the force is applied in the opposite direction)
Therefore, the net accelerating force = sum of all forces = 40,000 N + (-5000 N) = 35,000 N

(b) Total mass of the train = mass of engine + mass of each wagon = 8000kg + 5 × 2000kg
The total mass of the train is 18000 kg.
As per the second law of motion, F = ma (or: a = F/m)
Therefore, acceleration of the train = (net accelerating force) / (total mass of the train)
= 35,000/18,000 = 1.94 ms^-2
The acceleration of the train is 1.94 m.s^-2.

Question. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 ms^-2?
Answer.
Given, mass of the vehicle (m) = 1500 kg
Acceleration (a) = -1.7 ms^-2
As per the second law of motion, F = ma
F = 1500kg × (-1.7 ms^-2) = -2550 N
Therefore, a force of 2550 N must act on the vehicle in a direction opposite to that of its motion.

Question. What is the momentum of an object of mass m, moving with a velocity v?
(a) (mv)² (b) mv² (c) ½ mv² (d) mv
Answer.
Since momentum is defined as the product of mass and velocity, the correct answer is (d), mv.

Question. Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?
Answer.
Since the velocity of the cabinet is constant, its acceleration must be zero. Therefore, the effective force acting on it is also zero. This implies that the magnitude of opposing frictional force is equal to the force exerted on the cabinet, which is 200 N. Therefore, the total friction force is -200 N.

Question. Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 ms^-1 before the collision during which they stick together. What will be the velocity of the combined object after collision?
Answer.
Given, mass of the objects (m₁ and m₂) = 1.5kg
Initial velocity of the first object (u₁) = 2.5 m/s
Initial velocity of the second object which is moving in the opposite direction (u₂) = -2.5 m/s
When the two masses stick together, the resulting object has a mass of 3 kg (m₁ + m₂)
Velocity of the resulting object (v) =?
As per the law of conservation of momentum, the total momentum before the collision is equal to the total momentum after the collision.
Total momentum before the collision = m₁u₁ + m₂u₂
= (1.5kg) (2.5 m/s) + (1.5 kg) (-2.5 m/s) = 0
Therefore, total momentum after collision = (m₁+m₂) v = (3kg) v = 0
Therefore v = 0
This implies that the object formed after the collision has a velocity of 0 meters per second.

Question. According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
Answer.
Since the truck has a very high mass, the static friction between the road and the truck is high. When pushing the truck with a small force, the frictional force cancels out the applied force and the truck does not move. This implies that the two forces are equal in magnitude but opposite in direction (since the person pushing the truck is not displaced when the truck doesn’t move). Therefore, the student’s logic is correct.

Question. A hockey ball of mass 200 g travelling at 10 ms^–1 is struck by a hockey stick so as to return it along its original path with a velocity at 5 ms^–1. Calculate the magnitude of change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.
Answer.
Given, mass of the ball (m) = 200g
Initial velocity of the ball (u) = 10 m/s
Final velocity of the ball (v) = 5m/s
Initial momentum of the ball = mu = 200g × 10 ms^-1 = 2000 g.m.s^-1
Final momentum of the ball = mv = 200g × 5 ms^-1 = 1000 g.m.s^-1
Therefore, the change in momentum (mv – mu) = 1000 g.m.s^-1 – 2000 g.m.s^-1 = -1000 g.m.s^-1
This implies that the momentum of the ball reduces by 1000 g.m.s^-1 after being struck by the hockey stick.

Question. A bullet of mass 10 g travelling horizontally with a velocity of 150 m s^–1 strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.
Answer.
Given, mass of the bullet (m) = 10g (or 0.01 kg)
Initial velocity of the bullet (u) = 150 m/s
Terminal velocity of the bullet (v) = 0 m/s
Time period (t) = 0.03 s
To find the distance of penetration, the acceleration of the bullet must be calculated.

Therefore, force exerted by the wooden block on the bullet (F) = 0.01kg × (-5000 ms^-2)
= -50 N

This implies that the wooden block exerts a force of magnitude 50 N on the bullet in the direction that is opposite to the trajectory of the bullet.

Question. An object of mass 1 kg travelling in a straight line with a velocity of 10 ms^–1collides with, and sticks to, a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.
Answer.
Given, mass of the object (m1) = 1kg
Mass of the block (m₂) = 5kg
Initial velocity of the object (u₁) = 10 m/s
Initial velocity of the block (u₂) = 0
Mass of the resulting object = m₁ + m₂ = 6kg
Velocity of the resulting object (v) =?
Total momentum before the collision = m₁u₁ + m₂u₂ = (1kg) × (10m/s) + 0 = 10 kg.m.s^-1
As per the law of conservation of momentum, the total momentum before the collision is equal to the total momentum post the collision. Therefore, the total momentum post the collision is also 10 kg.m.s^-1
Now, (m₁ + m₂) × v = 10kg.m.s^-1

The resulting object moves with a velocity of 1.66 meters per second.

Question. Explain why- An inflated balloon lying on the surface of a floor moves forward when pierced with a pin.
Answer. The momentum of the inflated balloon is zero before it is pierced with a pin. Air comes out with a speed in the backward direction from balloon after it is pierced with a pin. The balloon moves in the forward direction to conserve the momentum.

Question. Explain our walking in terms of Newton’s third law of motion.
Answer. When we walk on ground, our foot pushes the ground backward and in return the ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.

Question. There are three solid balls, made up of aluminium,steel and wood of the same shape and volume. Which of them would have highest inertia? Why?
Answer. Steel ball because the density of steel is maximum. For the same shape and volume, steel ball will have maximum mass.

Question. State Newton’s three laws of motion.
Answer. Sir Isssac Newton further studied the idea of Galileo’s on force and motion and presented three laws of motion. These laws are as follows :
(i) First Law : A body remains in resting position unless it is not introduced with an unbalanced external force.
(ii) Second Law : The rate of change of momentum of a body is directly proportional to the applied unbalanced force and change takes place in the direction of the force.
(iii) Third law : Action and reaction are equal and opposite and they act on different bodies.

Question. What is frictional force?
Answer. The force that always opposes the motion of object is called force of friction.

Question. Give example to show the effects of force.
Answer. (i) Place a ball in the ground. Kick it with your foot. The ball starts moving. The ball moves because of the force applied to it.
(ii) If ball is coming towards you, you can kick it in any direction. The direction of motion of the ball changes because of the force applied to it.
(iii) Place a rubber on the ground. Press it with your foot. It is found that the ball is no longer round but takes the shape of an egg, i.e. it is oblong. The shape of the ball has changed because of the force applied on the ball.

Question. Why we tend to get thrown to one side when a motorcar makes a sharp turn at a high speed?
Answer. We tend to get thrown to one side when a motorcar makes a sharp turn at a high speed due to law of inertia. When we are sitting in moving car on a straight road, we tend to continue in our straight-line motion. But when an unbalanced force is applied on car to change the direction of motion, we slip to one side of the seat due to the inertia of our body.

Question. Why do fielders pull their hand gradually with the moving ball while holding a catch?
Answer. While catching a fast moving cricket ball, a fielder on the ground pulls his hands backwards with the moving ball. This is done so that the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the acceleration of the ball is decreased and therefore, the impact of catching the fast moving ball is reduced.

Question. What is inertia?
Answer.The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia.

Question. It is required to increase the velocity of a scooter of mass 80 kg from 5 to 25 ms–2 in 2 seconds. Calculate the force required.
Answer.
Given : m = 80 kg,
u = 5 ms^–2
v = 25 ms^–2
and t = 2 s
Now acceleration a = change in velocity time
= v – u/t
= 25 – 5/2 = 10 ms^–2
Force = mass × acceleration of F
= ma
Therefore, F = 80 × 10 = 800 N

Question. State Newton’s first law of motion.
Answer. An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

Question. How is force represented graphically?
Answer. Force is a vector quantity. So, force is represented by a line with an arrow head.
(i) The head of arrow represents the direction of the force.
(ii) The length of the line is proportional to the magnitude of the force.
(iii) This is done by choosing a convenient scale.
(iv) For example : A force of 10 N can be represented by a line with an arrow head.
(v) The length of the line depends upon the scale chosen. So, if a scale of 1 cm = 2 N is chosen, the force of 10 N is represented by a line of 5 cm in length.

Question. What are the disadvantages of friction?
or
Why friction is considered wasteful?
Answer. Friction is considered wasteful because :
(1) Friction leads to a loss of energy. Therefore, it reduces the efficiency of machines.
(2) Friction cause wear and tear of machine’s parts.

Question. The following is the distance-time table of an object in motion:

(a) What conclusion can you draw about the acceleration? Is it constant, increasing, decreasing, or zero?
(b) What do you infer about the forces acting on the object.
Answer. (a) The distance covered by the object at any time interval is greater than any of the distances covered in previous time intervals. Therefore, the acceleration of the object is increasing.
(b) As per the second law of motion, force = mass × acceleration. Since the mass of the object remains constant, the increasing acceleration implies that the force acting on the object is increasing as well

Question. Two persons manage to push a motorcar of mass 1200 kg at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of 0.2 m s^-2. With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort)
Answer. Given, mass of the car (m) = 1200kg
When the third person starts pushing the car, the acceleration (a) is 0.2ms^-2. Therefore, the force applied by the third person (F = ma) is given by:
F = 1200kg × 0.2 ms^-2 = 240N

The force applied by the third person on the car is 240 N. Since all 3 people push with the same muscular effort, the force applied by each person on the car is 240 N.

Question. A hammer of mass 500 g, moving at 50 m s^-1, strikes a nail. The nail stops the hammer in a very short time of 0.01 s. What is the force of the nail on the hammer?
Answer. Given, mass of the hammer (m) = 500g = 0.5kg
Initial velocity of the hammer (u) = 50 m/s
Terminal velocity of the hammer (v) = 0 (the hammer is stopped and reaches a position of rest).
Time period (t) = 0.01s

a = -5000ms^-2
Therefore, the force exerted by the hammer on the nail (F = ma) can be calculated as:
F = (0.5kg) * (-5000 ms^-2) = -2500 N
As per the third law of motion, the nail exerts an equal and opposite force on the hammer. Since the force exerted on the nail by the hammer is -2500 N, the force exerted on the hammer by the nail will be +2500 N.

Question. A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 km/h. Its velocity is slowed down to 18 km/h in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.
Answer. Given, mass of the car (m) = 1200kg
Initial velocity (u) = 90 km/hour = 25 meters/sec
Terminal velocity (v) = 18 km/hour = 5 meters/sec
Time period (t) = 4 seconds

Therefore, the acceleration of the car is -5 ms^-2.
Initial momentum of the car = m × u = (1200kg) × (25m/s) = 30,000 kg.m.s^-1
Final momentum of the car = m × v = (1200kg) × (5m/s) = 6,000 kg.m.s^-1
Therefore, change in momentum (final momentum – initial momentum) = (6,000 – 30,000) kg.m.s^-1
= -24,000 kg.m.s^-1
External force applied = mass of car × acceleration = (1200kg) × (-5 ms^-2) = -6000N
Therefore, the magnitude of force required to slow down the vehicle to 18 km/hour is 6000 N

Question. An object of mass 100 kg is accelerated uniformly from a velocity of 5 ms–1 to 8 ms^–1 in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.
Answer. Given, mass of the object (m) = 100kg
Initial velocity (u) = 5 m/s
Terminal velocity (v) = 8 m/s

Time period (t) = 6s
Now, initial momentum (m × u) = 100kg × 5m/s = 500 kg.m.s^-1
Final momentum (m × v) = 100kg × 8m/s = 800 kg.m.s^-1

Therefore, the object accelerates at 0.5 ms^-2. This implies that the force acting on the object (F = ma) is equal to:
F = (100kg) × (0.5 ms^-2) = 50 N
Therefore, a force of 50 N is applied on the 100kg object, which accelerates it by 0.5 ms^-2.

Question. Akhtar, Kiran, and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.
Answer. Kiran’s suggestion is correct. The mass of the insect is very small when compared to the mass of the car. As per the law of conservation of momentum, the total momentum before the collision between the insect and the car is equal to the total momentum after the collision. Therefore, the change in the momentum of the insect is much greater than the change in momentum of the car (since force is proportional to mass).
Akhtar’s suggestion is also correct. Since the mass of the car is very high, the force exerted on the insect during the collision is also very high.
Rahul’s suggestion is partially correct. As per the third law of motion, the force exerted by the insect on the car is equal and opposite to the force exerted by the car on the insect. However, Rahul’s suggestion that the change in the momentum is the same contradicts the law of conservation of momentum.

Question. How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be 10 ms^–2.
Answer. Given, mass of the dumb-bell (m) = 10kg
Distance covered (s) = 80cm = 0.8m
Initial velocity (u) = 0 (it is dropped from a position of rest)
Acceleration (a) = 10ms^-2
Terminal velocity (v) =?
Momentum of the dumb-bell when it hits the ground = mv

v = 4 m/s
The momentum transferred by the dumb-bell to the floor = (10kg) × (4 m/s) = 40 kg.m.s^-1

First Law of Motion

Frictional force
The force that opposes relative motion is called friction. It arises between the surfaces in contact.
Example: When we try to push a table and it does not move is because it is balanced by the frictional force.

Galileo’s Observation
• He observed the motion of objects on an inclined plane.
• When a marble is rolled down an inclined plane its velocity increases.

Galileo’s Arguments
• When a marble is rolled down from the left – It will go up on the opposite side up to the same height at which it is dropped down.

• If the inclination of planes is equal – The marble would travel equal distances while climbing up as travelled while rolling down.

• If we decrease the angle of inclination of the right plane – The marble would travel further until it reaches its original height.

• If the right side plane is made flat – Marble would travel forever to achieve the same height.

Galileo's Inference
• We need an unbalanced force to change the motion of the marble but no force is required when the marble is moving uniformly. In other words, objects move at a constant speed if no force acts upon them

First Law of Motion
A body continues to be in the state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. First Law is also called the Law of Inertia.

Inertia
Basically, all objects have a tendency to resist the change in the state of motion or rest. This tendency is called inertia. All bodies do not have the same inertia. Inertia depends on the mass of a body. Mass of an object is the measure of its inertia. More the mass → more inertia and vice versa.

Types of Inertia
It is of three types:
▪ Inertia of Rest
▪ Inertia of Motion
▪ Inertia of direction
Inertia of Rest: Is the inability of body to change the state of rest on its own.
Inertia of Motion: Is the inability of body to resist the change in state of motion on its own.
Inertia of direction: Is the inability of body to resist the change in direction of body on its own. Examples to understand more about Inertia

1. Have you noticed that passengers in a bus fall forward when bus stops suddenly.
REASON:- When the bus is moving, the bus and the passengers sitting in it are in motion. But when force is applied on the brakes, it stops, but due to inertia of motion the passengers remain in the state of motion, hence, fall forward.

2. Have you experienced that when carpet is beaten with a stick, the dust separates out
REASON:- Initially the carpet and dust are in state of rest. But when carpet is beaten with a stick, it comes in motion and moves backward. But due to inertia of rest, the dust particles remain in the state of rest hence, get separated.

3. Have noticed that leaves and fruits falls down when branch of tree is shaken vigorously
REASON:- Initially, the branch of a tree and its leaves and fruits are in state of rest. But when force is applied on the branch of a tree, the tree comes in motion but due to inertia of rest, the leaves and fruits remain in state of rest hence, falls down.

Exam Questions NCERT Class 9 Science Chapter 9 Force and Laws of Motion

Question. Mention the factors on which the force of friction depends?
Ans : (a) The force of friction is directly proportional to the weight of the body sliding over the surface.
(b) The force of friction depends on the surfaces in contact.

Question. What does first law of motion indicates when an unbalanced external force acts on an object?
Ans : First law of motion indicates that when an unbalanced external force acts on an object, its velocity changes i.e. the object gets acceleration.

Question. What happens to the velocity of an object when the force acting on the object are balanced?
Ans : An object can move with uniform velocity only when the forces (pushing force and friction force) acting on the object are balanced.

Question. What is resultant force?
Ans : The resultant force of several forces acting simultaneously on a body is that single force which produces the same effect on a body as all these forces produce together.

Question. How is inertia measured quantitatively?
Ans : Quantitatively the inertia of an object is measured by its mass.

Question. Why are roads on mountains inclined inwards at turns?
Ans : A vehicle moving on mountains is in the inertia of motion. At a sudden turn there is a tendency of vehicle to fall off the road due to sudden change in the line of motion hence the roads are inclined inwards so that the vehicle does not fall down the mountain.

Question. If a person jumps from a height on a concrete surface he gets hurt. Explain.
Ans :
When a person jumps from a height he is in state of inertia of motion. When he suddenly touches the ground he comes to rest in a very short time and hence the force exerted by the hard concrete surface on his body is very high, and the person gets hurt.

Question. When a force of 40 N is applied on a body it moves with an acceleration of 5 ms2. Calculate the mass of the body.
Ans :
Let m be the mass of the body.
Given : F = 40 N, a = 5 ms2
From the relation F = m a, we have
40 = m × 5
m= 40/5 = 8 kg

Question. Which will have more inertia a body of mass 10 kg or a body of mass 20 kg?
Ans : A body of mass 20 kg will have more inertia, as inertia is measure of mass.

Question. Name the factor on which the inertia of the body depends.
Ans : Inertia of a body depends upon the mass of the body.

Question. If a man jumps out from a boat, the boat moves backwards. Why?
Ans : When a man jumps out of a boat to the bank of the river, he supplies a force in the forward direction. Due to the reaction of this, the boat moves backwards.

Question. It is easier to push an empty box than to push the box full of books. Give reason.
Ans : This is because the empty box has less inertia of rest than the box full of books.

Question. Define SI unit of force.
                      or
What is one Newton force?
Ans : A Newton is that force which act on a body of mass of 1 kg to produce an acceleration of 1 ms^–2 in it.