CBSE Class 9 Science Force and Laws of Motion Assignment Set B

• Force is a push or pull upon an object resulting from the object's interaction with another object. The various effects of force are:
• Force can move a body initially at rest.
• Force can bring a moving body to rest.
• Force can change the direction of a moving body.
• Force can change the speed of a moving body.
• Force can change the shape of a body.
• Force can change the size of a body.

• Muscular force – It involves the action of muscles.
• Animals make use of muscular force to carry out their physical activities and other tasks.

• Friction – It is an opposing force that acts between surfaces in contact moving with respect to each other.
• Frictional force always acts between two moving objects, which are in contact with one another.
• Frictional force always acts opposite to the direction of motion.
• Frictional force depends on the nature of the surface in contact.

• Tension Force - This force appears in a string, attached to a rigid support, when an object is suspended by it.

• Mechanical Force - It involves the force generated by machines.

• Force exerted during collision - Two objects push each other with an equal but opposite forces if collision occurs between them. These forces are known as the force of action and force of reaction.

• Combined Forces - When two or more forces are acting on the same object.

• Non-contact force come into play even when the bodies are not in contact

• Magnetic force – Force acting between two magnets or a magnet and a magnetic material (eg. iron, steel, nickel, cobalt etc.). It can be attractive and repulsive.
• Electrostatic force – Force due to electric charges. It can be attractive and repulsive.
• Gravitational force – It is a kind of attractive force that comes into play because of the mass of a body. (eg. earth's gravitational attraction).

• First law of motion
• A body at rest remains at rest and a body in uniform motion continues its uniform motion unless an external force is applied.

• Inertia:
• It is the tendency of a body to resist any change in its state of rest or of uniform motion along a straight line.
• Mass of an object is the measure of its inertia, more is the mass more is the inertia.
• Types of inertia: Inertia of rest and motion
• Momentum is the product of the mass of the body and its velocity. It is a vector quantity.
• Momentum = Mass × Velocity
• SI unit of the momentum is kg m/s.

• Second law of motion
• The second law of motion says, when a force F is subjected to a body of mass m, an acceleration a is gained by the body in the direction of the force and the magnitude of acceleration is directly proportional to the F and inversely proportional to the m.
• Rate of change of momentum ∝ Applied unbalanced force
• Direction of change in momentum is the same as the direction of unbalanced force
F = ma
Unit of force is Newton
1 N = 1 kg × 1 m / s²

• Third law of motion
• For every action force there is an equal and opposite reaction force.
• The horse and the cart: From the third law of motion the pull by the horse in the forward direction is equal to the pull by the cart in the backward direction. The sum of these forces is therefore zero. Why should then the cart accelerate forward? Apart from the pulls of the horse and the cart there is frictional force and the reaction of the ground on the horse and the cart is also present. The resultant force of this normal reaction and the friction together helps the horse to move the cart in the forward direction.

• Universal law of gravitation

│F¯│ = G (m₁m₂/r₂)
F¯ = G (m₁m₂/r₂) (-r) = - G (m₁m₂/r₂) (r)

• For a point mass

The point of attraction between a hollow spherical shell of uniform density and a point mass situated outside is just as if the entire mass of the cell is concentrated at the center of the shell.
The force of attraction due to a hollow spherical shell of uniform density, on a point mass inside it is zero.

Gravitation Constant, G = 6.67 × 10^–11 Nm²/kg²

• Free Fall: A body is said to be free falling if it begins to fall downward towards the earth due to earth’s gravity only.
mg=GMmR2g=GMR2
‘g = 9.8 m/s²’ is the acceleration due to gravity, it is the acceleration of a freely falling body.

• Tips to solve numerical
• For upward motion take g = –9.8m/s2 and final velocity at the highest point as 0.
• For downward motion take g = 9.8m/s2 and for a freely falling body take initial velocity as 0. 

Equations of motion of an object under the influence of the earth’s gravity

for downward motion of the particle
v=u+gts=ut+12gt2v2=u2+2gs
for upward motion of the particle
v=u-gts=ut-12gt2v2=u2-2gs

• Mass: It is the amount of matter contained in the body

• Weight: It is the force exerted on a body due to the gravitational pull of another body such as the earth, the sun etc.

Weight of the object on the moon/Weight of the object on the earth = 1/6

Mark (1)

Q 1 Why does a boxer move his head backwards to minimize the effect of on coming punch?

Q 2 Briefly explain how an expert karate player breaks a slab of ice with a single blow.

Q 3 What is meant by balanced forces?

Q 4 A swimmer swims forward , even though he pushes water backward while swimming. Why?

Q 5 When a force acting on a body has an equal & opposite reaction, then why should the body move at all.

Q 6 What do you mean by impulsive force?

Q 7 Define impulse.

Q 8 Define momentum.

Q 9 What force is needed to produce an acceleration of 2 m/s2 in a body of mass 3kg?

Q 10 Give a simple experiment to illustrate the inertia of rest.

Marks (2)

Q 11 Why it is advised to tie the luggage with a rope on the roof of buses?

Q 12 In oil tankers some space is left at the top while filling them. Explain why?

Q 13 A person is prone to more serious injuries when falling from a certain height on a hard concrete floor than on a sandy surface. Explain why.

Q 14 A body is moving on a rough level road with a speed of 15m/s along a given direction. Does any force is needed to maintain this speed? Why?

Q 15 What happens when you shake a wet piece of cloth?

Q 16 An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle & road if the vehicle is to be stopped with a negative acceleration of 1.7 ms-2?

Q 17 What do you mean by the force of friction? How can it be minimised?

Q 18 Define force. Give its SI unit.

Marks (3)

Q 19 Explain why does a gun recoil when a shot is fired from it?

Q 20 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 ms -2 .With what force does each person push the motorcar?

Q 21 A force of 5 N gives a mass m, an acceleration of 10 m/s2 & on mass m2, an acceleration of 20 m/s2 , what acceleration would it give if both the masses were tied together?

Q 22 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?

Q 23 A car with a dead battery, is to be pushed for some time so as to start it why? What does this example signify?

Q 24 Which would require a greater force, accelerating 10g mass at 5 m/s2, or a 20 g mass at 2 m/s2?

Q 25 How many types of inertia do the material bodies have?

Q 26 State the three Newton‘s Laws of Motion.

Q 27 At the top of the oil tankers, some space is left while filling them. Explain, why.

Q 28 Two blocks made of different metals identical in shape and size are acted upon by equal forces which cause them to slide on a horizontal surface. The acceleration of the second block is found to be 4 times that of the first. What is the ratio of the mass of the first to second?

Marks (5)

Q 29 An 8000 kg engine pulls a train of 5 wagons, each wagon of mass 2000 kg, along a horizontal track. If the engine exerts a force of 40,000N & the track offers a friction force of 5,000 N.
Calculate
a) the net accelerating force,
b) the acceleration of the train, and
c) the force of wagon 1 on wagon 2.

Q 30 Two cars weighing 1500 kg are made to collide with a wall. The initial & final velocities of the car are - 15.0 m/s & 2.6 m/s respectively. If the collision lasts for 0.15 s, then find impulsive force exerted on the car.

Q 31 A bullet of mass 20 g and with the velocity of 150ms-1 moving horizontally strikes a wooden material and comes to rest in 0.02s. Calculate the magnitude of the force exerted by the wooden material on the bullet.

Q 32 Two football players of opposite teams collide while they are trying to hit a football on the ground and after colliding, they move off together. One with a mass of 60 kg was travelling with a velocity of 5.0 m s-1 and the other footballer with a mass of 55 kg was moving faster with a velocity 6.0 m s-1 towards the first player. What is the direction and the velocity with which they move after they become entangled?

Q 33 A large bus and a van, both moving with a velocity of magnitude v, have a head-on-collision and both the vehicles stop after the collision. If the time of the collision is 1 sec then,
a) Which vehicle experiences smaller force of impact?
b) Which vehicle experiences the smaller momentum change?
c) Which vehicle experiences the greater acceleration?
d) Why is it that the truck suffers less damage than the car?
 

Most Important Questions

Q 1 Define inertia.

Q 2 What force is needed to produce on acceleration of 2 m/sin a body of mass 3kg?

Q 3 What is meant by balanced forces?

Q 4 What is force?

Q 5 A person is prone to more serious injuries when falling from a certain height on a hard concrete floor than on a sandy surface. Explain why.

Q 6 Give a simple experiment to illustrate the inertia of rest.

Q 7 Write Newton‘s Laws of Motion.

Q 8 Which would require a greater force accelerating 10g mass at 5 m/s2, or a 20 g mass at 2 m/s2?

Q 9 Explain why does a gun recoil when a shot is fired from it?

Q 10 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?

Q 11 A hammer of mass 500 g, moving at 50 m/s, 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?

Q 12 Give the statement of second law of motion. Hence derive its mathematical formula.

Q 13 why we tend to fall forward when a bus suddenly stops?

Q 14 Why we tend to fall backward when a bus suddenly starts?

Q 15 A force changes the velocity of a box having mass 2kg from 2 m/sec to 5 m/sec in 3sec. Find the acceleration and the magnitude of force.

Q 16 A girl of mass 40kg having velocity 2m/sec jumps on a stationary cart of mass 4kg. Find the common velocity with which both will travel?

Q 17 Two spring balance are attached to each other as given in the figure:

If some force pulls the right spring balance will the left spring balance show the same reading? Why?

Q 18 A ball of mass 5 kg moving with velocity 3m/sec strikes a ball of mass 2 kg kept at rest. If the lighter ball moves with a velocity 2 m/sec after the collision find the velocity of the heavier ball?

Q 19 A trolley of mass 30kg is kept at rest. A force makes it to move with velocity 3m/sec in 2 sec. Find the force applied on it?

Q 20 If action is always equal and opposite to reaction, then how a horse is able to pull the cart?

Q 21 Define inertia.

Q 22 What force is needed to produce on acceleration of 2 m/s2 in a body of mass 3kg?

Q 23 What is meant by balanced forces?

Q 24 What is force?

Q 25 A person is prone to more serious injuries when falling from a certain height on a hard concrete floor than on a sandy surface. Explain why.

Q 26 Write Newton‘s Laws of Motion.

Q 27 Explain why does a gun recoil when a shot is fired from it?

Q 28 A hammer of mass 500 g, moving at 50 m/s, 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?

Q 29 why we tend to fall forward when a bus suddenly stops?

Q 30 Why we tend to fall backward when a bus suddenly starts?

Q 31 If action is always equal and opposite to reaction, then how a horse is able to pull the cart?

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

Question. Calculate the force required to impart to a car a velocity of 30 m/s in 10 s. The mass of the car is 1500 kg.
Ans :
u = 0, v = 30 m/s, t = 10 s
v = u + at
30 = 0 + a × 10
a = 3 ms–2
Now m = 1500 kg, a = 3 ms–2
Required Force = F = ma
1500 × 3 = 4500 N

Question. Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans :
Here u = 0 ms–1; v = 30 ms–1; t = 10 s; a = ?
Using                   v = u + at, we have
                         30 = 0 + a (10)
                           a = 3 ms–2
Now                   F = ma = 1,500 × 3
or                        F = 4,500 N

Question. An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans :
Given,
Acceleration, a = 8 ms–2
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion, s = ut + 2
1 at2, one gets
s = 0 × 1 + 2
1 × 8 × 12 = 4 m
The object travels a distance of 4 m.

Question. Name three non-SI units of force. Define them.
Ans : Some commonly used non-SI units force are :
Dyne : The dyne is the CGS unit of force. One dyne is the force which produces an acceleration of 1 cm/s2in a body mass 1 g, i.e.,
dyne = 1 G * 1 cm/s2
Gram-weight : It is the gravitational unit of force in CGS system. This is denoted as g-wt.
One gram weight is the force which produces an acceleration of 981 cm/s2 in a body of mass 1 g. Thus,
g-wt = 1 g * acceleration due to gravity (g)
Or, 1 g-wt = 1 g * 981 cm/s2 = 981 dynes
[a 1 g * 1 cm/s2 = 1 dynes]
Kilogram-weight : It is the gravitational unit of force in SI system. This unit is denoted as kg-wt and it is the force which produces an acceleration of 9.81 m/s2
in a body of mass 1 kg. Thus,
1 kg-wt = 1 kg * 9.81 m/s2

Question. Explain inertia and momentum.
Ans : Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position. Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the
product of the mass and velocity of the body and is denoted by p. p = mv
Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms–1).

Question. Give 3 examples in daily life which shows inertia.
Ans : Three examples of inertia in daily life are :
(i) When we are travelling in a vehicle and sudden brakes are applied we tend to fall forward.
(ii) When we shake the branch of a tree vigorously, leaves fall down.
(iii) If we want to remove the dust from carpet we beat the carpet so that dust fall down.

Question. Name the various types of forces.
Ans : Various types of forces are :
(i) Muscular force,
(ii) Tension,
(iii) Gravitational,
(iv) Reaction,
(v) Frictional forces,
(vi) Electrical forces,
(vii) Magnetic forces.

Question. Use law of conservation to explain why a gun recoils.
Ans : Gun and bullet both are at rest before firing. Hence, their initial velocities are zero. So, the initial momentum of the gun and the bullet is zero before fire. When a bullet is fired from the gun, it moves forward with a large velocity. The bullet imparts an equal and opposite momentum to the gun due to which the gun recoils backwards. Thus, the final momentum of the system is zero.

Question. Why is the movement of a rocket in the upward direction?
Ans : (i) The movement of a rocket in the upward direction can also be explained with the help of the law of conservation of momentum.
(ii) The momentum of a rocket before it is fired is zero. When the rocket is fired, gases are produced n the combustion chamber of the rocket due to the burning of fuel. These gases come out of the rear of the rocket with high speed. The direction of the Momentum of the gases coming out of the rocket is in the downward direction. To conserve the momentum of the system (rocket gases), the rocket moves upward with a momentum equal to the momentum of the gases. The rocket continues to move upward as long as the gases are ejected out of the rocket.

Question. What is momentum?
Ans : The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. The SI unit is kg m/s. (p = mv)

Question. State Newton’s third law of motion.
Ans : To every action, there is an equal and opposite reaction and they act on two different bodies.

Question. Which type of force does not change the state of rest or of motion of an object?
Ans : Balanced force.

Question. Why do athletes have a special posture with their right foot resting on a solid supporter for athletic races?
Ans : Athletes have to run the heats and they rest their foot on a solid supports before start so that duringthe start of the race the athlete pushes the support with lot of force and this support gives him equal and opposite push to start the race

KEY CONCEPTS

1 Motion (Uniform Motion And Non Uniform Motion, Acceleration and Velocity)

• A particle is a point-like object, has mass but infinitesimal size
• The object’s position is its location with respect to a chosen reference point, In the diagram, the road sign the reference point
• Motion occurs when an object changes its position. Both Distance and Time are important in describing motion.
• Sometimes you know motion has occurred even if you didn’t see it happen. (mail truck)
• Relative motion: when two objects are moving in a plane (either in same direction or opposite) each have relative motion with respect to second. e.g. a person sitting in a train and watching a tree, in this case tree is stable but is assumed to be moving but with respect to train.

Distance vs. Displacement

• Distance: How far an object has moved. It has only magnitude without direction. (total)
• Displacement: How far and in what direction an object has moved from its start position. i.e. the direct distance between two points.

Speed

• Speed = the distance an object travels in a given amount of time
• Speed = distance / time\
• sI unit of speed is m/s

Types of Speed

• Constant speed: speed doesn’t change (set your car on cruise control)
• Changing speed: Riding a bike for 5 km. Take off and increase speed, slow down up hill, speed up down hill, stop for stop sign. The trip took you 15 min (.25 h)

• Average speed: total distance /total time

• Instantaneous speed: speed at any given time.

Velocity

• Velocity: includes speed and DIRECTION
• Storm is moving at 20km/hr.
• Should you be seeking shelter?
• Suppose two trains are going with the same speed in opposite direction so they are having different velocities.
• Race car going around an oval track might have constant speed, but different velocities at each point.

Acceleration

• Any change in velocity over a period of time is called acceleration.
• The sign (+ or -) of indicates its direction. + sign shows the acceleration and – sign shows de-acceleration.
• Uniform (constant) acceleration equation
• Images of car are equally spaced.
• The car is moving with constant positive velocity (shown by red arrows maintaining the same size) .
• The acceleration equals to zero
• Images of car become farther apart as time increases
• Velocity and acceleration are in the same direction
• Acceleration is uniform (Arrows below the car maintain the same length)
• Velocity is increasing (Arrows above the car are getting longer)
• This shows positive acceleration and positive velocity

The instant speed at points of equal elevations is the same.

The velocities are different because they are in opposite
Free Fall & Air Resistance

Galileo Galilei Italian physicist and astronomer

Formulated laws of motion for objects in free fall

• A freely falling object is any object moving freely under the influence of gravity alone.
• It does not depend upon the initial motion of the object
• Dropped – released from rest
• Thrown downward
• Thrown upward
• The acceleration of an object in free fall is directed downward, regardless of the initial motion
• The magnitude of free fall acceleration (gravitational acceleration) is g = 9.80 m/s2
• g decreases with increasing altitude
• g varies with latitude, height and depth from earth surface.
• 9.80 m/s2 is the average at the Earth’s surface
• The italicized g will be used for the acceleration due to gravity
• Not to be confused with g for grams
• With negligible air resistance, falling objects can be considered freely falling. objects of different shapes accelerate differently (stone vs feather)
• Speed both upward and downward

• The path is symmetrical.
• Acceleration is constant.
• The magnitude of the velocities is the same at equal heights.
• Images become closer together as time increases
• Acceleration and velocity are in opposite directions when ball goes upward.
• Acceleration is uniform (violet arrows maintain the same length)
• Velocity is decreasing in upward motion (red arrows are getting shorter)
• Positive velocity and negative acceleration
• Velocity becomes zero at maximum height.
• Time duration flight in going upward and coming back is always same.

Test Yourself :

1. What is SI Unit of displacement?
2. Name the quantity which represents rate of change of velocity. 
3. A particle describes a semicircle of radius l 14m. What are its distance and displacement covered?

2 Graphical Representation Of Motion & Graphs

Test Yourself :

1. What does slope of Position – Time graph represent?
2. If velocity –time graph is parallel to time axis, what type of motion does it represent?

3 Equation of motion

(1) When object is moving in straight line-

• v = vo + at
• x = xo + vot + ½ at2
• v2 = vo 2 + 2a(Δx)
• Average acceleration describes how fast the velocity is changing with respect to
  time.

• where: a_ave = average acceleration
• v = change in velocity
• x = displacement
• t = elapsed time

(2) when object is coming vertically downward-

• v = vo + gt
• h = vot + ½ gt²
• v² = vo² + 2ah

(3) when object is coming vertically upwardv
= vo - gt
h = vot - ½ gt²
v² = vo
2 - 2gh

• The SI unit of velocity is the m/s.

Average acceleration is + or – depending on direction.

• Instantaneous Acceleration

• Instantaneous acceleration is the limit of Δv/Δt as Δt approaches zero.
• Instantaneous acceleration is zero where slope is constant
• Instantaneous acceleration is positive where curve is concave up
• Instantaneous acceleration is negative where curve is concave down