# Force And Laws Of Motion Chapter 9 Class 9 Science Assignments

Please refer to Force And Laws Of Motion Chapter 9 Class 9 Science Assignments below. We have provided important questions and answers for Force And Laws Of Motion which is an important chapter in Class 9 Science. Students should go through the notes and also learn the solved assignment with solved questions provided below. All examination and class tests questions are as per the latest syllabus and books issued by CBSE, NCERT, and KVS. We have also provided Class 9 Science Assignments for all chapters on our website.

## Chapter 9 Force And Laws Of Motion Class 9 Science Assignments

Question. Why action and reaction do not cancel each other?

Action and reaction act simultaneous but on different objects. Hence, they do not cancel each other.

Question. If a man jumps out from a boat, the boat moves backwards. Why?

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. 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?

Steel ball because the density of steel is maximum.For the same shape and volume, steel ball will have maximum mass.

Question. Which will have more inertia a body of mass 10 kg or a body of mass 20 kg?

A body of mass 20 kg will have more inertia, as inertia is measure of mass.

Question. It is easier to push an empty box than to push the box full of books. Give reason.

This is because the empty box has less inertia of rest than the box full of books.

Question. No force is required to move an object with a constant velocity. Why?

For an object moving with a constant velocity, a = 0, so F = ma = 0

Question. Is force a scalar quantity or a vector quantity?

Force is a vector quantity. It has both magnitude and direction.

Question. Explain our walking in terms of Newton’s third law of motion.

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. Define SI unit of force.
or
What is one Newton force?

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.

Question. Define force.

It is a push or pull on an object that produces acceleration in the body on which it acts.

Question. What is balanced force?

When 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.

Question. Two similar trucks are moving with the same velocity on a road. One of them is loaded while the other one is empty. Which of the two will require a larger force to stop it?

The loaded truck will require a larger force to stop because it has greater momentum.

Question. State Newton’s second law of motion.

The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

Question. Name the scientist who introduced the property of momentum.

Newton.

Question. What is the direction of momentum?

The direction of momentum is the same as that of velocity.

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

Balanced force.

Question. Why is it dangerous to jump out from a moving bus?
Answer. While moving in a bus our body is in motion. On jumping out of a moving bus our feet touches the ground and come to rest. While the upper part of our body fall in motion and moves forward due to inertia of motion and hence we can fall in forward direction.
Thus, to avoid this we need to run forward in the direction of bus.

Question. What happens when you shake a wet piece of cloth? Explain your observation.
Answer. When a wet piece of cloth is shaken, small droplets of water fall down. This is because in the beginning both water and the piece of cloth were at rest. When the cloth is moved, the water in it tends to remain in the state of rest due to inertia of rest. As a result, the droplets of water fall down and the clothes dry quickly.

Question. Give difference between balanced and unbalanced forces.

Question. Why is it necessary to bend knees while jumping from greater height?
(i) Our feet come to rest at once during the jump.
(ii) Due to the less time a large force acts on the feet.
(iii) If we bend out knees, the time of impact increases.
Hence, less force acts on the feet.

Question. Why is it difficult to achieve a zero unbalanced force in practical situations? In practice what happens to a rolling marble? How can we reduce the effect of friction on a marble?
(i) It is difficult to achieve a zero unbalanced force because of the presence of the frictional force acting opposite to the direction of motion.
(ii) In practice, the rolling marble stops after travelling some distance.
(iii) The effect of frictional force may be reduced by using a smoother marble and a smoother plane and providing a lubricant.

Question. Use law of conservation to explain why a gun recoils.
Answer. 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 do the passengers fall backward when a bus accelerates from rest?
Answer. The passengers fall backward when a bus accelerates from rest due to their inertia. The passengers tend to remain in their state of rest even when the bus starts moving.

Question. What do you understand by the gravitational unit of force?
Answer. A gravitational unit of force is defined as that force which produces an acceleration equal to the acceleration due to gravity in a body of one unit mass. Gram-weight and kilogram-weight are the gravitational unit of force.

Question. Mention the factors on which the force of friction depends?
(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. Why are tyres corrugated and made rough?
(1) This provides better grip on the road.
(2) It also prevents the vehicles from slipping.

Question. What does first law of motion indicates when an unbalanced external force acts on an object?
Answer. 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. How are safety belts helpful in preventing any accidents?
or
Why do you think it is necessary to fasten your seat belts while travelling in your vehicle?
Answer. While travelling in a moving car, our body remains in the state of rest with respect to the seat. But when driver applies sudden breaks in the car, our body tends to continue in the same state of motion because of its inertia. Therefore, this sudden break may cause injury to us by impact or collision. Hence, safety belt exerts a force on our body to make the forward motion slower.

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

Question. Give 3 examples in daily life which shows inertia.
Answer. 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. What is resultant force?
Answer. 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. Write the effect of force.
or
What change will be in a body by force?
Ans : Force can produce three effects :
(a) It can change the magnitude of velocity of an object (i.e. to make the object move faster or slower).
(b) It can change the direction of motion of an object.
(c) It can change the shape and size of an object.

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

Question. What are the changes possible on an object at rest if we apply on it?
(a) A balanced force?
(b) An unbalanced force?
(a) Object may changes its shape or size.
(b) Objects changes its speed, velocity or accelerates.

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. When we hold a bag full of books in our hand steady at some height above the ground, what kind of forces are acting on it?
Answer. The bag full of books is steady. This means its position is fixed. So, it must be under balanced forces. The forces acting on it are :
(i) Weight (or the gravitational pull) downwards,
(ii) Upward force applied by the person holding the bag.

Question. State the law of conservation of momentum.
(i) Momentum of two bodies before collision is equal to the momentum after collision.
(ii) In an isolated system, the total momentum remains conserved.

Question. Give example to show the effects of force.
(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. Name three non-SI units of force. Define them.
Answer. 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/s2 in 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/sin a body of mass 1 kg. Thus,
1 kg-wt = 1 kg * 9.81 m/s2

Question. (a) What is friction?
(b) What are the advantages of friction?
or
Why is friction necessary?
(a) The force which opposes the motion of one body over the surface of another is called friction or the force of friction.
(b) Friction is very desirable and an important force in our daily life.
Some advantages of friction are :
(i) The nails and screws hold the wooden boards together due to friction.
(ii) It is due to the friction between the ground/road and the soles of our shoes that we are able to walk.
(iii) The friction between the road and the surface of the tyres permits safe driving.
(iv) We are able to write on paper because of the friction between the pen/pencil and the paper.That is why, it is very difficult to write on a glazed/waxed paper.
(v) The application of brakes to stop a cycle, scooter or car, etc., is possible due to friction between the brake-lining and the rim of the wheel.

Question.(i) What are the causes of friction?
(ii) How can friction between any two surfaces can be reduced?
or
Describe some method for reducing friction.
Answer. The friction rises due to the following factors :
1. Due to the force of attraction between the molecules of the two surfaces in contact. This is called the force of adhesion.
2. Due to the interlocking of the surface irregularities.
3. Thus, friction is due to the roughness of the two surfaces in contact.
4. The friction between two surfaces can be reduced by following methods :
(i) By polishing surfaces : Rough surfaces can be made smooth by polishing. Therefore, polishing reduces friction.
(ii) By applying oil or grease on the surfaces : Oil/ grease forms a thin layer between the two surfaces and reduces friction.

Numerical Questions

Question. Which would require greater force : accelerating a 10 g mass at 5 ms–2 or 20 g mass at 2 ms–2?
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. A truck starts from rest and rolls down a hill with constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 metric ton.
Here u = 0 ms–1 (Starting from rest);
s = 400 m; t = 20 s; a = ?
Using s = ut + 1/2 at2,
We have 400 = 0 × 20 + 1/2 × a × 202 = 200 a
a = 400/200
or a = 2 ms–2
Now mass m = 7 metric ton = 7,000 kg; F = ?
F = ma = 7,000 x 2
or F = 14,000 N

Question. A force of 5 N gives a mass m1, an acceleration of 8 ms–2 and a mass m2, an acceleration of 24 ms–2. What acceleration would it give if both the masses are tied together?
Let us first find mass m1 and m2.
F = m1a1
5 = ml (8)
or m1 = 8/5 kg
F = m2a2
5 = m2 (24) or m2 = 5/24 kg
Total mass M = (m1 + m2)

Let A be the acceleration produced in mass M.
F = m × a

Hence, the acceleration of the combination is 6 ms 2.

Question. A car of mass 1,000 kg moving with a velocity of 40 km h–1 collides with a tree and comes to stop in 5 s.What will be the force exerted by car on the tree?
Here, m = 1,000 kg
u = 40 kmh–1 = 100 -1 /9 ms
v = 0; t = 5 s
F = ma

Question. A car of mass 1,000 kg moving with a velocity of 40 km h–1 collides with a tree and comes to stop in 5 s.What will be the force exerted by car on the tree?
Here, m = 1,000 kg
u = 40 kmh–1 = 100 -1 /9 ms
v = 0; t = 5 s
F = ma
or F = – 2,222 N

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 atrest, 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. An iron sphere of mass 10 kg is dropped from a height of 80 cm. If the downward acceleration of the ball is 10 ms–2, calculate the momentum transferred to the ground by the ball.
Here, initial velocity of sphere, u = 0
Distance travelled, s = 80 cm = 0.8 m
Acceleration of sphere, a = 10 ms–2

Step 1. Final velocity of sphere when it just reaches the ground can be calculated using
v2 – u2 = 2as
u2 – 0 = 2 × 10 ms–2 × 0.8 m
= 16 m2s–2

Momentum of the sphere just before it touches the ground = mv = 10 kg × 4 ms–1 = 40 kg ms–1

Step 2. On reaching the ground, the iron sphere comes to rest, so its final momentum = 0
According to the law of conservation of momentum,Momentum transferred to the ground = momentum of the sphere just before it comes to rest = 40 kg ms–1.

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.
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. A boy of mass 60 kg running at 3 m/s jumps on to a trolley of mass 140 kg moving with a velocity of 1.5 m/s in the same direction. What is their common velocity?
For boy, m1 = 60 kg, u1 = 3 ms–1, v1 = v
For trolley, m2 = 140 kg, u2 = 1.5 ms–1, v2 = v
By conservation of momentum,
m1u1 + m2u2 = (m1 + m2) v
60 × 3 + 140 × 1.5 = (60 + 140) v

= 1.95 ms–1

Question. A bullet of mass 20 g moving with a speed of 500 ms–1 strikes a wooden block of mass 1 kg and gets embedded in it. Find the speed with which block moves along with the bullet.
For bullet, m1 = 20 g = 0.2 kg, u1 = 500 ms–1,
v1 = v
For block, m2 = 1 kg, u2 = 0 ms–1, v2 = v
By conservation of momentum,
m1u1 + m2u2 = (m1 + m2) v

Question. A force of 0.6 N acting on a body increases its velocity from 5 m/s to 6 m/s in 2 s. Calculate the mass of the body.
F = 0.6 N, u = 5 ms–1, v = 6 ms–1,
t = 2 s

Question. Calculate the force needed to speed up a car with a rate of 5 ms–2, if the mass of the car is 1000 kg.