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

Students should practice questions given in Force And Laws Of Motion Chapter 9 Class 9 Science Worksheets. These worksheets for Class 9 Science have a good collection of important questions and answers which are expected to come in your class tests and examinations. You should learn these solved worksheet questions for Science Class 9 as it will help you to understand all topics and give you more marks.

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

Please refer to below questions and answers for Force And Laws Of Motion Chapter 9 Class 9 Science Worksheets. Prepared by expert teachers for Standard 9 Science

Question. How is inertia measured quantitatively?

Quantitatively the inertia of an object is measured by its mass.

Question. The fruits fall off the branches when a strong wind blows. Give reason.

Fruits tend to continue in the state of rest on account of inertia while branches suddenly come into motion.

Question. Why do athletes run some distance before jumping?

Athlete has the inertia of motion and thus continues to move past the line.

Question. Name the physical quantity which is determined by the rate of change of linear momentum.

Force.

Question. What is frictional force?

The force that always opposes the motion of object is called force of friction.

Question. What is inertia?

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

Question. If a ball is thrown up in a moving train, it comes back to the person’s hands. Why?

This is because no horizontal force acts on it. It moves with the same horizontal speed.

Question. Which type of force brings an object in motion?

Unbalanced force.

Question. State Newton’s first law of motion.

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. State Newton’s third law of motion.

To every action, there is an equal and opposite reaction and they act on two different bodies.

Question. Why are road accidents at high speeds very much worse than accidents at low speeds?

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 the factor on which the inertia of the body depends.

Inertia of a body depends upon the mass of the body.

Question. Name two factors which determine the momentum of a body.

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?

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

Question. What is momentum?

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. If a person jumps from a height on a concrete surface he gets hurt. Explain.
Answer. 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. What is the relation between Newton’s three laws of motion?
(i) Newton’s first law explains about the unbalanced force required to bring change in the position of the body.
(ii) Second law explains about the amount of force required to produce a given acceleration.
(iii) While Newton’s third law explains how these forces acting on a body are interrelated.

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. Why are athletes made to fall either on a cushioned bed or on a sand bed in a high jump athletic event?
Answer. In a high jump athletic event, athletes are made to fall either on a cushioned bed or on a sand bed so as to increase the time of the athlete’s fall to stop after making the jump. This decreases the rate of change of momentum and hence the force.

Question.Why are roads on mountains inclined inwards at turns?
Answer. 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. Why do athletes have a special posture with their right foot resting on a solid supporter for athletic races?
Answer. Athletes have to run the heats and they rest their foot on a solid supports before start so that during the 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.

Question. Why you get hurt by hitting a stone while when you kick a football it flies away?
Answer. This is because stone is heavier than football and heavier objects offer larger inertia. When we kick a football its mass is less and inertia is also less so force applied by our kick acts on it and hence it shows larger displacement but in case of stone, it has more mass and offers larger inertia. When we kick (action) the stone it exerts an equal and opposite force (reaction) and hence it hurts the foot.

Question. Give any three examples in daily life which are based on Newton’s third law of motion.
Answer. Three examples based on Newton’s third law are :
Swimming : We push the water backward to move forward.
(i) Action – water is pushed behind
(ii) Reaction – water pushes the swimmer ahead
Firing gun : A bullet fired from a gun and the gun recoils.
(i) Action – gun exerts force on the bullet
(ii) Reaction – bullet exerts an equal and opposite force on the gun Launching of rocket :
(i) Action – hot gases from the rocket are released
(ii) Reaction – the gases exert upward push to the rocket

Question. Why does a ball rebound after striking against a floor?
Answer. When a ball strikes against a floor, it exerts a force on the floor. According to Newton’s third law of motion, the floor exerts an equal and opposite force on the ball. Due to this reaction, the ball rebounds.

Question. How do we swim?
Answer. While swimming, a swimmer pushes the water backward with his hands. The reaction offered by the water to the swimmer pushes him forward.

Question. Which concept is behind the phenomenon-boatman pushes the river bank with a bamboo pole to take his boat into the river”.
Ans : When the boatman pushes the river bank with a bamboo pole, the river bank offers an equal and opposite reaction. This reaction helps the boat to move into the river.

Question. Why does a fireman struggle to hold a hose-pipe?
Answer. A fireman has to make a great effort to hold a hosepipe to throw a stream of water on fire to extinguish it. This is because the stream of water rushing through the hose-pipe in the forward direction with a large speed exerts a large force on the hose-pipe in the backward direction.

Question. Why is the movement of a rocket in the upward direction?
(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 in 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 happens when a quick jerk is given to a smooth thick cardboard placed on a tumbler with a small coin placed on the cardboard? The coin will fall in the tumbler. Why?
Answer. The coin was initially at rest. When the cardboard moves because of the jerk, the coin tends to remain at rest due to inertia of rest. When the cardboard leaves contact with the coin, the coin falls in the tumbler on account of gravity.

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. How can force change the state of motion of the objects?
Answer. Force can bring objects into motion by pushing hitting and pulling them.

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 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. Why all cars are provided with seat belts?
Answer. Sudden movement of the vehicle results in the sudden change in the state of motion of the vehicle when our feet are in contact with it. But the rest of our body opposes this change due to its inertia and tends to remain where it was. Seat belts are provided to protect the passengers from falling backward or forward during such situation.

Question. State all 3 Newton’s law of motion.
Answer. Newton’s I law of motion : An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.
Newton’s II 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. Newton’s III law of motion : To every action, there is an equal and opposite reaction and they act on two different bodies.

Question. Explain inertia and momentum.
Answer.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. 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. 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 its inertia. 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 applied on it.

Numerical Questions

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.
Answer. 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. An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
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 +1/2 at2, one gets
s = 0 × 1 + 1/2 × 8 × 12 = 4 m
The object travels a distance of 4 m.

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.
Given : m = 80 kg,
u = 5 ms–2
v = 25 ms–2
and t = 2 s
Now acceleration a = change in velocity time

Force = mass × acceleration of F
= ma
Therefore, F = 80 × 10 = 800 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.
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. A cricket ball of mass 70 g moving with a velocity of 0.5 ms–1 is stopped by player in 0.5 s. What is the force applied by player to stop the ball?
Here m = 70 g = 0.070 kg;
u = 0.5 ms–1; v = 0; t = 0.5 s

or F =- 0.07 newton
The negative sign indicates that the force exerted by the player is opposite to the direction of motion of the ball.

Question. What will be acceleration of a body of mass 5 kg if a force of 200 N is applied to it?
Here m = 5 kg; F = 200 N
F = ma or a =F/m

a = 40 ms–2

Question. A bullet of mass 10 g is fired from a rifle. The bullet takes 0.003 s to move through its barrel and leaves with a velocity of 300 ms–1. What is the force exerted on the bullet by the rifle?
Here m = 10 g = 0.010 kg; u = 0; v = 300 ms–1
t = 0.003 s, F = ?
F =m (v – u)/t
F =0.010(300 – 0)/ 0.003
or F = 1,000 N

Question. What force would be needed to produce an acceleration of 1 ms–2 on a ball of mass 1 kg?
Here m = 1 kg; a = 1 ms– 2; F = ?
Now F = ma
= 1 × 1
or F = 1 Newton

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

Question. How long should a force of 100 N act on a body of 20 kg so that it acquires a velocity of 100 ms–1?