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

Question. What is Newton's First Law known as? The Law of
A. Friction.
B. Mass.
C. Inertia.
D. Gravity
Answer: Inertia

Question. Motions need an unbalanced net force to maintain
A. True.
B. False
Answer: true

Question. Which has more Inertia? (so need more force to move)
A. Truck.
B. Ant.
C.A cat.
D.A skateboard
Answer: Truck

Question. What is Newton's First Law
A. F=ma
B. Every action has an equal and opposite reaction
C. An object at rest stays at rest, an object in motion stays in motion.
D. Friction
Answer: An object at rest stays at rest, an object in motion stays in motion.

Question. friction is a force that acts
A. in the same direction as the motion
B. opposite to the direction of the motion
C. opposite to the applied forces
D. the same direction as the applied force
Answer: opposite to the direction of the motion

Question. What is the name for a push or pull?
A. A movement.
B. A force.
C. Speed.
D. Density
Answer: B

Question. What can a force do?
A. start a motion
B. stop a motion
C. change the direction of the motion
D. All of the above
Answer: all of the above

Question. Which type of force(s) will cause a change in an object's motion?
A. Gravity.
B. zero net force.
C. balanced forces.
D. unbalanced forces
Answer: Unbalanced force

Question. What kind(s) of objects have inertia?
A. all objects with mass.
B. only objects at rest.
C. only objects in motion.
D. only objects whose motion is being changed
Answer: All objects with mass

Question. Which one needs a greater net force to move, a kilogram of feathers or a kilogram of iron?
A. feathers
B. iron.
C. same.
D. depends on their friction
Answer: Depends on their friction

Question. an object with a zero net force could be moving
A. true.
B. false
Answer: False

Question. an object with a zero net force could be moving in a circle at a constant speed
A. false.
B. true
Answer: True

Question. Which object has more inertia — a Bowling ball or a tennis ball?
Answer: A bowling ball

Question. The passenger in a running bus tends to lean forward , when the bus stops suddenly. This is due to
A. inertia of motion.
B. Inertia in rest
C. Inertia with direction
D. None of the above
Answer: Inertia if motion

Question. The moment the ball was thrown, the ball was also in motion alone with the person who has thrown the ball. This is the example of :-
A. Newton’s second law of motion
B. Newton’s mass law of motion
C. Newton’s first law of motion
D. All of the above
Answer: Newton’s first law of motion.

Question. Why are tyres corrugated and made rough?
Answer: Tyres are corrugated and made rough to increase friction.
(1) This provides better grip on the road.
(2) It also prevents the vehicles from slipping.

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. State the law of conservation of momentum.
Answer: (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. 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. 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. 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. 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. Which concept is behind the phenomenon- “A boatman pushes the river bank with a bamboo pole to take his boat into the river”.
Answer: 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. 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. Why is it necessary to bend knees while jumping from greater height?
Answer:
(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?
Answer:
(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. A batsman has a choice to use heavy or light bat, while facing a fast bowler. He will prefer:
(A) light bat, because handling it is easy
(B) heavy bat, so that he can handle it firmly
(C) heavy bat, because it will recoilless
(D) none of the above.

Answer: C

Question. A man while running at a constant acceleration of 3.5 ms–2, develops a force of 280 N. The mass of man is:
(A) 80 kg
(B) 85 kg
(C) 75 kg
(D) 60 kg

Answer: A

Question. The mass of an aeroplane is 2.5 t. Its engine develops a force of 8750 N before taking off. The acceleration of the aeroplane at the time of take off is :
(A) 3.45 ms^–2
(B) 3.65 ms^–2
(C) 3.50 ms^–2
(D) 3.60 ms^–2

Answer: C

Question. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg along a horizontal track. If the engine exerts a force of 40,000 N and track offers a friction of 5000 N, then net accelerating force acting on the system is :
(A) 45,000 N
(B) 40,000 N
(C) 35,000 N
(D) none of the above

Answer: C

Question. A body is moving with a constant momentum. The motion of the body is :
(A) uniform velocity
(B) accelerated
(C) de-accelerated
(D) none of the above.

Answer: D

Question. A horse while running at a constant velocity of 15 ms^–1, develops a momentum of 3000 Ns. The mass of horse is:
(A) 180 kg
(B) 300 kg
(C) 200 kg
(D) 250 kg

Answer: C

Question. A boy of mass 30 kg while running at constant velocity has a momentum of 180 Ns. The constant velocity of the boy is :
(A) 3 ms^–1
(B) 6 ms^–1
(C) 18 ms^–1
(D) 12 ms^–1

Answer: B

Question. An electron of mass 9 x 10^–31 kg is moving in a straight line path with a velocity of 6 × 10⁷ ms^–1. The momentum of electron is :
(A) 5.4 × 10^–23 Ns
(B) 5.4 × 10^–24 Ns
(C) 4.5 × 10^–23 Ns
(D) 0.5 × 10^–24 Ns

Answer: A

Question. A vehicle has a mass of 1500 kg. If the vehicle is to be stopped with a negative acceleration of 1.7 ms–2, the force of friction between the vehicle and road is :
(A) –2250 N
(B) –2050 N
(C) –2550 N
(D) none of the above.

Answer: C

Question. A bullet of mass 0.01 kg is fired from a rifle. The bullet takes 0.003 s to move through the barrel and leaves with a velocity of 300 ms^–1. The acceleration acting on the bullet is :
(A) 10,000 ms^–2
(B) 100,000 ms^–2
(C) 1000,000 ms^–2
(D) 1000 ms^–2

Answer: B

Question. By Newton's second law, the physical quantity which can be calculated, if we know the magnitude of force on a given mass is :
(A) velocity
(B) momentum
(C) acceleration
(D) none of the above.

Answer: B

Question. A truck starts from rest and rolls down the hill with a constant acceleration.
It travels 400 m in 20 s. If the mass of truck is 7 metric tonnes, the force acting on it is:
(A) 28,000 N
(B) 14,000 N
(C) 1400 N
(D) 24,000 N

Answer: B

Question. A cracker at rest explodes into two equal parts. These parts will move in :
(A) opposite direction with different velocities
(B) same direction with different velocities
(C) same direction with same velocities
(D) opposite direction with same velocities

Answer: D

More Question..

Multiple Choice Questions

1. Which of the following statement is not correct for an object moving along a straight path in an accelerated motion?
(a) Its speed keeps changing
(b) Its velocity always changes
(c) It always goes away from the earth
(d) A force is always acting on it

2. The forces of action and reaction are
(a) always equal only
(b) always equal and opposite
(c) always equal but in same direction
(d) always unequal and opposite.

3. According to the third law of motion, action and reaction
(a) always act on the same body
(b) always act on different bodies in opposite directions
(c) have same magnitude and directions
(d) act on either body at normal to each other

4. The action and reaction forces at
(a) on different bodies always
(b) on some body always
(c) on same body, sometimes
(d) on different bodies, sometimes

5. A goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goal keeper to
(a) exert larger force on the ball
(b) reduce the force exerted by the ball on hands
(c) increase the rate of change of momentum
(d) decrease the rate of change of momentum

6. The inertia of an object tends to cause the object
(a) to increase its speed
(b) to decrease its speed
(c) to resist any change in its state of motion
(d) to decelerate due to friction

7. Principle of conservation of linear momentum is deduced from
(a) Newton’s first law
(b) Newton’s second law
(c) Newton’s third law
(d) none of the above

8. The function of mud guards is based on
(a) inertia of rest
(b) inertia of direction
(c) inertia of motion
(d) none of the above

9. The force of action and reaction
(a) always cancel each other
(b) never cancel
(c) cancel sometimes
(d) cannot say

10. A passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is
(a) accelerated
(b) uniform
(c) retarded
(d) along circular tracks

11. An object of mass 2 kg is sliding with a constant velocity of 4 m s–1 on a frictionless horizontal table. The force required to keep the object moving with the same velocity is
(a) 32N
(b) 0 N
(c) 2 N
(d) 8 N

12. Rocket works on the principle of conservation of
(a) mass
(b) energy
(c) momentum
(d) velocity

13. A water tanker filled up to 2/3 of its height is moving with a uniform speed. On sudden application of the brake, the water in the tank would
(a) move backward
(b) move forward
(c) be unaffected
(d) rise upwards

14. Inertia of a body in linear motion is measured by its
(a) mass
(b) momentum
(c) velocity
(d) none of the above

15. What mass of a body can attain an acceleration of 5m/s2 under a force of 250 N?
(a) 5 kg
(b) 250 kg
(c) 50 kg
(d) 10 kg

Question. What is the direction of momentum?
Answer. The direction of momentum is the same as that of velocity.

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

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

Question. What is balanced force?
Answer. 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. No force is required to move an object with a constant velocity. Why?
Answer. For an object moving with a constant velocity, a = 0,
so F = ma = 0

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

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 he 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. 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. Write the effect of force.
or
What change will be in a body by force?
Answer. 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. 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.Which of the following has more inertia: (a) a rubber ball and a stone of the same size? (b) a bicycle and a train? (c) a five-rupee coin and a one-rupee coin?
Answer.
Since inertia is dependent on the mass of the object, the object with the greater mass will hold greater inertia. The following objects hold greater inertia because of their mass.
Stone
Train
Five-Rupee coin

Question. In the following example, try to identify the number of times the velocity of the ball changes: “A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team”. Also identify the agent supplying the force in each case.
Answer.
Let the two football teams be team A and team B respectively. Initially, the football is at rest. Now, the football is kicked by a player on team A to another player of team A. (The velocity of the ball has changed 1 time so far). This change is brought on by the force applied by the player who kicked the ball.
The football is now kicked by the other team A player towards the goal. (The velocity of the ball has changed 2 times so far). This change is also brought on due to the force applied by the team A player who kicked the ball.
The goalkeeper of team B stops the ball, bringing it to rest. (the velocity of the ball has changed 3 times so far). This change is brought on by the force applied by the goalkeeper of team B to stop the ball.
Finally, the goalkeeper of team B kicks the ball towards another player of team B. the velocity of the ball changes for a final time, bringing the total count to 4. This change is also brought on by the goalkeeper of team B, who applies the force to kick the ball forward.
Therefore, the velocity of the ball changes 4 times in this example.

Question. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Answer.
When the branch of the tree is shaken, the branch moves in a to-and-fro motion. However, the inertia of the leaves in attached to the branch resists the motion of the branch. Therefore, the leaves that are weakly attached to the branch fall off due to inertia whereas the leaves that are firmly attached to the branch remain attached.

Question. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?
Answer.
Initially, when the bus accelerates in a forward direction from a state of rest, the passengers experience a force exerted on them in the backward direction due to their inertia opposing the forward motion.
Once the bus starts moving, the passengers are in a state of motion in the forward direction. When the brakes are applied, the bus moves towards a position of rest. Now, a force in the forward direction is applied on the passengers because their inertia resists the change in the motion of the bus. This causes the passengers to fall forwards when the brakes are applied.

Question. If action is always equal to the reaction, explain how a horse can pull a cart.
Answer.
When the horse walks in the forward direction (with the cart attached to it), it exerts a force in the backward direction on the Earth. An equal force in the opposite direction (forward direction) is applied on the horse by the Earth. This force moves the horse and the cart forward.
The velocity at which the horse can move by applying a force on the earth depends on the mass of the horse (and the cart attached to it). The heavier the cart, the slower the motion of the horse (for a given amount of force applied by the horse on the Earth). If the cart is too heavy, the force exerted by the horse on the Earth will be insufficient to even overcome the force of inertia. In this case, the horse will not be able to pull the cart.

Question. Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
Answer.
For the hose to eject water at high velocities, a force must be applied on the water (which is usually done with the help of a pump or a motor). Now, the water applies an equal and opposite force on the hose. For the fireman to hold this hose, he must apply a force on it to overcome the force applied on the hose by the water. The higher the quantity and velocity of the water coming out of the hose, the greater the force that must be applied by the fireman to hold it steady.

Question. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s^–1. Calculate the initial recoil velocity of the rifle.
Answer.
Given, the Bullet’s mass (m₁) = 50 g
The rifle’s mass (m₂) = 4kg = 4000g
Initial velocity of the fired bullet (v₁) = 35 m/s
Let the recoil velocity be v₂.
Since the rifle was initially at rest, the initial momentum of the rifle = 0
Total momentum of the rifle and bullet after firing = m₁v₁ + m₂v₂
As per the law of conservation of momentum, the total momentum of the rifle and the bullet after firing = 0 (same as initial momentum)
Therefore, m₁v₁ + m₂v₂ = 0

Therefore, the recoil velocity of the rifle is 0.4375 meters per second in the direction opposite to the trajectory of the bullet (backward direction).

Question. Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 ms^–1 and 1 ms^–1, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 ms^–1. Determine the velocity of the second object.
Answer.
Assuming that the first object is object A and the second one is object B, it is given that:
Mass of A (m₁) = 100g
Mass of B (m₂) = 200g
Initial velocity of A (u₁) = 2 m/s
Initial velocity of B (u₂) = 1 m/s
Final velocity of A (v₁) = 1.67 m/s
Final velocity of B (v₂) =?
Total initial momentum = Initial momentum of A + initial momentum of B
= m₁u₁ + m₂u₂
= (100g) × (2m/s) + (200g) × (1m/s) = 400 g.m.sec^-1
As per the law of conservation of momentum, the total momentum before collision must be equal to the total momentum post collision.

v₂ = 1.165 m/s
Therefore, the velocity of object B after the collision is 1.165 meters per second.