Conceptual Questions on Force and Laws of Motion (with Solution)

Conceptual Questions (with solution)

Question 1. A cricket player lowers his hand while catching the ball. Why?

Answer.

When a cricket player lowers his hands, the time to stop the ball increases, so the rate of change of momentum decrease and hence less force has to be applied to stop the ball, due to less force the hands of the player are not injured.

That is why, a cricket player lowers his hands while catching the ball.

Question 2. A person falling on a cemented floor gets injured but a person falling on a heap of sand is not injured. Why?

Answer.

When a person falls on a cemented floor, the rate of change of momentum is very high because the person is abruptly stopped i.e. stopped in a very small interval of time.

Hence the person gets injured due to the application of a large force on his body by to the floor. On the other hand, when a person falls on a heap of sand, the sand yields under the weight of the person.

As a result of this, the person comes to rest in a longer period of time. So the change in momentum takes place at longer interval of time. Hence a small force is exerted on the body of the person when he falls on a heap of sand. Therefore, he does not get injured.

Question 3. A bullet fired from a gun makes hole in the window pane while passing through it, but the stone striking the window pane breaks it into pieces. Why?

Answer.

In case of bullet, only a small portion of the window pane where the bullet strikes comes in motion because the bullet makes contact with the pane for a very short time due to its high speed. On the other hand, the remaining portion of the window pane remains at rest due to inertia of rest. Thus, a small hole is made by the bullet in the window pane.

In case of a stone, the speed of the stone is very low compared to the speed of the bullet. So the stone makes contact with the window pane for longer period of time. During this long time, the whole window pane comes in motion and hence breaks into pieces.

Question 4. Our hand hurts more when we hit the wall than when we hit a sponge seat of a car. Why?

Answer.

When we hit the wall with our hand, then the hand comes to rest in small interval of time. So large change in momentum of our hand takes place in small interval of time. Hence, large force is exerted on our hand. On the other hand, when we hit the sponge seat of a car, the seat yields and the hand comes to rest after long interval of time. So, the change in momentum of the hand takes place in large interval of time. Hence, less force is exerted on our hand.

Question 5. A karate player breaks a pile of tiles or bricks with a single blow. How?

Answer.

When a karate player strikes the pile of tiles with his hands, he des so as fast as possible. In other words, the time taken to strike the pile of tiles is very small. As the momentum of the hand of a karate player reduces to zero when his hand strikes the pile of tiles in a very small interval of time, therefore, the rate of change of momentum is very large and hence a very large force is exerted on the pile of tiles. This force is enough to break the pile of tiles.

Question 6. “Action and reaction are equal and opposite but even then they do not cancel each other.” Explain, Why?

Answer.

Two equal and opposite forces can cancel each other if they act on the same body. But action and reaction do not act on the same body. Action acts on one body and the reaction acts on another body. Hence they cannot cancel each other.

Question 7. It is difficult to balance our body when we accidently step on a peel of banana. Explain. Why?

Answer.

When we walk on the ground, our foot pushes the ground in the backward direction (Action). On the other hand, ground pushes our foot in the forward direction (Reaction). This reaction of the ground helps us to move in the forward direction. But, when our foot falls on a peel of banana, then our foot slips on the peel of banana and cannot push the round in the backward direction. Consequently, no reaction force acts on our foot. Hence we lose the balance and fall down.

Question 8. 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 falls in the tumbler. Whey?

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 9. Which has more inertia, a bicycle or a train and why?

Answer.

A train because of its large mass.

Question 10. What type of force is acting on the body whose motion is shown in the following v–t graph.

Answer.

(a) Since the velocity is constant, acceleration is zero. Therefore no force is acting on the body.

(b) This is a case of constant retardation.

Therefore, a constant retarding force is acting on the body.

(c) This is a case of constant acceleration.

Therefore, a constant accelerating force is acting on the body.

Question 11. A truck and car moving with same velocity come to halt after head on collision. If the collision lasts for 10 s.

(a) Which vehicle experiences the greater force of impact?

(b) Which vehicle experiences the greater acceleration?

(c) Which vehicle experiences the greater change in momentum?

(d) Why is the car damaged more than the truck?

Answer.

(a) Since momentum of truck is more than the momentum of the car, because of its greater mass so the car will experience greater force of impact exerted by the truck.

(b) Since, so both will have same acceleration. Where a = acceleration, v = final velocity, u = initial velocity, t = time taken to change in velocity

(c) The truck experiences a large change in momentum because its initial momentum is more than the initial momentum of the car.

(d) The car gets damaged more than the truck because the truck transfer more momentum to the car.