Question

an example of a force applied against the direction of motion is A. Opening door B. Applying brakes suddenly moving car C. A batsman hitting a cricket ball D. Drawing water from a well

Answer 1

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Force And Laws of Motion - NCERT Questions
Q 1.
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-rupees coin and a one-rupee coin?

SOLUTION:
(A) A stone of the same size as rubber ball has more mass. As mass is a measure of inertia, therefore, stone has more inertia than a rubber ball.
(B) A train has greater inertia than a bicycle, because train is much heavier than a bicycle.
(C) A five-rupees coin is heavier than a one-rupee coin. Therefore, inertia of five-rupees coin is more than the inertia of one-rupee coin.

Q 2.
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.

SOLUTION:
(i) Velocity of the football changes when first player kicks the ball towards another player of his team.
(ii) Velocity of the football also changes when another player kicks the football towards the goal.
(iii) Velocity of the football also changes when the goalkeeper of the opposite team stops the football by collecting it.
(iv) Velocity of the football changes when the goalkeeper kicks it towards a player of his team.
Thus, the velocity of the ball changes four times in this case.
In first and second cases, the force is supplied by the foot of players. In third case, force is supplied by the hands of the goalkeeper. In fourth case, as the goalkeeper hits the football with his foot, so the foot of the goalkeeper supplies the force.

Q 3.
Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

SOLUTION:
When a tree is vigorously shaken, the branches of the tree come in motion but the leaves tend to continue in their state of rest due to inertia of rest. As a result of this, leaves get separated from the branches of the tree and hence fall down.

Q 4.
Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

SOLUTION:
When a moving bus brakes to a stop, our feet come to rest with the bus. But upper part of our body continues to move forward on account of inertia of motion. That is why we tend to fall in the forward direction.
However, when the bus accelerates from rest, lower part of our body moves with the bus. The upper part of the body tries to maintain itself at rest on account of inertia of rest. Therefore, we tend to fall backwards.

Q 5.
If action is always equal to the reaction, explain how a horse can pull a cart.

SOLUTION:
It is true that when a horse pulls a cart with a certain force, then the cart also pulls the horse with the same force and the system of horse and cart should not move.
However, if we analyse the problem carefully, then horse not only applies force on the cart, but also pushes the earth backward with his feet. In this action of the horse, the earth reacts back and pushes it in the forward direction. Now, the force applied by the horse on the earth is insufficient to move the earth, but the force applied by the earth is sufficient to make the horse to move in the forward direction.
It is this unbalanced force applied by the earth, which makes the system of horse and cart to move in the forward direction.

Q 6.
Explain, why is it difficult for a fireman to hold a hose, which ejects large amount of water at a high velocity ?

SOLUTION:
The ejection of a large amount of water at a high velocity from a hosepipe results in the development of an equal force of reaction on the hosepipe in the backward direction. That is why it becomes difficult for the fireman to hold the hosepipe.

Q 7.
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.


Hopefully this helps you

Answer 2

Answer:

its all about frictional force.

Explanation:

These examples makes us know that it moves forward.

Thus saying it as forward motion..

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I think it helps you..