Question

class 9 chapter force and law of motion​

Answer 1

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(ii) the time taken by the bullet to come to rest is :

${ {v}^{2} = u}^{2} + 2as$

$0 = ( {10}^{3 )^{2} } + 2 \times a \times (5 \times {10}^{ - 2} )$

$a = { - 10}^{7} m/ {s}^{2} </strong></p><p><strong>[tex]a = { - 10}^{7} m/ {s}^{2}$

$v=u+at$

$0 = {10}^{3} - {10}^{7} \times t$

$t = {10}^{ - 4} s$

(i) the resistive force exerted by sand on bullet is :-

$F = m \times a$

$F = {10}^{ - 2} \times {10}^{7}$

$F = {10}^{5} N$

$F = 100k N$

Answer 2

Answer:

Explanation:

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 of 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.

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

Solution

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.

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

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.

Intext Questions – 2 Page: 126,127

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

Solution

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