Question

A ball collides elastically with an immovable wall fixed to the earth’s surface. Which statement is false? 1. The ball's speed is unchanged by the collision.2. The ball does no net work on the wall.3. The ball's kinetic energy is unchanged by the collision.4. the wall doos no net work on the ball.5. the ball's size and shape are unchanged by the collision.6. All of these statements are true7. The ball's momentum is unchanged by the collision

Answer 1

Answer:

Statements 4, 6 & 7 are false.

Explanation:

For any elastic collision the vector sum of the total momentum of the system is zero.

Here we have an elastic collision between the ball and a fixed wall. The velocity of the ball will always remain zero whereas the velocity (hence momentum) of the ball will totally revert after the collision.

Initial momentum of the ball is given as:

$p=m.v$

where:

m = mass of the ball

v = initial velocity of the body

after the elastic collision:

$p=m.(-v)$

• Here the direction of the velocity is reversed, hence the momentum is changed only in terms of direction violates the statement 7.

• At the moment of collision the ball and the wall exert an equal and opposite force mutually on each other. The wall does not moves in any case whereas the ball moves in the direction of the reaction force of the wall hence it does some work on the ball violating the statement 4.

• Since the collision is elastic the shape and size of the ball remains intact.

• From the above explanation it is clear that that not all the given statements are correct hence statement 6 goes violated.