# Speed of a Ball After Elastic Collision with a Wall

## Explanation:

**In an elastic collision between a ball and a wall, both the momentum and the kinetic energy are conserved.**

Let's denote the initial velocity of the ball as V1 and the final velocity as V2. Since the ball is moving along the normal to the wall, the velocity of the wall doesn't affect the collision. Therefore, we can consider the wall as stationary.

Using the conservation of momentum, we can set up the equation: m * V1 = m * V2

Since the mass of the ball is small compared to the mass of the wall, we can assume that the wall is immovable, and the mass of the wall doesn't affect the equation. Therefore, the final velocity of the ball after the collision is equal to the negative of its initial velocity: V2 = -V1.

Given that V1 = 4 m/s, the speed of the ball after the elastic collision with the wall is 4 m/s in the opposite direction, away from the wall. Therefore, the correct answer is option A. 5 m/s away from the wall.