Zero velocity means zero acceleration

Below is a picture of a billiard ball which rolls into a wall and bounces back. The arrows indicate the direction of motion of the ball. Students are asked to draw velocity and acceleration vectors for the ball at each of the following times: before striking the wall, at the instant ball is in contact with the wall, and after striking the wall. If the acceleration is zero, explicitly state that. Explain your answers.

 

 

 Before Striking the Wall:

Velocity:

Acceleration:

If the student assumes that there is friction between the ball and the ground, the ball will be slowing down. Thus the acceleration vector will point opposite the velocity vector.

If the student assumes that ground is a frictionless surface, the ball will be traveling at a constant speed. Thus the acceleration will be zero.

 

Instant of Contact:

Velocity: Zero.

Acceleration: As the ball strikes the wall, its velocity is changing from to. Since the acceleration vector points in the same direction as the change in velocity ( - ), it must point to the left.

 

After Striking the Wall:

Velocity:

Acceleration:

 If the student assumes that there is friction between the ball and the ground, the ball will be slowing down. Thus the acceleration vector will point opposite the velocity vector.

If the student assumes that ground is a frictionless surface, the ball will be traveling at a constant speed. Thus the acceleration will be zero.