Question

A tennis ball with a velocity of 9.86 m/s to
the right is thrown perpendicularly at a wall.
After striking the wall, the ball rebounds in
the opposite direction with a velocity of −7.07
m/s to the left.
If the ball is in contact with the wall for
0.011 s, what is the average acceleration of
the ball while it is in contact with the wall?
Answer in units of m/s^2