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Average acceleration

Hi. Here's my problem:

"A tennis ball with a speed of 11.8 m/s is thrown perpendicularly at a wall. After striking the wall, the ball rebounds in the opposite direction with a speed of 8.51 m/s. If the ball is in contact with the wall for 0.00175 s, what is the average acceleration of the ball while it is in contact with the wall? Define the initial direction of the ball's motion to be positive."

I thought I could solve this problem pretty easily using the average acceleration equation (change in velocity divided by time elapsed), but I didn't get the right answer. Here's what I did:

(8.51 m/s - 11.8 m/s) / 0.01175 m/s = -280 m/s^2

Can you tell me what I'm doing wrong here and the correct equation for this problem/the correct way to approach this problem?

Thank you!

Solution Preview

<br>You have made good efforts.
<br>In such problems use vector method you will never make mistake.
<br>initial velocity = 11.8 m/s (->)
<br>final vlocity = 8.51 m/s (<-)
<br>so if the direction of initial velocity is ...