Physics Help Forum Need help with this question regarding kinematics.

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 Sep 2nd 2018, 09:27 AM #1 Junior Member   Join Date: Sep 2018 Posts: 5 Need help with this question regarding kinematics. A ball is traveling at 3m/s then bounces off of a tennis racket and returns at 2m/s. If the ball is in contact with the racket for 12ms, what is the average acceleration? I converted 12ms to 0.012s. Then I listed what is known in the problem. The initial velocity is 3m/s and the final velocity is -2m/s. I know the equation for acceleration is Vf-Vi/Tf-Ti. But the answer that I get is -417m/s^2. The question is multiple choice and none of the answers match this. What am I doing wrong here?
 Sep 3rd 2018, 06:42 AM #2 Senior Member     Join Date: Jun 2016 Location: England Posts: 935 Could you post the other multiple choice options for us please? This might let us identify what is missing from your analysis of the problem. __________________ ~\o/~
Sep 3rd 2018, 01:01 PM   #3

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 Originally Posted by Palilth A ball is traveling at 3m/s then bounces off of a tennis racket and returns at 2m/s. If the ball is in contact with the racket for 12ms, what is the average acceleration? I converted 12ms to 0.012s. Then I listed what is known in the problem. The initial velocity is 3m/s and the final velocity is -2m/s. I know the equation for acceleration is Vf-Vi/Tf-Ti. But the answer that I get is -417m/s^2. The question is multiple choice and none of the answers match this. What am I doing wrong here?
I agree with you.

Quick point: $\displaystyle v_f - v_i / t_f - t_i$ is incorrect. You need parenthesis: $\displaystyle (v_f - v_i)/(t_f - t_i)$. You probably think I'm being picky about the notation (along with another post I made to you) but you have to get the notation right in order to speak clearly about it.

-Dan
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