# Need help with this question regarding kinematics.

#### 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?

#### Woody

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.

#### topsquark

Forum Staff
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