# Vectors help needed!

1. Oct 13, 2013

### ahmed1237

A tennis ball crossing a net at 35m/s [S10 degrees W] strikes a person's racket and her volley returns it at 30m/s [N 30 degrees W]. If the person's racket was in contact with the ball for 0.005s then what is acceleration of the ball?

For this question is my vi=35m/s and vf=30m/s? If so, when I draw my vectors do I subtract them or add them. The equation for acceleration is a=vf-vi/t right?

2. Oct 13, 2013

### Staff: Mentor

The ball's velocity is in the opposite direction after it is struck by the racket.

(vf - vi)/t -- notice the parentheses - is the change in velocity. That's not the same as the acceleration, although over short time intervals, the change in velocity divided by the change in time is closer to the instantaneous velocity. In other words, if Δt is reasonably small, Δv/Δt ≈ dv/dt = a.
No, the way it works here is that you show us what you got, and we'll let you know if it's correct or not.

Last edited: Oct 13, 2013
3. Oct 13, 2013

### ahmed1237

so my answer is 4600m/s2 (w 21 degrees n) is that correct?

4. Oct 13, 2013

### Staff: Mentor

The units should be m2/sec2, but the acceleration value seems way high to me.

5. Oct 13, 2013

### ahmed1237

so then how do I go about doing this?