# Physics: When a 58-g tennis ball is served?

1. Sep 22, 2009

### triplel777

1. The problem statement, all variables and given/known data

When a 58-g tennis ball is served, it accelerates from rest to a speed of 36 m/s. The impact with the racket gives the ball a constant acceleration over a distance of 36 cm. What is the magnitude of the net force acting on the ball?

2. Relevant equations

3. The attempt at a solution

Fnet= ma
to find a i need time
so i used v=d/t to find the time. 36=36/t so t=1sec
now to find a i used V-V_0=at so 36=a*1 so a=36
now i plugged it in
Fnet= 58g*36=2088

what am i doing wrong?

2. Sep 22, 2009

### w3390

You are very sloppy with your units. Go back through your calculations making sure the units are correct. I can see at least two major place where you units are incorrect. See if this fixes the answer.

3. Sep 22, 2009

### triplel777

ok so i changed the m=0.058kg, and d=0.36m
so t=0.01s which makes a=3600m/s2 so fnet= 3600*0.058=208.8. did i do a conversion wrong/

4. Sep 22, 2009

### willem2

The speed of the ball isn't 36 m/s during the entire acceleration, but only at the end of it, so you can't use v = d/t and the time you computed is wrong.
use x = (1/2) a t^2 to find out when the ball has moved 36 cm.

5. Sep 22, 2009

### triplel777

how can i use x= 1/2 at^2 if i dont know the t or a? what do i use for t? i tried using v^2-V_0^2=2a(x-x_0) but that didnt work out?

6. Sep 22, 2009

### willem2

You know x = 0.36m = (1/2)at^2 and v = at = 36 m/s

7. Sep 22, 2009

### triplel777

oh ok thanks its 104.4