# Terminal velocity of a ping pong ball

1. Nov 15, 2004

### disruptors

Hey ppl, just a quick question..

You have a 10 g ping pong ball with a diameter of 3.5 cm...
a)The ball is shot straight up at twice its terminal speed. What is it’s
initial acceleration?

i know D=W at terminal speed but when u double it do you just assume D=4D and if you do how come?

The answer is a=-5g as summation of Fy=-D-w=-4w-w=-5mg=ma

the -4w is just throwing me off...

Thanks

2. Nov 15, 2004

### Tide

If $v = 2 v_t$ then $v^2 = 4 v_t^2$ which accounts for your factor of 4.