# Help me understand centripetal acceleration?

1. Feb 24, 2017

1. The problem statement, all variables and given/known data
A child swings a tennis ball attached to a .750 m-string in a horizontal circle above his head at a rate of 5.00 rev/s What is the centripetal acceleration of the tennis ball?

2. Relevant equations
angular speed $\omega$ = 2pi/T
speed = r$\omega$

$a_c = -v^2 /r$
$a_c = -\omega ^2 r$

3. The attempt at a solution
So its 5 rev/s, or .2s in 1 revolution. Using angular speed I get 10pi radians per second
Plugging in,

$a_c = -(10pi)^2 (.750 m) = -740 m/s^2$

But my books answer is 740 m/s^2.

I don't understand why my answer is negative, even though I used the correct formulas. Does anyone know why?

2. Feb 24, 2017

### haruspex

The minus sign is in the formula because the acceleration vector is in the opposite direction to the radius vector. But you are working with scalars, not vectors, so you can drop the minus sign and just say in words which way the acceleration is (if you need to specify it, which you probably do not here).
By the way, the answer is not quite right. The given distance is the length of the string, not the radius of the circle. Gravity should be taken into account.

3. Feb 24, 2017

What does gravity have to do with the length of the string??

4. Feb 24, 2017

### haruspex

It does not alter the length of the string, but it does alter the radius of the circle.

5. Feb 24, 2017