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## Homework Statement

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?

## Homework Equations

angular speed [itex]\omega[/itex] = 2pi/T

speed = r[itex]\omega[/itex]

[itex]a_c = -v^2 /r [/itex]

[itex]a_c = -\omega ^2 r [/itex]

## 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,

[itex]a_c = -(10pi)^2 (.750 m) = -740 m/s^2 [/itex]

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?