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 [itex]\omega[/itex] = 2pi/T speed = r[itex]\omega[/itex] [itex]a_c = -v^2 /r [/itex] [itex]a_c = -\omega ^2 r [/itex] 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, [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?