Why Does My Calculation of Centripetal Force Keep Giving the Wrong Answer?

[melopelo]

Your roommate is working on his bicycle and has the bike upside down. He spins the 60-cm-diameter wheel, and you notice that a pebble stuck in the tread goes by four times every second. What is the centripetal force? i know the speed is 7.5 m/s and the equation is a=v^2/r but it keeps on giving me the incorrect answer of 1.875. Help!

 

[melopelo]

I mean acceleration my b

 

[StevieTNZ]

Please do not post those sorts of questions in this forum. This section is for introductions only. :)

 

[Evo]

melopelo, please post your homework in the proper homework section.

Thank you.