Calculating centripetal force when radius is unknown

[BogMonkey]

Heres the question
"A ball with a mass of 250 g has a centripetal acceleration of 5 m/s2. What is the centripetal force acting on this ball? "

Since radius depends on the string length I have no idea how to derive it from these 2 variables. The only formula I know for getting centripetal force is F = m * V^2 / r

 

[bp_psy]

If the acceleration is already given why do you want to calculate it?

 

[Nabeshin]

If the acceleration is already given why do you want to calculate it?

Precisely. Elaborating, one rewrites the equation (F=ma) in that form (F=mv^2/r) because usually one knows velocity and radius. In this case, you know neither, so why rewrite it at all? :)