The acceleration formula is
[tex]a_{c}=\frac{v^{2}}{r}[/tex]
We are given
[tex]v=4.5\ m/s[/tex]
[tex]r=1.35\ m[/tex]
Plug in:
[tex]a_{c}=\frac{(4.5\ m/s)^{2}}{1.35\ m}=15\ m/s^{2}[/tex]
The formula for force is
[tex]F=ma[/tex]
We now have
[tex]a=15\ m/s^{2}[/tex]
[tex]m=0.65\ kg[/tex]
Plug in:
[tex]F=15\ m/s^{2}*0.65\ kg=9.75\ N[/tex]
So the centripetal force is 9.75 Newtons.