F向1=mv1^2/R
F向2=mv2^2/R
F向2-F向1=mV2^2/R-mV1^2/R
=m/R(V2^2-V1^2)
=m/R(4v1^2-v1^2)
=3mv1^2/R=45
mv1^2/R=15N