Find the force on an object on an inclined plane

[ritwik06]

Homework Statement

A body is projected up an inclined plane with a speed Vo. The inclination of the incline is \theta. The surface of th incline is a tangent to a circle of radius R. Find the force acting on th object when it is at the top of the sphere provied that Vo= 2*the minimum velocity need to reach the top of the sphere.
The length of incline = L
Assume that the block does not lose contact with the track.

The Attempt at a Solution

The minimum velocity required, as I calculated was:
\sqrt{2g[R(1-cos \theta)+ L sin \theta} which was correct.
But the Force I calculated was diffrent.

At the top of the phere, the velocity will become Vo/2. Why oesnt (mV^2)/R give me the correct answer? as the body perform circular motion. Help me please.

 

[ritwik06]

I am posting a picture of the situation. Please help me now.
http://img178.imageshack.us/img178/4315/diagav4.th.jpg

 

[Dick]

The minimum velocity required to reach the top satisfies (1/2)mv0^2=mgh (where h is the total height of the top of the sphere) - use conservation of energy. If you leave with twice that velocity your energy at the top is (1/2)m(2*v0)^2-mgh. Equate that to kinetic energy at the top and solve for velocity. You DON'T arrive with half the initial velocity.