A rock rolling down an incline-velocity and force

[NDbogan]

The question is:
a spherical rock weighing 20 tons starts rolling down a steep incline from a height of 400 metres. Calculate the velocity when it reaches the ground level. What is the force exerted on the rock?


I've tried to work this out but I just can't seem to grasp exactly what I'm missing.

Any help will be really appreciated.

 

[semc]

Conservation of energy ::

 

[NDbogan]

hmm...could you explain it a bit further.

 

[semc]

Use conservation of energy. The energy the rock has at the start and what is the energy converted to when it reaches the bottom