1. The problem statement, all variables and given/known data A 200 kg boulder falls from rest off a 120.0 m high cliff. If the boulder experiences a frictional force of 280 N as it falls, what would be the velocity just before it hits the ground? 2. Relevant equations kinetic energy = (1/2)mv2 gravitational potential energy = mgh work = (net force)(Δd) 3. The attempt at a solution Total force = mgh = 200kg*9.8N/kg*120m = 235200 J Wfriction = FΔd Wfriction = 280 N*120 m Wfriction = 33600 J total energy = kinetic energy + work done by friction kinetic energy = 235200 J - 33600 J kinetic energy = 201600 J 201600 J = (1/2)mv2 v = 45 m/s However, according to my teacher, this answer is incorrect.