Velocity, Force of Friction, and Total Energy in a System Question

[Killic]

Homework Statement

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?

Homework Equations

kinetic energy = (1/2)mv²
gravitational potential energy = mgh
work = (net force)(Δd)

The Attempt at a Solution

Total force = mgh = 200kg*9.8N/kg*120m = 235200 J

W_friction = FΔd
W_friction = 280 N*120 m
W_friction = 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.

 

[Steely Dan]

What does your notation "N" represent? If, as I suspect, it is saying that the friction force is proportional to velocity (this is typical for air drag), then your treatment of the friction as constant is not appropriate here.