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

1. Apr 19, 2012

Killic

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.

2. Apr 19, 2012

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.