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

AI Thread Summary
A 200 kg boulder falling from a 120 m cliff experiences a frictional force of 280 N as it descends. The total gravitational potential energy of the boulder is calculated to be 235,200 J, while the work done against friction amounts to 33,600 J. Subtracting the work done by friction from the total energy yields a kinetic energy of 201,600 J, leading to a calculated velocity of 45 m/s just before impact. However, the teacher points out a potential error, suggesting that the notation "N[U]" may indicate a velocity-dependent friction force, which would require a different approach to the calculations. This highlights the importance of accurately interpreting forces in physics problems.
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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

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.
 
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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.
 

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