# Help correcting test problem

ConfusedStudent
Hi, everyone...we just got a test back and I'm correcting my problems so I have the right answers for the final...

An inert physics student slides down a plane inclined at 20 degrees above the horizontal. He feels a constant retarding friction force of 100 N as he slides 10 m down the plane. What is the student's speed at the bottom of the plane?

I'm not going to bother to show you guys my work since I got the question wrong :(

Thanks all

Homework Helper
There's not enough information to answer this.

PrudensOptimus
Assuming the skater started out with 0 initial velocity then the final velocity is:

v = 2sqrt(100&mu;k)

Homework Helper
Originally posted by PrudensOptimus
Assuming the skater started out with 0 initial velocity then the final velocity is:

v = 2sqrt(100&mu;k)

We know that the retarding force is 100 N but have no idea what the student's mass is, so we have no idea about inertia or gravity. I'd like to see the work for this one.

Homework Helper
Missing information:
Student's mass

Also, is that 10m vertical drop, or 10m along the slide?

Staff Emeritus
Gold Member
Nate,

I concur. You also need to know either the mass, or the coefficient of kinetic friction.

The easiest way to solve the problem is via th conservation of energy:

kinetic energy at the bottom = potential energy lost in the fall - work done by friction during the fall

1/2 m v2 = d m g sin 20 - d &mu;k m g

- Warren

ConfusedStudent
I'm so sorry, I somehow left out the mass, it was given to be 120kg...

So sorry

Also that is 10m down the slide,.

Last edited:
Staff Emeritus