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Homework Statement
Which forces does work?
http://www.screencast.com/users/ntrinh3/folders/Jing/media/3f09a85d-142c-4f73-a8a8-1ad506d5c2b8
Homework Equations
W=fdcos(theta)
The Attempt at a Solution
I always thought normal and friction does no work because it is perpendicular to motion. What gives?