# Help with total work done by friction problem

## Homework Statement

A Block of mass (m) slides down a plank of length (s) from a height (h) down to the ground. The friction between the plank and the block is uk. Derive and expression for the total work done by the force of friction done by force of friction on the block

W=Fd
F=W/d
d=W/F
Wtotal=1/2 mv^2

## The Attempt at a Solution

at top PE = M=mgh
honestly ive been staring at this damn problem for a half an hour and thats all ive got, any help would be very appreciated

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cepheid
Staff Emeritus
Gold Member
Work equals force times distance. Can you figure out what the magnitude of the friction force on the block is?

ok, so mg(mk) is the magnitude of the friction force
so w = mg(mk) * s?

i mean mg (uk)

cepheid
Staff Emeritus
Gold Member
Careful. The friction force is equal to the normal force times uk. Is the normal force on the block equal to its weight in this situation?

ok, so F of friction is equal to sin (theta) x uk x mg?

ok so this is what i've worked out so far

W= F x D
D= -SF=uk x m x g

but i dont think that's right, i cant figure out how to incorporate the height, nor do i know how to get the normal force

cepheid
Staff Emeritus