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I'm having trouble solving a problem related to work done on an object with friction present. Any help would be appreciated. Here's the question:

A 15.0 kg block is dragged over a rough, horizontal surface by a 82.0 N force acting at 20.0° above the horizontal. The block is displaced 5.50 m, and the coefficient of kinetic friction is 0.300.

a) Find the work done on the block by the 82 N force.

This was easy enough. Since the force is constant, I used the equation [sum]W = F*d*cos([the]). I got the right answer, which was

423.8014 J.

b) Find the work done on the block by the normal force

and

c) Find the work done on the block by the gravitational force.

Since the displacement in the vertical direction was 0, no work was done by these forces.

Here's the part that I'm having trouble with:

d) What is the increase in internal energy of the block-surface system due to friction?

I figured that since friction is not a conservative force, I could say that the increase in internal energy would be equal to the work done by the frictional force. I was wrong, and I'm not sure what concept I'm missing here.

A 15.0 kg block is dragged over a rough, horizontal surface by a 82.0 N force acting at 20.0° above the horizontal. The block is displaced 5.50 m, and the coefficient of kinetic friction is 0.300.

a) Find the work done on the block by the 82 N force.

This was easy enough. Since the force is constant, I used the equation [sum]W = F*d*cos([the]). I got the right answer, which was

423.8014 J.

b) Find the work done on the block by the normal force

and

c) Find the work done on the block by the gravitational force.

Since the displacement in the vertical direction was 0, no work was done by these forces.

Here's the part that I'm having trouble with:

d) What is the increase in internal energy of the block-surface system due to friction?

I figured that since friction is not a conservative force, I could say that the increase in internal energy would be equal to the work done by the frictional force. I was wrong, and I'm not sure what concept I'm missing here.

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