r/AskPhysics • u/kotobukiii • 1d ago

Confused about exam practice question

My mechanics professor provided us practice questions to study for our midterm. I’m confused about this one:

Your friend is on her way cross-country skiing and has to do 3000 J of work by pushing against the ground with her skis. In addition, friction does -800 J of work on her. With all her gear her mass is 60 kg. Her initial speed was 5 m/s and her final speed is 3 m/s.

a) Calculate the change in kinetic energy of the skier.

Since net work is just change in kinetic energy, this should be really simple. What I don’t understand is the skier doing 3000J of work by pushing against the ground with her skis - wouldn’t that be perpendicular to the displacement? How is it doing work?

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u/mehmin 1d ago

The perpendicular component does no work, sure, but there's still the parallel component which does require friction, but it's still there.

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u/kotobukiii 1d ago

wouldn’t that be covered by the -800J of work by friction?

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u/mehmin 1d ago

This is getting too nitpicky, I think...

There's static friction that propels you forward when you push yourself off the ground; we don't usually call this the work by friction since at the point of contact there's no displacement. It only acts to transfer the work you do from your muscles to overall forward movement.

There's kinetic friction that opposes your movement, this one point backward; this is what we call the work by friction, usually.

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u/acd124 14h ago

Net work is not just the change in kinetic energy, there can also be a change in potential energy.

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u/drydorn 1d ago

I have no idea, I failed college physics 101, mostly because I never went to class, or opened the book, or studied... I mostly just stayed in my door room and played Master of Orion 1.

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u/kotobukiii 1d ago

thanks for sharing?

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u/drydorn 1d ago

You're welcome!