I have a question about a skateboard problem. The athlete starts at the top of a half-pipe, in a crouched position, and goes to the bottom of the pipe. His center of mass moves along a circle with a radius of 6.3m.(adsbygoogle = window.adsbygoogle || []).push({});

Right after he reaches the bottom, he straightens out of his crouch, making his center of mass move along a circle of radius 5.85 m.

We haven't covered center of mass yet, but I have a question about this.

I am having trouble visualizing how the work done by the skateboarder straightening out of his crouch increases the potential energy which then goes into kinetic energy, increasing his velocity on the way up the other side of the half-pipe. It seems to me that this is perpendicular to the tangential motion of the skater, and so shouldn't have any effect on that part of the equation.

So if I need to calculate the velocity of the skater when he reaches the other top of the half pipe, I thought all I had to do was:

Kbottom = Utop + Ktop

And solve for the v in Ktop. But this results in an answer which is too small (according to the answer in the back of the book), which obviously used:

Kbottom + W_legs = Utop + Ktop

So I'm trying to understand why this would be right.

Thanks!

Dorothy

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# Homework Help: Skateboarding physics

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