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Anderson Video - Work Done by Friction

Professor Anderson
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So let's see how this works, right. If I have a box that's sliding along a surface and it's going at some V i and then it eventually comes to a stop. So V f equals zero. We know that there's got to be some sort of frictional force here that is slowing down the box. And as the box moves along, what direction is that frictional force? It's got to be to the left. That box is sliding to the right, the force to the left is trying to slow it down. Okay, but we also know that the box moved to the right. So that's our Delta X. So what is work? Work is F cosine theta Delta X. The F here is frictional force F k cosine of the angle between them and then Delta X. What do I need to put in there for the Delta for the theta? Okay, I need to put in 180 degrees right because Delta X is this way but F is that way. And so the angle between them is in fact a hundred and eighty degrees. What's the cosine of 180 degrees? Negative one. And so this whole thing becomes negative F k Delta X. Work due to dissipative forces is negative because the force is always opposite the motion What about the work? Here we were talking about kinetic friction but what's the work due to static friction Why don't you guys turn on your mics for a second. Let's have a little chat about this one. What do you guys think right here? What's the work due to static friction? What are your names? [Inaudible] Okay, Alisha and Hannah. What do you guys think? >> (student speaking) It should be zero because there's no friction. >> Okay. I think you're right because the key is right there, right? Static friction. Static friction means no movement. So work is F cosine theta Delta X. But Delta X is zero. Okay, so static friction cannot do any work. Only kinetic friction can do work. And of course rolling friction could do work.