Climate Change. Climate Feedback. Ocean Acidification. Rising Sea Level. Ask Question. Asked 6 years, 3 months ago. Active 5 years, 3 months ago. Viewed 35k times. Improve this question.
Add a comment. Active Oldest Votes. Improve this answer. Alex Alex 5, 2 2 gold badges 25 25 silver badges 54 54 bronze badges. Timothy Timothy 1, 1 1 gold badge 12 12 silver badges 20 20 bronze badges. Well, the way we do it, is we just say that this force of friction is doing negative work on the penguin.
And we know the work is negative because the force of friction is directed in the opposite direction to the penguin's motion. The other way we could see this is that, we could just use the formula for work done by any force. That formula's f d cosine theta. If we want to find the work done by the force of friction, we would plug in the force of friction for our force, the magnitude of it, times the distance the penguin slid to the right, and then this theta in cosine theta is always the angle between the force and the direction of motion.
So this penguin's sliding to the right, the forces directed to the left, you might think that's zero, but that's not zero. Think about it, the angle between leftward and rightward is not zero, that's actually degrees. So this angle would be right here, or pi radians. And cosine of is going to give you a negative one, so the work done by the force of friction on this penguin is going to be negative f k d.
Negative the force of friction, times the distance the penguin slid to the right. But this still doesn't answer Walter's question. Where did the kinetic energy go? Friction may have done negative work on this penguin, but where did that energy end up? And you probably have a good idea, 'cause when two surfaces rub together, some of that energy of motion is going to get transformed into thermal energy in those two surfaces.
In other words, this sheet of ice is going to have a little more thermal energy, it's going to heat up just a little bit. And Walter's feathery coat is going to heat up just a little bit, and they're going to have more thermal energy to end with, than what they started with.
Just like when you rub your hands together vigorously on a cold day to get warm, you're turning some of that kinetic energy into thermal energy that warms up your hands.
And you might be like, "Alright, that's all well and good, "but how do we put this all together? And we can add the external work that was done, which we've just figured that out. We know the external work would be the work done by friction, so we'd have a minus, 'cause it was negative work, f k d, and it's negative again because this force is taking energy out of the system. But some people might object, they might say, "Wait a minute, we just said there was thermal energy "to end with, how come we didn't include that "in our final energy?
In other words, Walter, and only his motional energy, his kinetic energy, was the only energy we were keeping track of, that's why we said that, initially, there was just Walter's kinetic energy.
And this sheet of ice was external to our system, not part of our system, that's why it exerted a negative external work, removed the energy from the system, and Walter ended up with no kinetic energy. But there's an alternate way to go about this calculation. You could say, "Alright, instead of just considering "Walter and Walter alone to be part of our system, "let's go ahead and include the ice as part of our system.
So an alternate way to solve these problems, is to use this same formula, but now, Walter and the ice are both part of our system. Our system would still start with the kinetic energy that Walter had at the beginning, that doesn't change.
But now there would be no external work, not because force of friction isn't acting, there's still a force of friction, but that's an internal force between objects in our system. So there's no external work done now. That might be a new or confusing idea to some people, so let me just say, if there's forces between objects within your system, then those forces cannot exert external work and they cannot change the total energy of your system.
Only forces exerted on objects within your system from outside of your system, can change the total energy of your system. So when this ice was not part of our system, it was exerting an outside external force on Walter, and the energy of our system changed. We started with kinetic energy, we ended with no energy. This can cause damage to a machine. Any device that has moving parts can wear out rapidly due to friction. Lubrication is used not only to allow parts to move easier but also to prevent them from wearing out.
Some examples of materials wearing out due to friction include automobile ties, the soles of your shoes, a pencil eraser, as well as machinery. With friction caused by molecular adhesion, wear is caused when molecules get torn loose from a material.
With surface roughness, small objects may get torn loose from the surface. When friction is caused by deformations in a material, the frequent changes on shape can affect the materials. Friction can result in an extra force needed to move an object.
This means more energy is required.
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