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# Thermal energy from friction

In this video David shows how to find the energy generated by the force of friction and solves an example conservation of energy problem involving thermal energy. Created by David SantoPietro.

## Want to join the conversation?

• if we apply newton's first law then if we threw an object in space then that object will go on forever in the direction that we threw. At this point the displacement would be infinite right? then the work we done on that object is infinite?
• No, you wouldn't continuously work just because the object never stops, you only have to apply force once when throwing the object and it is that force that equals work.
• So when someone says that "energy changes", does that mean that energy physically from one form to another or is it just the way in which we use energy that changes, for example, say that a box is being dropped from a certain height, when the box is falling, does the energy that was stationary and had the potential to do work (PE) making the box fall(KE) or is it because the energy physically or materially changes?
• Hi Hafsa,

When we start the box has potential energy.

When it is dropped the potential energy is converted to kinetic energy.

One minutia before the box strikes the ground the potential energy is zero and the kinetic energy has reached some maximum.

Finally we arrive at the answer to your question. When the box strikes the ground the kinetic energy is converted to heat.

Regards,

APD
• Why is the thermal energy equals Fk X d ?
• Thermal energy is due to the two surfaces (i.e Walter's fur and the ice) rubbing against each other, which is basically friction. So the work done by friction is W-Fk*d (Fk is the force of friction) and it is equal to the thermal energy.
I Hope i was clear enough!
• A bit confused. In the last video, we had seen that the formula (W=Fdcos(theta)) only applies to constant forces.
How is it that we are using this for friction? Wouldn't friction be a varying force?
• no, we assume that friction is given by the equation (coefficient of friction)*normal force. That doesn't vary.
also the formula works as long as you use average force. Doesn't have to be constant
• How does energy change into heat? Heat is something physical, that which we can feel but energy isn't,right?
Thanks
• Energy is in many forms. Heat is just one form of energy.

Energy changes from one form to another and so it can (and usually does) change into heat energy
• At , I am confused for the distance that David deduced form law of conservation. Suppose the hill is more steeper, that would mean we have more distance on the horizontal but with the law of conservation we will have same values every time. This is indigestible.
• The hill is frictionless. it doesn't matter how steep it is, the amount of kinetic energy (speed) the penguin has at the bottom will always be the same as the amount of potential energy it had at the top off the hill. there for, if the penguin has to come to a stop, the same amount of kinetic energy must be dissipated as heat, which would take the same distance, if the coefficient of friction is still the same.
• At , david says force of kinetic energy is equal to coefficient of kinetic energy multiplied by the normal force. But how? Can you please post a derivation or a link where he explains it?
(1 vote)
• This is a good question. I have been using this formula for quite some time and have never seen a derivation either. My best educated guess would be that it was experimentally determined, meaning physicists investigated the kinetic friction force and found that it was proportional to the normal force by some factor, and they named the factor the coefficient of kinetic friction.
• If David is taking the force of gravity to be mg then how come he does not take the normal force to be -mg, but rather he has the normal force also equalling mg?
(1 vote)
• He's using the magnitude of the normal force.