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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.

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• 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? • 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 ? • 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? • How does energy change into heat? Heat is something physical, that which we can feel but energy isn't,right?
Thanks • 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) • why friction is considered as an internal force when we took ice to be a part of our system.why internal force doesn't do work
(1 vote) • The relative motion between the penguin and the ice block causes equal and opposite frictional forces: one from the penguin on the ice block and the other from the ice block on the penguin. But when we take both penguin and ice block to be our system, these two forces become internal to our system which means they can't change the state of the entire system (block+penguin) because they cancel out.
Also, the frictional force between the penguin and the block does do work. That's how the kinetic energy of the system changes to thermal energy but the amount of thermal energy produced equals the kinetic energy that the system had. The total energy of the system remains conserved. 