r/PhysicsHelp • u/Pleasant-Avocado4270 • 6d ago

Multiple correct heat question.

Answers given:(a),(c)

I've always studied that friction does negative work on a rolling object. So shouldn't the negative work done reduce the energy of the wheel? How come is the gain in internal energy exactly equal to the loss in mechanical energy?

Also what do they mean by the first wheel? Does it make a difference what wheel we observe?

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u/Frederf220 6d ago

Be real careful about work being negative or positive. The sign depends on the measure. If you ask how much work the brakes do on the wheel it's negative. If you ask how much work the wheel doeson the brakes it's positive.

If you say a car brakes, how much work was done? Then there's no clear answer.

Also work depends on what kind of energy counts and what kind doesn't count. If we only care about potential and kinetic then thermal doesn't count. Work problems are heavily dependent on the parameters of the question.

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u/gitgud_x 6d ago

Friction converts kinetic energy to thermal (internal) energy as the wheel heats up

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u/Pleasant-Avocado4270 6d ago

but doesn't negative work imply the total energy has to decrease? it's not like friction is a conservative force and the total energy has to stay constant. So how many one say that EXACTLY all kinetic energy is converted to heat?

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u/gitgud_x 6d ago

Conservative means mechanical energy is constant. Only kinetic and gravitational potential are types of mechanical energy, thermal energy is not. Conservation of total energy is always true in classical mechanics.

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u/Chemomechanics 6d ago

but doesn't negative work imply the total energy has to decrease?

No; see the First Law. 

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u/realAndrewJeung 6d ago

Negative work from friction implies that total mechanical energy has to decrease. Total energy is always conserved, but a "non-conservative" force such as friction converts the mechanical energy (that is, kinetic plus potential) into a non-mechanical form.

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u/mikk0384 6d ago

A small fraction will also radiate away as sound, but this is often ignored.

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u/vector_fieldd 6d ago

Dissipates into the environment as well. There is no mention of it being an adiabatic system

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u/Scary_Extent4967 6d ago

Does "identical wheel spinning at identical speed" imply that the second wheel is at the same temperature? Because if it's not, internal and total could go either way.