KE vs KE+Rotational energy

[IlyaR]

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I'm going over the physics SA and have a question:
I recall from the wiki premed cards that if you have a block (on a frictionless ramp) and a ball (on a ramp that has enough static friction for it to roll without skidding), the block will get to the bottom first since the initial potential energy MGH is equal to KE, but the ball has KE as well as rotational energy too.
One question on the SA basically asks if a tire is moving at a speed x, how does the work of the friction of the brakes relate to the translational energy of the tire.

I chose that it is MORE than the translational energy of the tire due to the tire having rotational energy also, but the answer says that it is the same (which is what I would have picked had I not seen that flashcard lol)

 

[jetsfan1234]

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[Odi]

The question is pretty vague but I'm assuming it's strictly asking about the brakes of the car and the translational energy as being your system--not anything to do with the road.

Thus, the brakes of a car use friction when they come in contact with the tire to convert the cars translational energy into thermal energy in the brake pads to slow down the car. Conservation of energy tells us that the energy supplied by the translational energy (kinetic energy plus rotational energy) to the work in the friction pads should be equal or else we would have violated our energy laws.

Work done by break friction is supplied by the translational energy (energy
equilibrium).

If the work attained by friction was LESS than the translational energy being supplied, we would have lost energy somewhere, thus violating the conservation laws. If the work attained from friction was MORE than the translational energy being supplied, we could have created energy out of nowhere, thus attaining God like powers.

 

[IlyaR]

The question is pretty vague but I'm assuming it's strictly asking about the brakes of the car and the translational energy as being your system--not anything to do with the road.

Thus, the brakes of a car use friction when they come in contact with the tire to convert the cars translational energy into thermal energy in the brake pads to slow down the car. Conservation of energy tells us that the energy supplied by the translational energy (kinetic energy plus rotational energy) to the work in the friction pads should be equal or else we would have violated our energy laws.
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I was under the assumption that translational energy did NOT include rotational energy.

The total kinetic energy of an extended object can be expressed as the sum of the translational kinetic energy of the center of mass and the rotational kinetic energy about the center of mass.

http://hyperphysics.phy-astr.gsu.edu/hbase/rke.html

 

[Odi]

I was under the assumption that translational energy did NOT include rotational energy.


http://hyperphysics.phy-astr.gsu.edu/hbase/rke.html

Hm, yeah I mixed up total kinetic energy with translational.

So only the KE is being transferred to the work of friction through the break pads.

RE is not. I've been trying to think of the logic to that but no light bulb is clicking. I'm assuming it has something to do with static friction at the point of contact. There is no instantaneous velocity between the tire and the break maybe.

 

[Odi]

The answer is the work energy theorem... W = KEf - KEi.

The KEf = 0 so the work done is simply equal to the magnitude of KEi as we already established.

But now I don't understand why no work is done by the breaks on rotational energy? Thanks OP lol

Anyone?

 

[IlyaR]

But now I don't understand why no work is done by the breaks on rotational energy? Thanks OP lol

Anyone?

Haha sorry bud

 

[QuinnTheEskimo]

I am reluctant to say this...but I think this question might be a mistake! I woudn't worry too much about it. If you are worried, look over the rotational energy chapter of your physics textbook.