rubbing hands - work done?

[dadasolee]

By definition, shouldn't work involve an object moving in the direction of force? W = F * d. Then How is it that rubbing your hands together is considered work? The EK explanation says that kinetic energy is transferred to heat, but still don't understand how it fits to the definition of W=Fd...

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[Isoprop]

When you slide your hand in one direction, you are doing work on your hand in that direction by F*d (where F is the force you exert on the hand, and d is the distance). The friction from your other hand is doing work on your hand equal to the normal force * coef of kinetic friction * distance. Some of the work done by friction is lost to the system, which generates heat, sound, etc.

 

[ChemEngSoonMD]

By definition, shouldn't work involve an object moving in the direction of force? W = F * d. Then How is it that rubbing your hands together is considered work? The EK explanation says that kinetic energy is transferred to heat, but still don't understand how it fits to the definition of W=Fd...

F = Did you use your MUSCLE?
D = DID YOUR HAND MOVE?

1+1=?

 

[ODorDO]

but how would you explain the case of sin0= 90?

 

[Isoprop]

but how would you explain the case of sin0= 90?

umm... what? sin of theta can only be between 0 and 1. I'm guessing you mean something else?

But since you are talking about angles, I will say that only the force parallel to the direction of the direction of motion does work. So the equation for work is really:

W = F*d*cos(theta)

where theta is the angle between the force and the displacement.

 

[boaz]

Besides for the mechanical there is also some electrical work done. Which is why the hands warm up when doing so.

 

[ChemEngSoonMD]

Besides for the mechanical there is also some electrical work done. Which is why the hands warm up when doing so.

Thats not the reason your hands warm up. Mechanical -> Friction -> Heat. Don't go spreading ignorance.

 

[boaz]

Thats not the reason your hands warm up. Mechanical -> Friction -> Heat. Don't go spreading ignorance.

Don't go spreading vitriol.

"Friction force" is an electromagnetic force. So it's mechanical->electromagnetic->heat.

 

[boaz]

F = Did you use your MUSCLE?
D = DID YOUR HAND MOVE?

1+1=?

The fact that the muscle was flexed/hand moved does not tell us much about friction; i.e. it could have moved through that distance without rubbing against the other hand...

The OP is rightly asking about the work resulting from kinetic friction force because it is different from that resulting from regular push/pull force. The fact that it's "inhibitory" would have one think that the "d" is missing from the W=Fd equation.

 

[Isoprop]

Don't go spreading vitriol.

"Friction force" is an electromagnetic force. So it's mechanical->electromagnetic->heat.

Yes, all contact forces are electromagnetic forces! Unfortunately, this is beyond the scope of the MCAT, and I would suspect that anyone who went along with this type of reasoning on the MCAT would "overthink" a problem and get the wrong answer (unless it was specifically mentioned in a passage).