Just a little thought

[ebasappa]

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Hey everyone, I was just thinkin about a couple things before I went to bed and thought up a couple physics scenarios that I need help on:


I] assume a block of mass M is being pushed at a constant velocity across a surface with no friction, is any work being done on it as it is moving?

II] how would that ^ scenario change if there was friction, and still at a constant velocity?

III] let's say I raise a book of mass M over my head by 1meter, so obviously I did work on it right? Because w=fd , but now it has a new pot energy of (1)mg ,( assume my head to be the reference point), and W=PE change correct, so does that F x D = the change in PE ? No right?

Sorry if I worded that weird haha...

 

[Charles Darwin]

Hey everyone, I was just thinkin about a couple things before I went to bed and thought up a couple physics scenarios that I need help on:


I] assume a block of mass M is being pushed at a constant velocity across a surface with no friction, is any work being done on it as it is moving?

II] how would that ^ scenario change if there was friction, and still at a constant velocity?

III] let's say I raise a book of mass M over my head by 1meter, so obviously I did work on it right? Because w=fd , but now it has a new pot energy of (1)mg ,( assume my head to be the reference point), and W=PE change correct, so does that F x D = the change in PE ? No right?

Sorry if I worded that weird haha...

1) No
2) The friction force would reduce the velocity to zero, and the work done by the force of friction would be equivalent to the change in kinetic energy of the block.
3) Think about it mathematically: If F*d = m*g*h, and your head is the reference point, then d = h, thus giving F = m*g which is a true statement. (You can ignore the initial potential energy of the block because h = 0.)

EDIT: I *conveniently* didn't read the end of question #2 so ignore my response for that one. Milski's response is more correct. If the velocity remains constant even after friction is introduced, the force of friction is equal in magnitude but opposite in direction than the force that's moving the block. Thus, the work on the block is zero.

 

[milski]

If the velocity is constant, the net force on the block is 0 and the total work done is zero as well. Keep in mind that you could have two forces equal by magnitude but opposite in direction acting on the block - in that case the total work is still zero the forces are doing W and -W work correspondingly.

If there's friction and velocity is still constant - look the second part of above. The net work on the block is still 0 (and thus its KE=const), the work done by friction is -W (friction is always opposite of direction) and the force pushing it has to be equal to friction by magnitude, so the work done by it is W.

In that case, ΔPE=W can be true if the force that you applied was parallel to the force related to the potential energy. In other words, you did work W on the block, gravity did -W and as a result its potential energy increased by W. If you had moved the block up and slightly to the side, say pushing it up on an incline with friction, you would have done more work than the change in PE.

 

[ebasappa]

Thanks for the responses guys! Totally makes sense now. One more thing haha, so, if a question were in scenario 2 and asked if there was work being done BY the person pushing the block, you would say yes right? Because even though there is a net work/ force of zero ON the block, the person is still applying some amount of work?

 

[milski]

Correct. Which is why it's very important to use the proper phraseology when talking about work. It's always "work done by a force on an object" or "net work on an object."