Work and Friction

[Hemichordate]

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Sorry I had a brain fart today, so I just can't think clearly, but my question is:

If one object is moved from A to B with certain amount of work, and when its moved back from B to A, it requires less work, that does not violate any laws of physics right? Is the work going into things like heat and friction on the way back?

 

[Hemichordate]

Also, does friction count when you say that work is the same independent of path? Meaning if there is friction, will work by the same when going from A to B compared to B to A?

 

[collegestud2013]

Friction is a non-conservative force so how much work it does DEPENDS on the path taken. So yes it is possible that W = deltaKE = deltaPE is not true if there is any nonconservative force in play.

 

[Hemichordate]

I guess i'm still a little confused. So if I push a cart up a ramp with friction, is the work that I do pushing the cart up still equal to the work that I do to push the cart down (just the magnitude)?

I know if there is no friction, then it is the same, but with friction..?

 

[collegestud2013]

the minimum force you must need to apply to push the cart UP the ramp is F = mgsin(theta) + (us)mgcos(theta).

Depending on the the incline of the ramp and the coefficient of static friction btwn the ramp and the cart you may or may not need to apply a force at all to move it DOWN the ramp. However the component of gravity will apply try to move the cart down the ramp always has a force of F = mgsin(theta) with or without friction.

So assuming you're moving the same distance up and down the ramp, then with friction you do more work to push it up the ramp than gravity does to move it down (assuming us < tan(theta) is true).

 

[sangria1986]

I guess i'm still a little confused. So if I push a cart up a ramp with friction, is the work that I do pushing the cart up still equal to the work that I do to push the cart down (just the magnitude)?

I know if there is no friction, then it is the same, but with friction..?

I think I understand ur confusion but you are not helping yourself by asking the wrog question-the one you asked above won't help you much if I am correct about what confuses you

when you are doing work-whether or not it equals the work for in reverse depends if it is conservatvr or not. Thus when you ask is the work the same going a-b and b-a the answer is yes and no. For conservative forces such as gravitational force or a spring the answer is no the work from a-b is
not the same as b-a. In these instances the magnitude is the same. But the direction is NOT as a result in one direction the magnitude is positive but in the reverse it is negative. It does nt matter whichdirection you take as positive or negative. The sign is not dependent on the path-ie If u are going left right up or down but on the storing of potential energy. For example if u lift an object up u increase it's potential energy because you are going AGAINST the force on it (which is gravity and is down). Because u went against gravity and since gravity is dependent on height u have increased the force gravity can do on it an therefore the work gravity does on the object. When you bring it down u decrease the potential energy-opposite of above.

now for non conservative forces the work is always the same (assuming same force applied) from a-b and b-a. Friction is not conservative. This means it is dependent on path. For example if you drag the objec left a distance x the friction work done is Fx where F represents the force of friction=uN. If you go to the left the work done is still Fx. It doesn't matter that you went left or right. All that matters is the distance you went. Thus if you drag an object a to b and back b-a and the distance between the two is x the total work done is 2*Fx

Also remeber the concepts of conservative
forces and the relationships between energy work and their relationship w
conservation. I suggest reading this up