Work done

[Primarygun]

My teacher said that a box 1kg leave up by a man 50m, then he just said the potential energy= work done , so PE=500 J. The explanation may be a bit vague that cause me think of the work done is by human efforts against the gravitational force. Contradiction arises as F by human not = mg, otherwise, no movement or continuous movement.

 

Here is my illustration. I think that after a certain distance rose, if it is released, the earth pull F=mg , this is the source of the work done.

And where have the human efforts gone? Some stored in object as PE, where are the others, KE?

 

and

1.Potential energy is defined as the work done by gravity, right?

2.Energy gain refers to potential energy gains,right? but no kinetic energy gain.

[swansont]

Almost right. Potential energy is the work done against gravity.

 

Work done can also result in KE, so you have to account for that, too. But often problems are worded so that the speed is zero or constant, so there either is no KE or it has not changed.

[Primarygun]

Thank you.

How about a man holding a book is walking, what's the work done on the book by him?

Is this should be accounted into the work done by friction but not by the man? Cheers

[blakeja]

If the man is walking at a constant speed and is not accelerateing the book the work done is 0

[JaKiri]

If the man is walking at a constant speed and is not accelerateing the book the work done is 0

 

Only in a frictionless environment.

 

If we draw a free body diagram... (click to enlarge)

 

 

R is the normal reaction from the man's hand.

mg is the gravity.

Fa is the friction from the air (and other assorted things on the book)

Fm is the friction from the man's hand.

 

Since the book is not accelerating, R=mg and Fa=Fm. This means that the work being done by the man is equal to the retardive forces on the book multiplied by the distance carried.

 

NB: This doesn't take into account actual biology. For the purpose of that result, we needed to assume that the man (and his hand) was a rigid body, which is not the case. As is fairly obvious, the body has pivots in, and to get the complete answer (which is unbelieveably complicated) we'd need to know things like resistance in motion of a wrist joint, and the efficiency of the man's muscles, as he works to counteract the moment.

[Primarygun]

I think a better explanation is work done is applied by the friction but not the man:P