0 Members and 1 Guest are viewing this topic.
The picture is wrong. Gravity affects the box, aswell as the arm that keeps the box above the ground.
Gravity acts down and the force from the arm acts up so the resultant force is zero.
Another way of seeing this is that the distance moved is zero so the work done is zero.
Yet another way is to note that the arm could be replaced by a shelf. The shelf cannot do work and so the work done must be zero.
QuoteGravity acts down and the force from the arm acts up so the resultant force is zero.Exactly.The arm does apply energy, to keep the box up. QuoteAnother way of seeing this is that the distance moved is zero so the work done is zero.Distance moved is, indeed, zero. Work is done in order to make the box not move. Therefore, work is done. The amount of energy used on the task is above zero.QuoteYet another way is to note that the arm could be replaced by a shelf. The shelf cannot do work and so the work done must be zero.I disagree.The shelf is doing work in order to keep it up. It is, however, not the same kind of work a human would do.The shelf is a platform. It can either carry the load, or it breaks down. Humans are thinkers. We can either carry it for a given time period, or let it go. No human will carry something until it kills him.
Yet another way is to note that the arm could be replaced by a shelf. The shelf cYou are wrong on every count.Try studying a bit more about physics.Work is done when a force moves through a distance. There is no movement so there is no work.
what about isometric exercise building muscle?
It is down to how human muscles work. Our muscles can't just set in one position unsupported and stay that way. To hold the box "still" our muscles actually relax and contract minutely over and over. So while the net work done on the box is zero, the net work done by the muscles is not.
Intensive breath makes work. Radiation of a additional heat by the body moves molecules of air. It is work also.
Quote from: simplified on 19/07/2011 07:38:01Intensive breath makes work. Radiation of a additional heat by the body moves molecules of air. It is work also.True. In physics when you talk about work, you have to specify two things: 1) what is performing the work and 2) the receiver of the work. To be specific, the person in the drawing is doing no work on the box. But you could also say that the various cells within the person's body are doing work since they have to transport fuel and waste to and from the cells as a result of the muscle strain of holding up the box. (Someone who knows physiology better could even tell you what parts of those cells are doing work on what chemicals.)
Also, it seems that I have to complain to you that Geezer's use of the verb "elevate" is unorthodox. I think it means to move something upwards (which does need work) and he seems to think it means to hold something up (which doesn't).