# I don't understand work

I understand all of these concepts but still don't understand a couple of things. I realize that holding an object in the air should not require energy, but why does it seem like it would? If I put a belt on that weighed 400 lbs I would become tired faster. Is it simply because we are actually making small motions in order to balance the object that require force and more mass requires more force resulting in more energy?

Redbelly98
Staff Emeritus
Homework Helper
I understand all of these concepts but still don't understand a couple of things. I realize that holding an object in the air should not require energy, but why does it seem like it would? If I put a belt on that weighed 400 lbs I would become tired faster. Is it simply because we are actually making small motions in order to balance the object that require force and more mass requires more force resulting in more energy?
Yes.
While zero net work is done on the object, different muscles are all doing work, so you will get tired. The sum of the work done by all the muscles involved is still zero.

rcgldr
Homework Helper
I realize that holding an object in the air should not require energy, but why does it seem like it would?
Muscles consume energy in order to generate forces, even when there is no movement. This is chemical energy being consumed, I'm not sure how much heat is lost in the process.

You get tired when holding 400lbs because you are extremely inefficent at doing that kind of thing. Somewhere, somehow, on some biological level some force and motion does occur. That's just a limitation of the human body. Just set the 400lb weight on a table and it will hold it just the same without ever getting tired (assuming it's strong enough).

russ_watters
Mentor
Yes.
While zero net work is done on the object, different muscles are all doing work, so you will get tired. The sum of the work done by all the muscles involved is still zero.
I don't think you got that quite right. Even if you aren't moving at all you are dissipating energy and your muscles require energy to generate a static force. There need not be any motion or output work. It is better imo to view it as a matter of efficiency. In some instances, the body is 0% efficient.

Redbelly98
Staff Emeritus
Homework Helper
Yes.
While zero net work is done on the object, different muscles are all doing work, so you will get tired. The sum of the work done by all the muscles involved is still zero.
I'll concede this point, based on rcgldr's and Russ's posts.

"just put it this way, just imagine when you are holding an object while you are walking that object experiences a lot of external forces acting on it and the sum of those forces or what we call the net force would be zero because the forces that are acting on it are like strings that holds up that object just like a puppet where in if you view it as a particle [spherical in shape] all the forces are well distributed in all parts of the surface area of that object. from the study of vectors, taking the sum of them all [all the vectors] would eventually result to zero."

-R. Laride

The short answer I tell people is: doing work on an object is how we change it's kinetic energy.

Here I am talking about the total (or net) work done by the net force acting on an object. As long as the object moves while a nonzero net force acts on it, it will either speed up or slow down (have a change in its kinetic energy).

We can also talk about the work done by individual forces, even if the net force does zero work.
only oin few cases we change ke... in cases where we take even the 2-d and 3-d (rarely) in consideration, then , even the p.e changes.. and the change is represented as a func of displacement (s) or instantaneous time (t) or variable force [F(t)] etc. also momentum changes.. obviously..