# Work done as a function of time

1. Dec 20, 2012

### s0ft

Hey guys, can the work done on a body be described as the function of time during which a force acts upon it?

2. Dec 20, 2012

### Staff: Mentor

You can do this.
It might be more useful to consider power, but that depends on the setup.

3. Dec 20, 2012

### s0ft

I mean in a situation like where you hold an object against, say, gravity for some time would there be any work done? From what we know, until there is no motion, the work done is zero. But would it be possible to look at this problem from the "time" point of view?
For example you have a mass 'm' being attracted to another mass 'M'. You want to keep 'm' from getting to 'M'. So, say you use a rocket to push 'm', against the direction of its motion. If you manage to stop it and hold it against the gravity of 'M', would any work be done? If yes, work against what?

4. Dec 20, 2012

### Staff: Mentor

Power is force times velocity, and power is the derivative of work done. With zero velocity, power is zero and therefore work done does not change.

A rocket has to do work in your setup, but that is used to accelerate its reaction mass.

5. Dec 20, 2012

### s0ft

Reaction mass?

6. Dec 20, 2012

### Staff: Mentor

That stuff which gets ejected. In chemical rockets, it is the same as its fuel. In other designs (like ion drives), it can be different.
See this explanation for details.

7. Dec 20, 2012

### s0ft

Thanks. So basically the work done to throw the reaction mass is the work done to hold the 'm' in its place?

8. Dec 20, 2012

### Staff: Mentor

Well, it is work done by the rocket to counter some force (here: gravity) - this just shows that rockets are a bad idea if you want to hold something in a fixed distance. A simple connection gives the same result and does not have to do work.