Work or Net Work when calculating Power?

[brawler234]

Homework Statement

Calculate the power.
Forces: 5N east, 2N west
Displacement: 3m east
Time: 3s

Relevant Equations

Work, Power

When calculating power, which is Work/time, do we consider NET work (work calculated based on net force) or just work done by the individual force that goes in the same direction as the displacement?

If it's the first, then power is 3W.
If it's the second, then power is 5W.

Which one is correct?

 

[haruspex]

Homework Statement:: Calculate the power.
Forces: 5N east, 2N west
Displacement: 3m east
Time: 3s
Homework Equations:: Work, Power

When calculating power, which is Work/time, do we consider NET work (work calculated based on net force) or just work done by the individual force that goes in the same direction as the displacement?

If it's the first, then power is 3W.
If it's the second, then power is 5W.

Which one is correct?

It depends what the question asks for. Please quote the whole question.

 

[brawler234]

It depends what the question asks for. Please quote the whole question.

It basically just asks us to calculate the power.

 

[haruspex]

It basically just asks us to calculate the power.

If the information you posted is really all you have, and a single answer is wanted, then I would go for net power.
The alternative is to say the first force exerts power 5W while the second exerts power -2W.

 

[brawler234]

If the information you posted is really all you have, and a single answer is wanted, then I would go for net power.
The alternative is to say the first force exerts power 5W while the second exerts power -2W.

But I thought that force needs to be in the same direction as displacement, otherwise work isn't done.

 

[haruspex]

But I thought that force needs to be in the same direction as displacement, otherwise work isn't done.

yes, but that includes the case of opposite direction.
If a force F advances a distance s then it does work Fs. This is true even when F or s is negative,
If I push against an object with force 2N and it yields 1m I have done work 2J on it.
If, at the same time, you were pushing the opposite way with force 1N then the object has done work 1J on you (and you have done work -1J on the object). The net work done on the object is 2-1=1J, and this will take the form of gained KE.

If, in the question, the second force had been to the N, all else the same, it would have done no work (no displacement in the NS direction).