Does a constant force = Constant Power?

[torquemada]

Hey I'm trying to teach myself physics using H&R. Unfortunately I didn't take trig physics in high school so every concept is new to me. But I have taken a year of calc (up to but not including Multivariable) so i understand most of what I read (at least superficially). But i have to ruminate over things before I understand them (who doesn't i guess). I'm trying to understand if a force is constant and there is a displacement over time, then will the power increase with time or will the power be constant. In other words, will the scalar quantity u get by multiplying displacement with force always turn into the same value once its divided by time elapsed? Or does the scalar value increase faster than time passes, thus yielding an increase in power over time even with a constant force? thanks for your help

 

[Doc Al]

Power is work/time = force*distance/time = force*speed. So as the speed increases, so does the power.

 

[torquemada]

Power is work/time = force*distance/time = force*speed. So as the speed increases, so does the power.

Ok thanks! :)

And the speed increase would be caused by the constant acceleration of a constant force yes?

 

[Doc Al]

And the speed increase would be caused by the constant acceleration of a constant force yes?

Right. (Presuming that the force in question is the only force or the net force.)

 

[torquemada]

Ok so the idea of power is a bit confusing. If an object is moving at a certain speed and a force is accelerating it, then you multiply those two quantities to get power. But that is the power of what? The power of the system? Or the power of the force acting on the system? Or something else?

 

[torquemada]

ok so i did more reading and research and it seems the power is a characteristic of the force itself. So if a force is constant then that force's power, it's ability to do X work in a given time interval, would increase.

 

[russ_watters]

A force does not have a single specific power associated with it. Force times velocity is power. So a force may generate any power (including none) depending on the motion of what it is acting on. It can even vary depending on the frame of reference from which you observe it!

 

[Matterwave]

Power is not a characteristic of only the force, if you had 2 equal and opposite forces acting on an object at rest, for example, no power would be done by either force.

 

[Doc Al]

Ok so the idea of power is a bit confusing. If an object is moving at a certain speed and a force is accelerating it, then you multiply those two quantities to get power. But that is the power of what? The power of the system? Or the power of the force acting on the system? Or something else?

The power is just a measure of the rate at which that force is doing work on the object, thus transferring mechanical energy to the object.

ok so i did more reading and research and it seems the power is a characteristic of the force itself.

As others have explained, just having a force implies nothing about power without knowing it's displacement. For that force to do work on something, there must be a displacement in the direction of the force. At any point, the instantaneous power will be force*speed (more precisely, P = \vec{F}\cdot\vec{v}).

So if a force is constant then that force's power, it's ability to do X work in a given time interval, would increase.

That's true only because the object accelerates (which we stipulated earlier). But you can certainly have situations where a force does no work at all, or where it's power is constant.

 

[torquemada]

But you can certainly have situations where a force does no work at all, or where it's power is constant.

Pushing against the wall I've read is an example of the former. What would be an example of the latter? Would one example be where a constant horizontal force across a surface yields a constant velocity due to an equal force of kinetic friction from the floor?

 

[Doc Al]

Pushing against the wall I've read is an example of the former.

Good.

What would be an example of the latter? Would one example be where a constant horizontal force across a surface yields a constant velocity due to an equal force of kinetic friction from the floor?

Yes. In that case, the work done ends up as 'heat' (thermal energy) due to work against friction, instead of increasing the speed (and kinetic energy) of the object.

 

[torquemada]

Ok - it's starting to coalesce in my mind. Thanks for all your help guys.