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P=Fv equation for power
gg173
#1 Posted : Friday, June 24, 2016 3:57:06 PM
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I have a question regarding the P=Fv equation for power. The CC states that this equation is useful when we are asked about the power of something moving at a constant velocity. How can an object have constant velocity if a force is being applied on it?

P=W/time
but W=Fd
therefore
P=Fd/t=Fv


Since there is a net force applied on the object (because work is being done on the object), doesn't that mean the object is experiencing a net acceleration? And if so, how can the object in question have a 'constant velocity'?

faisaluddin
#2 Posted : Saturday, June 25, 2016 4:44:11 AM
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gg173 wrote:
I have a question regarding the P=Fv equation for power. The CC states that this equation is useful when we are asked about the power of something moving at a constant velocity. How can an object have constant velocity if a force is being applied on it?

P=W/time
but W=Fd
therefore
P=Fd/t=Fv


Since there is a net force applied on the object (because work is being done on the object), doesn't that mean the object is experiencing a net acceleration? And if so, how can the object in question have a 'constant velocity'?




Hello gg173,

You understanding is definitely correct. The CC should not say "CONSTANT" velocity. It should say the "AVERAGE" velocity.

As you correctly identified, an object under the influence of a net force will have an acceleration and, thus, a changing velocity.

However, if we take the average velocity of that object through the displacement, we can plug that into P=Fv and get the power.

I hope this answers your question!
-Faisal
faisaluddin
#3 Posted : Monday, June 27, 2016 3:20:03 PM
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Just want to add that there are some question types where there might be a constant velocity and you might have to use P=Fv in some way. These questions will have more than 1 object transferring energy to/from each other. For example, a 1000kg car travels at constant 10 m/s driven by an engine developing 10kW. The net force on the car is zero but solving for F=P/v=10000Watts/10m/s, you get 1000N. This force represents the the resistive forces (friction) that the engine has to overcome to achieve constant velocity for the car.

Your thinking is still correct, as in the engine there are net forces and accelerations within it so it produce a power output, but in this case the energy is being transferred to power another object (the car) to give it a only constant velocity with no acceleration.

-Faisal
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