TPR Science Workbook Physics #158

[UpQuark]

Question: A object weighing 100N is travelling vertically (no air resistance) at constant velocity of 5m/s. What is the power required to keep the object in motion?

A) 0 W
B) 20 W
C) 200 W
D) 500 W

If velocity is constant, then acceleration is 0m/s^2 right, so the force should be zero, and consequently the power?

But TPR explanation is that since the velocity is constant, so is the force, so power would simply be P = W/t = Fd/t = Fv so 100*5? I'm still having trouble understanding it... any help?

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[ridethecliche]

I guess that the way I'd think about this is that to pick an object up and keep it going, you still have to counteract gravity, so you're doing work by picking it up. As the potential energy is going up as well, somethings going on, right?

So power= w/t

Change in potential energy every second is mgh=10x10x5=500J

500J/1sec=500W

To bring the system to a higher state of potential energy, some work has to be done on it, no?

Edit: Corrected above.

 

[ilovemcat]

Question: A object weighing 100N is travelling vertically (no air resistance) at constant velocity of 5m/s. What is the power required to keep the object in motion?

A) 0 W
B) 20 W
C) 200 W
D) 500 W

If velocity is constant, then acceleration is 0m/s^2 right, so the force should be zero, and consequently the power?

But TPR explanation is that since the velocity is constant, so is the force, so power would simply be P = W/t = Fd/t = Fv so 100*5? I'm still having trouble understanding it... any help?

For future questions regarding problems, you should post then in the Q&A forum located here: http://forums.studentdoctor.net/forumdisplay.php?f=134

For this question, draw a force diagram. The weight of the object is acting downwards on the object and some unspecified force is applied upwards. We know since the object is traveling at constant velocity, that the applied force is equal in magnitude to the weight of gravity. Therefore, the applied force is 100N upwards.

I think this question could be re-phrased a little better. Instead of asking, "What is the power to keep the object in motion," they should instead asked "What is the power is required to maintain constant velocity." Because we know the force applied upwards is needed to main constant velocity (otherwise it would accelerate downwards) - we use that to find the power output.

Power = Work / time = Force x Distance / time = Force x Velocity
Power = 100N x 5 m/s
Power = 500 Watt

EDIT: Had they asked: "What is the overall Power output of the object," then Power would be 0 W (because the net work is 0J - Gravity does Negative Work, Applied Force does Positive Work). But they are referring to the power needed to "Keep the object in motion" which is essentially asking, what is the Power of the force being applied to the object. Does that make sense?