About the transformation of energy and the power..............

[albertlee]

We know that Work = Force*Distance, and Power = Work/Time

If Potential Energy of a book is 50 J....And the mass of the book is 1 kg....and the distance is 5m.....

How do i calculate the power of the book? in watts?

What i do is to transform the the P.E. to K.E., hence the kinatic force is also 50 J..........

In this case, what i want to know is the time.......
and the formula for calculating the K.E. is W = 1/2mv^2
so, 50 = 1/2(1)(v^2), 100 = v^2, and v = 10
Since i know the final velocity, and the average velocity would be 5m/s
Since i know the distance, it is 5m, then d/v = t, therefore the time is 1s...
Finally, 50J/1s = 50 watts.......

Is my solution correct? Whether is right or wrong, please give me more complete answers to my aiming question:How do i calculate the power of the book? in watts?

Please responds, cheers

[albertlee]

Any body?

[albertlee]

Anybody please help......................... :confused:

[YT2095]

reverse engineer the problem :)

a joule is 100 grams raised perpendicular to gravitys pull, do it in one second however and that would = 1 watt (if I remember correctly).

pe=50j mass=1000gr height=5m

50j divided by 5 meter = 10
so that`s 10 joules per meter

a joule per meter = 100 gr, so if we multiply 100gr by 10 we get 1000gr, or 1Kilo :)

and all of this is accomplished in 1 second, so it will be 50Watts :)

sorry if it seemed long winded answer, but from this reply, you`ll be able to work out any further calculations for yourself :)


yes you were 100% correct :))

[swansont]

reverse engineer the problem :)

a joule is 100 grams raised perpendicular to gravitys pull, do it in one second however and that would = 1 watt (if I remember correctly).


1 Watt = 1 Joule/second, but g is 9.8, not 10.

The original problem has an error, since a 1kg book 5m off the ground has a PE of 49J, not 50J. "What is the power of the book?" is also a poorly formed question - The book isn't really doing anything. Power is the rate at which work is being done, and the book isn't doing work on anything other than the earth. Also, the ~50 W is the average power, but since the book is accelerating it's not the instantaneous value. Calculate again for 0.5 seconds, and the distance is only one quarter, so the answer changes. The work being done isn't happening at a constant rate, so you need to know whether you want the instantaneous value or the average.