## Simple physics question I can't seem to solve

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Mon Mar 12, 2012 3:10 am
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PiesAreSquared says...

Ok, physics questions. (I do my other subjects quite well, but some things in physics seem to snag me a little, though it’s my favorite subject)

A constant torque of 200 N turns a wheel about its center. The moment of inertia about this axis is 100 kg m sq. Find the kinetic energy gained after 20 revs.

Ok, here’s the problem. I think that the formula to use is ½ * I * w, the problem is, how do I find w? (w is the angular velocity)

A flywheel has a kinetic energy of 200 J, Calculate the no. of revs. it makes before coming to rest if a constant opposing couple of 5 N m is applied to the flywheel.
If the moment of inertia about its center is 4 kg m sq. how long does it take to come to rest?

This is really an easy question, I don’t know why I trip up on it.

Water flows steadily along a uniform flow tube of cross-section 30 cm sq. Static pressure is 1.2 *10 ^5, Total pressure is 1.28 * 10 ^ 5, Calculate the flow velocity and mass of water per second flowing past a section of the cube, density of water being 1000 kg m-3.

I’ve found the answers to this question. Mass of water is found by multiplying velocity by the cross section (after converting to m sq.) The problem I have is with finding velocity. For every question with this type, I’ve tried this formula, and it worked, always. Is it the correct formula?

Velocity pressure (I don’t find another way to call it, either it’s this or water pressure) divided by (half * 10 ^3 ). 10 being gravity. The answer to this is square rooted for the velocity. How is this linked to ½ * p * v^2 ?

I know these questions are really easy, and I would really like to find the solutions, thanks!
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Thu Mar 22, 2012 11:12 pm
AlfredSymon says...

Algebra!

Algebra can help you here. Just substitute the values from the given to the formula and do the opposite operation. If it needs to be multiplied, then divide it. Something like that. I would really like to give you the solution to these problems, but it's really hard to explain in this situation!
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Sat Mar 24, 2012 10:14 am
PiesAreSquared says...

Sorry Alfred, I don't understand you. However, I have solved the first two questions!
The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other. C. S. Lewis

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Sun Mar 25, 2012 6:02 am
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AlfredSymon says...

I know! It's just that it's very complicated! I wish I could show it to ya!
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Sun Mar 25, 2012 10:42 am
PiesAreSquared says...

Thanks anyway!
The moment you say that one set of moral ideas can be better than another, you are, in fact, measuring them both by a standard, saying that one of them conforms to that standard more nearly than the other. C. S. Lewis

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Sun Mar 25, 2012 3:04 pm
Rosendorn says...

alfredsymon wrote:I know! It's just that it's very complicated! I wish I could show it to ya!

Write it out on paper, scan/take picture of paper, upload paper as attachments.
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