# Re: conservation of energy problem(?)

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1. Oct 1, 2014

### yugeci

1. The problem statement, all variables and given/known data

(apologies for the bad picture, but it was the only one I could find online)

2. Relevant equations

Conservation of energy, KE1 + PE1 = KE2 + PE2
PE = mgh
KE = 0.5mv²

3. The attempt at a solution

I'm not sure where to begin. I know the answer but I don't know how you get it. Can't you simply use conservation of energy here? At the top PE = mgr and KE = 0... and at the bottom PE = 0 and KE = 0.5mv². If I do that v = √2gr, but the answer is actually √(gr(π/2 + 4/π)). I'm not sure what to do with the mass per unit length (density?) either and how it's relevant to this equation.

Steps on how to solve this problem would be appreciated.

2. Oct 1, 2014

### BiGyElLoWhAt

So you have an expression for the energy from the gravitational force on the TOP link, are there any other forces acting on the top link?

3. Oct 1, 2014

### Staff: Mentor

Why not?

That would be true if all the mass were concentrated at a point at the top, but that's not the case. The mass is spread out over the channel.

Hint: How would you measure the gravitational PE of an extended object?

4. Oct 2, 2014

### yugeci

mgY where Y is the Y-coordinate of the center of mass. So that would mean

initial PE = mg(r/2)
initial KE = 0 (?) (KE isn't affected by length?)
final PE = mg(-π/4)
final KE = 0.5mv²

Took the bottom right corner of the quadrant as the datum. When using the equation initial PE + initial KE = final KE + final PE I still don't get the right answer here. I am assuming my expressions for the KE is wrong.

5. Oct 2, 2014

### Staff: Mentor

Good.

Are you sure that's where the center of mass is?

6. Oct 2, 2014

### yugeci

On second thought, no. Would it be halfway down the mass? Is this diagram correct?

So doing some trigonometry there I get the Y coordinate as r sin (π/4) which ends up being r sqrt(2) upon 2, but I still get the wrong answer when substituting that in my equation above.

7. Oct 2, 2014

### haruspex

No, that's not the right way to find the centre of mass either. Have you not been taught how to find a centre of mass?

8. Oct 2, 2014

### yugeci

Um, no. Not really? I just assumed it would be half-way down the length of the string. Where would it be?

Interesting my answer is almost correct (but not quite and probably just by luck).

9. Oct 2, 2014

### Staff: Mentor

To find the center of mass, you'll have to use a bit of calculus. (Or look it up somewhere.)

Since the chain is curved, the center of mass won't be on the chain itself.

10. Oct 2, 2014

### yugeci

OK so after applying some integration (never studied this in class for center of mass, weird) + verifying it online the coordinates seem to be (-4R/3pi, 4R/3pi). Putting the Y coordinate into my equation I wrote above and making v the subject I get:

v = √gr((π/2) + (8π/3))

But the answer is √gr((π/2) + (4/π))

11. Oct 2, 2014

### Staff: Mentor

Dare I ask you to show how you got that? It doesn't seem right. How did you verify it? (All you care about is the y-coordinate.)

12. Oct 2, 2014

### haruspex

You seem to have found the centre of mass of a quarter disc, not a quarter circle arc.

13. Oct 3, 2014

### yugeci

Yep. Whoops. That was silly of me. I found the arc center of mass as -2r/π, 2r/π and I seem to get the right answer with it now.

Thanks for all of the help.