# Homework Help: Calculating how long it takes for a comet to reach a sun of radius zero

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1. Jul 26, 2012

### xdrgnh

1. The problem statement, all variables and given/known data
Consider the extreme case that the comet is released from rest at a distance R_max from the sun. In this case L is actually zero. Use the technique described in connection with 4.58 to find how long the comet takes to reach the sun. The radius of the sun for now is zero.

2. Relevant equations
Equation 4.58 is T=∫dr/(E-U(r))^(1/2)

E should be -GM_sM_c/R_max

and U(r) should be -GM_sM_c/r

3. The attempt at a solution

So I intergrate this and my bound are 0 to R_max. I just want to know if I'm on the right track becasue the integral is very messy. Thanks.

Last edited: Jul 26, 2012
2. Jul 26, 2012

### vela

Staff Emeritus
Is T supposed to be the time? If so, equation 4.58 doesn't seem to work out unit-wise.

3. Jul 26, 2012

### xdrgnh

Yes it is. Well I forgot to mult that integral by a (m/2)^(1/2). But that doesn't change the difficulty of the intergrand.

4. Jul 26, 2012

### vela

Staff Emeritus
Without that factor, the units of T are 1/force. Multiplying by mass1/2 won't fix it.

In any case, I don't see why you think the integrand is a mess. It looks like it should succumb to a simple u-substitution.

5. Jul 26, 2012

### xdrgnh

Darn I made another mistake while typing I'll fix the OP re look at it. I added a square root.

6. Jul 26, 2012

### cepheid

Staff Emeritus
$$v = \frac{dr}{dt} \Rightarrow dt = dr/v$$ $$v = \sqrt{2K/m} = \sqrt{2[E - U(r)]/m}$$ $$t = \int\,dt = \int v^{-1} dr = \int \left(2\frac{E - U(r)}{m}\right)^{-1/2}\,dr$$

Is that the equation?

7. Jul 26, 2012

### xdrgnh

Yep.

8. Jul 27, 2012

### xdrgnh

Any word if my E-U(r) is correct?

9. Jul 28, 2012

### cepheid

Staff Emeritus
Yeah, the expression $$- G\frac{mM}{R} + G\frac{mM}{r}$$ looks okay to me. So the integrand would become $$\left[2GM \left(\frac{1}{r} - \frac{1}{R}\right) \right]^{-1/2}$$ Did you have any luck computing it?

10. Jul 28, 2012

### xdrgnh

Not much luck. I haven't integrated any complicated integral since I took calc II two year ago. But I feel a trig sub might work. But I was mainly concerned with the physics part rather then the math. I doubt a question like this would appear on the exam and I',m only doing it for my own enrichment. But see how the trig goes if I can figure out how it would work. If not I might try a u sub. I know I should simplify it first.

Last edited: Jul 28, 2012