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- Thread starter Izzhov
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Doc Al

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Edited: Please note my correction to the original equation!

Firstly, that equation is not quite right. The integral (under the square root sign) should be:

[tex] \int_{r_1}^{r_2} \frac{GM}{r^2} dr = \left[ - \frac{GM}{r} \right]_{r_1}^{r_2}[/tex]

So your equation should read:

[tex] v= \sqrt{2 \int_{r_1}^{r_2} \frac{GM}{r^2} dr} [/tex]

The corrected equation represents the instantaneous speed of the falling object at r1, assuming it started from rest at r2 (where r2 > r1).My question is: does this equation represent the instantaneous velocity at r1 or the average velocity?

The integral comes from calculating the increase in KE of the two mass system. If the smaller mass is a tiny fraction of the larger's mass, then all that energy becomes KE of the smaller mass--as in the given equation for v. (That's where that equation comes from.) But if you want to include the motion of the larger mass as well, you'll have to distribute that KE across both masses and apply conservation of momentum to find the relative speed of the masses.Also, how can this equation be changed to include the force of gravity of the smaller object on the larger one?

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So, if I wanted to include the KE that the smaller mass' force of gravity produces, would the new equation be [tex] v= \sqrt{2 \int_{r_1}^{r_2} \frac{GMm}{r}} [/tex] where m is the smaller mass?But if you want to include the motion of the larger mass as well, you'll have to distribute that KE across both masses and apply conservation of momentum to find the relative speed of the masses.

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Doc Al

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Edited: Please note my corrections!

No. Check and you'll see that the units don't match across that equation.

Before you worry about modifying the original equation, first understand how it was obtained; it starts with this statement of conservation of energy:

[tex] \frac{1}{2}mv^2 = \int_{r_1}^{r_2} \frac{GMm}{r^2} dr [/tex]

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Would the KE be distributed evenly?

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Gib Z

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EDIT: MY BAD!! Classical Physics section >.< Sorry

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Doc Al

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Note to Izzhov: I just realized (last night, after logging off) that there is an error in your original equation, which I have let propagate into my own. D'oh! So I will revise my answers accordingly. Stay tuned. (Same basic idea, though; perhaps you just miscopied the equation.)

Edited: Please note my corrections in posts 2 and 4!

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Doc Al

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The distribution of KE depends on the relative size of the masses. If they were equal, then the KE would be distributed evenly. If they were wildly different, like a bowling ball or rocket compared to the earth, then the bowling ball or rocket would get just about all of the KE. (That's the assumption made in your orginal equation.)Would the KE be distributed evenly?

The key is that both energy

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So essentially what you're saying is that if [tex] \int_{r_1}^{r_2} \frac{GMm}{r^2} dr [/tex] is the KE, then [tex] \frac{M}{M+m} \ast \int_{r_1}^{r_2} \frac{GMm}{r^2} dr [/tex] is the amount of KE that the smaller mass gets, and [tex] \frac{m}{M+m} \ast \int_{r_1}^{r_2} \frac{GMm}{r^2} dr [/tex] is the larger mass's KE, and that this can be derived using conservation of momentum. Is this correct?

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Doc Al

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Exactly right!

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