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## #1 2005-08-05 22:24:18

wcy
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### Orbit of moon around the earth

This is a physics question, but it actually requires no background knowledge.

Assume there is a planet Zo, which is spherical in shape. It has a moon, 10 times smaller in volume, which is also spherical in shape. The moon revolves Zo in an elliptical orbit. Assuming the density of the planet and the moon is equal, and that they are 100km apart, does the moon revolve around the planet with constant velocity?

you have 50% of getting it correct, either yes or no but please post your explanation as well

## #2 2005-08-05 23:02:14

mathsyperson
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### Re: Orbit of moon around the earth

I'm confused. If the moon orbits the planet elliptically, how can you say that they are 100km apart? Wouldn't the distance keep changing?
Anyway, I guess yes

Why did the vector cross the road?
It wanted to be normal.

## #3 2005-08-06 09:51:14

MathsIsFun

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### Re: Orbit of moon around the earth

I have played with this on the PC years ago.

I wrote a little "orbit simulator" - you have two points with different masses and velocities and the program loops around calculating the new positions and plotting the points on the screen every loop.

If you have a large and small mass, the small one moves fastest, and when it gets really close to the other it REALLY speeds up and then gets "slingshotted" away.

So, I would say that in an elliptical orbit the moon's velocity speeds up when it is closer to the planet.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #4 2005-08-06 18:42:03

wcy
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### Re: Orbit of moon around the earth

haha this is meant to be a trick question, sorry abt the confusion

Ans:
Velocity is a vector, and since the direction of the moon keeps changing, velocity is not constant!

## #5 2005-08-08 09:13:14

NIH
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### Re: Orbit of moon around the earth

#### wcy wrote:

haha this is meant to be a trick question...

Then you should have specified a circular orbit!

For an elliptical orbit, both the speed and the velocity change.  But for a circular orbit, although the speed is constant, the velocity changes, thus greatly increasing your chances of pulling off the trick!

2 + 2 = 5, for large values of 2.