Find Density Given Period of Orbit

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1. Jun 12, 2016

Just144Ice

1. The problem statement, all variables and given/known data
A satellite is in a circular orbit very close to the surface of a spherical planet. The period of the orbit is 2.53 hours. What is the density of the planet? Assume that the planet has a uniform density.

2. Relevant equations
T^2=(4pi^2r^3)/GM
V=4/3piR^3
Density= Mass/ Volume

3. The attempt at a solution
So far, I have calculated the amount of seconds in the orbit, but that is where my understanding of this comes to an abrupt halt.
How could I get the radius without the speed of the satellite? Is it not true that the satellite could orbit ANY planet in 2.53 hours if it had enough speed?
Some equations call for the gravity of planet, which is not given.
I guess you could say that I need a little "Barney-style" walkthrough on this one.

2. Jun 12, 2016

Try writing $M=density \cdot V$ and put it into your T^2 equation. Also $R$ and $r$ are the same in this problem. Do you see how the $R's$ will cancel? You can look up the value for $G$. (It should be in your textbook). When $G$ is M.K.S., the density will be in kilograms/meter^3. (assuming T is in seconds).

3. Jun 12, 2016

Keith_McClary

Eliminate R from the equations:
$T^2=(4\pi^2R^3)/GM$
$V=\frac{4}{3}\pi R^3$

4. Jun 12, 2016

Just144Ice

Ok so,
I've got:
9108^2=G((4pi^2)/(D*(4/3)pi))

5. Jun 12, 2016

G=6.67 E-11 in M.K.S. units if I remember it correctly. You can solve for D, but your $G$ belongs in the denominator.

6. Jun 12, 2016

Just144Ice

Where the heck did you find that?

7. Jun 12, 2016

Just144Ice

Gettin there:
9108^2=(4pi^2)/(6.67e11(D)(4/3)pi)

8. Jun 12, 2016

Just144Ice

Pardon my slow advance on the problem, this is my first time taking physics, and i've been out of school for 8 years.
I'm not too savvy on how to isolate the D out of the denominator.

9. Jun 12, 2016

You multiply both sides of the equation by $D$. This will put the $D$ on the left side and the $D's$ on the right side (upstairs and downstairs) will cancel. Meanwhile your $\pi^2/\pi=\pi$ and $\pi=3.14159$. Looks like your algebra may be a little rusty, but this is a good calculation to get some practice with it...Notice also how the 4's cancel on the right side (4/4=1), and the "3" (in the 4/3) will wind up in the numerator(upstairs) on the right side. (The $G$ that you asked about is the universal gravitational constant and it is $G=6.67 E-11$).

Last edited: Jun 12, 2016
10. Jun 12, 2016

Just144Ice

I got 1.70333034e-19
Doesn't sound right.

11. Jun 13, 2016

jbriggs444

Keep everything symbolic until you have a formula for the quantity you are after. Measured numeric values in a formula make the algebra more difficult.

12. Jun 13, 2016

The G is a 6.67 E-11 in the denominator. (I think you might have incorrectly used 6.67 E+11. G=6.67 E-11). It will get multiplied by the 9108^2 and the result is approximately 5.2 E-3 in the denominator. There is a $3 \pi$ in the numerator so the result will be ((3)(3.14))/(5.2 E-3). Next step is to convert this to a single number. Please try to verify these estimates. The result is in kilograms per cubic meter. (To compare to the density of water, you need to convert to grams per cubic centimeter. Water has density 1 gram per cubic centimeter. That should tell you if your answer makes sense.)

Last edited: Jun 13, 2016
13. Jun 13, 2016

Staff: Mentor

It is a fundamental constant, you have to either know it or look it up. Your textbook, Wikipedia, WolframAlpha, Google, ... all have the value.
This really helps. Also, keep the units once you plug in values, that gives an additional cross-check that everything is right.