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gravitation

can you please help me we this question. I'm not sure how I should approach this question.

Calculate period of a small satellite which is making a circular orbit just outside a planet of density 4.0 x 103 kg· m−3. Provide the answer in SI units. Compare this to hours.

Any help will be much appreciated
Original post by ahmed2017
can you please help me we this question. I'm not sure how I should approach this question.

Calculate period of a small satellite which is making a circular orbit just outside a planet of density 4.0 x 103 kg· m−3. Provide the answer in SI units. Compare this to hours.

Any help will be much appreciated

Surely there must be more data provided, like the radius of the orbit or somethings else.
If you have this you can do this:
-combine the eqquation for circular motion with the equation for gravitational force:
mv^2/r = GMm/r^2
-then by rearranging:
v^2=GM/r
-From circular motion we know that, v = 2*pi*r/T (T=time period)
(2*pi*r/T)^2=GM/r
4pi^2r^2/T^2 = GM/r
4*pi^2*r^3/GM = T^2

Voila

**M = mass of planet
m = mass of satellite (but it doesn't matter as it gets cancelled out)
G = gravitational constant
Reply 2
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ahmed2017
OP
10
yes that's what I did. thankyou
Reply 3
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RichE
15
Original post by ahmed2017
yes that's what I did. thankyou

But if r (the orbit's radius) is much the same as r (the planet's) radius, can't you work out M = (4pi/3)r^3 x density and the r terms cancel.
Original post by RichE
But if r (the orbit's radius) is much the same as r (the planet's) radius, can't you work out M = (4pi/3)r^3 x density and the r terms cancel.

Good shout yeh

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