# Easy gravitational problem

solo
i have no idea how to do this :(

Venus is an average distance of 1.08x10^8 km from the sun. Estimate the length of the Venutian year given that the Eearth is 1.50x10^8 from the sun on the average?

one more :(

the rings of saturn are composed of chunks of ice that orbit the planet. the inner radius of teh rings is 73,000 km, while the outer radius is 170,000 km. The mass of saturn is 5.69x10^26 kg.

a) find period of orbiting chunk of ice at inner r
b) find period of orbiting chunk of ice at outer r

im thikning im going to use the formula: F = m1m2 / r^2 but im not sure how to approach it

help would be appreciated

thanks dudes

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## Answers and Replies

Ambitwistor
For all of these problems, start with Newton's gravitational force law (which you gave, except you left out the gravitational constant G); assume that the orbits are circular, so that the gravitational force is a centripetal force. Use the centripetal force to find the orbital velocity, and with that and the radius, you can find the period.

solo
F = m v^2 / r is centripetal force right

and thanks for the help

Ambitwistor
Yes, that's the right formula for centripetal force.

solo
coudl you please clarify how to use the radius and orbital velocity to find the period?

my texctbook doesn't give a good explanation?

thanks

F = m v^2 / r

so it would be G m m(e) / r^2 = m v^2 / r

what would m(e) be

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Ambitwistor
coudl you please clarify how to use the radius and orbital velocity to find the period?

If you know the distance something travels (e.g., the circumference of the orbit), and its speed, you can find out how long it takes to travel that far.

F = m v^2 / r
so it would be G m m(e) / r^2 = m v^2 / r
what would m(e) be

In this context, it's the mass of the gravitating body about which the body of mass m is orbiting.

solo
yes thank you it is finally clear

G ( 5.69 x 1-^26) / (170,000,000)^2 = v^2/r

finally

i got it :D