What is the Angular Momentum of a Planet Rotating and Orbiting a Star?

AI Thread Summary
The discussion focuses on calculating the angular momentum of a planet that rotates and orbits a star. For part (a), the moment of inertia is calculated using the formula I=2/5MR², yielding a value of 1.01 x 10^38 kg·m². The angular velocity is derived from the planet's rotation period, resulting in an angular momentum of 7.34 x 10^33 kg·m²/s about its rotation axis. In part (b), the angular momentum for the planet's orbit is to be calculated by treating it as a point mass, using the formula L = m*r²*ω, where ω is the orbital angular velocity. The discussion emphasizes the need to apply the correct formulas for both rotational and orbital angular momentum.
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Homework Statement


Consider a planet that rotates one revolution in one day and orbits a star in one year. The planet has mass = 5.6 1024 kg, radius = 6.7 106 m, and is 1.7 1011 km from the star.
(a) Determine the angular momentum for the rotating planet about its rotation axis (assume a uniform sphere).
kg · m2/s

(b) Determine the angular momentum for the planet in its orbit around its star (treat the planet as a particle orbiting the star).
kg · m2/s






Homework Equations





The Attempt at a Solution



I started this problem, but I am not sure how to do part b.
a)
I=2/5MR^2
I=2/5(5.6*10^24)(6.7*10^6)^2
I=1.01*10^38
w=1rev/day*2pi/1rev*1day/24hrs*1hr/60min*1min/60sec=7.27*10^-5
L=Iw
L=(1.01*10^38)(7.27*10^-5)
L=7.34*10^33
 
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The moment of inertia that you calculated in a) is for the rotation about the axis of the planet.

In b) they are asking you to calculate the angular momentum about it's star. They also suggest to treat it as a point mass.

I = m*r²

L = m*r²*ω = 2π*m*r²/t
 
Thanks!
 
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Thread 'Variable mass system : water sprayed into a moving container'

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