- #1

ShaunPereira

- 40

- 4

- Homework Statement
- If the earth were to triple its present its present distance from the sun then the number of days in one year will be

- Relevant Equations
- T^2 α r^3

I1ω1= I2ω2

The classic way to go about this problem would be to use Kepler's laws and thus find the new time period of earth.

However I encountered this question in a test on rotational motion which deals with conservation of angular momentum.

The equation used here would be I

Replacing I with MR

MR

which gives us T α R

Why shouldn't I use conservation of angular momentum in a problem like this even when it makes sense to do so as there is no net torque acting on the planet?

Is this because the force acting on the planet is gravity which is an inverse square force or is there something else that I am not able to understand?

However I encountered this question in a test on rotational motion which deals with conservation of angular momentum.

The equation used here would be I

_{1}ω_{1}= I_{2}ω_{2}Replacing I with MR

^{2}and ω with 2π/T we get,MR

_{1}^{2}2π/T_{1}= MR_{2}^{2}2π/T_{2}which gives us T α R

^{2}which is different from what Kepler's third law gives us.Why shouldn't I use conservation of angular momentum in a problem like this even when it makes sense to do so as there is no net torque acting on the planet?

Is this because the force acting on the planet is gravity which is an inverse square force or is there something else that I am not able to understand?