# Gravitational Orbits

1. Feb 22, 2010

### Fredley_Banyo

1. Which of the following statements about a circular orbit are true? (The planet is orbiting around the star.)

a. At any instant the momentum of the planet is tangent to the planet's trajectory.

b. The magnitude of the planet's momentum is constant.

c. At every instant, dvector p/dt points from the planet to the star.

d. The direction of the planet's momentum is changing at every instant.

e.The gravitational force on the planet due to the star always acts at a right angle to the planet's momentum.

2. Which of the following statements about an elliptical orbit are true? (The planet is orbiting around the star.)

a. At any instant the momentum of the planet is tangent to the planet's trajectory.

b. The magnitude of the planet's momentum is constant.

c. At every instant, dvector p/dt points from the planet to the star.

d. The direction of the planet's momentum is changing at every instant.

e.The gravitational force on the planet due to the star always acts at a right angle to the planet's momentum.

I missed this problem on my homework and i would like to know what the correct statements are for each so that i can understand the concepts.

2. Feb 22, 2010

### Delphi51

Welcome to PF, Fredley!
The purpose of the forum is to help you work out the answers yourself, so we are not supposed to just give you answers. If you share your thinking or work, we'll be very glad to point out any errors or omissions and aim you in the right direction.

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