Elliptical Orbit Homework: Momentum, Direction & d/dt

[davamr]

Homework Statement

Based on your observations of the behavior of your computer model of a planet orbiting a star, and on your reading in the textbook, which of the following statements about an elliptical orbit are true?

At any instant the momentum of the planet is tangent to the planet's trajectory.
The magnitude of the planet's momentum is constant.
The direction of the planet's momentum is changing at every instant.
The gravitational force on the planet due to the star always acts at a right angle to the planet's momentum.
At every instant, d/dt points from the planet to the star.

Homework Equations

The Attempt at a Solution

The correct answer is :

-At any instant the momentum of the planet is tangent to the planet's trajectory.
-The direction of the planet's momentum is changing at every instant.
-At every instant, d/dt points from the planet to the star.

Although I do not understand why at every instant, d/dt from the planet to the star.

Can someone please explain this to me?

 

[HallsofIvy]

No one can answer your question because "d/dt" is meaningless. The derivative with respect to t of what?

 

[davamr]

dP/dt

 

[gneill]

dP/dt

So, what does dP/dt yield? If I tell you that a body of mass M is undergoing a change in momentum of some given value, what does that tell you about what's going on?

 

[davamr]

Thanks guys, I actually figured it out. I watched a video on KhanAcademy, which explained exactly what I was not understanding.

If anyone else wants to check it out go to khanacademy's website- scroll down to Physics - then click "Centripetal Force and Acceleration Intuition".