# Calculating Speed of Asteroid at its Perihelion

This is not part of my coursework; I am preparing for an exam and this is a question from a past paper to which answers are not given.

1. Homework Statement

An asteroid orbiting the sun has its perihelion at 1AU and its aphelion at 3AU. Calculate the speed of the asteroid at its Perihelion.

2. Homework Equations
Kepler s
Universal Gravitation
UCM ?
3. The Attempt at a Solution

I am a bit stuck on this. First I calculated its semi-major axis as I assume it will be needed,

$a=\frac{r_a + r_b}{2}=\frac{(1.49 \times 10^{11})+(3 \times 1.49 \times10^{11}}{2} = 2.98 \times 10^{11} m$

Could I use the uniform circular motion equation $F_c= \frac{mv^2}{r}$ in this situation? I ask as its an a moderately eccentric ellipse.

I was thinking of equating that to the force of gravity to get $v$, but if I could use it in this situation, what would I use as $r$, the semi-major axis, or 1AU?

If I cannot use that UCM equation then I am more stuck than I realised, so would appreciate any help.

Thanks,

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#### TSny

Homework Helper
Gold Member
Since the motion is not UCM, then you would need to justify using the UCM equation with the semi-major axis. I'm pretty sure it won't lead to the right result.

Can you think of a couple of quantities that are conserved in the orbital motion?

What units are you using for distance? The 1015 factors look strange to me.

Since the motion is not UCM, then you would need to justify using the UCM equation with the semi-major axis. I'm pretty sure it won't lead to the right result.

Can you think of a couple of quantities that are conserved in the orbital motion?

What units are you using for distance? The 1015 factors look strange to me.
Sorry, I remembered incorrectly, it is 1011 in metres.

Angular momentum? But I cannot see how I can work with that, not knowing the asteroids shape etc.

#### TSny

Homework Helper
Gold Member
Orbital angular momentum is conserved. Treat the asteroid as a point particle.

You will need a second conserved quantity.

Orbital angular momentum is conserved. Treat the asteroid as a point particle.

You will need a second conserved quantity.
Ok thanks. Have not done many problems in angular momentum, could I use the Earth?

EDIT: Typo in op, the aphelion is 3AU, not 2.

Homework Helper
Gold Member

#### gneill

Mentor
Hi FaraDazed. If I might offer a hint: what do you know about the relationship between the semi-major axis of an orbit and its specific mechanical energy?

The earth is not relevant to this question. You just have the asteroid orbiting the sun.

See if this link helps refresh your memory concerning orbital angular momentum: http://hyperphysics.phy-astr.gsu.edu/hbase/amom.html
Thanks for the link. I didn't think I could, I just couldn't think what else, as it's not like it has any moons (the only question i did before with regarding the Earth Moon system.