# Angular momentum of asteroid

1. Oct 28, 2006

### lizzyb

A planet is hit by an asteroid whose mass is M, velocity v, and it's velocity vector is 22 degrees below the Eastward horizontal. The planet has a radius R.

I used the equation $$l = r m v sin \phi$$ but the answer was wrong. Am I not doing this correctly?

2. Oct 28, 2006

### OlderDan

You have not told us the question, or anything we need to know about the planet. Please post the full statement of the problem.

3. Oct 28, 2006

### lizzyb

ok - I didn't think the values regarding the planet were pertinent.

A planet has a mass $$M_p$$ and radius $$R_p$$ and we may approximate it as solid ball of uniform density. It rotates on its axis once every T (in hours but its easy to convert). The asteriod has a mass $$M_a$$ and a speed $$v_a$$ (relative to the planet's center); its velocity vector points $$\theta$$ below the Eastward horizontal. The impact happens at an equatorial location.

We first calculated the angular momentum of the planet before the impact - I got that right.

the next question regarding the same description is "Calculate the asteroid's angular momentum relative to the planetary axis".

So I just did $$l = r m v \sin \theta = R_p M_a v_a \sin \theta$$ but this was wrong.

4. Oct 28, 2006

### Staff: Mentor

Where did that $$\sin \theta$$ come from?

5. Oct 28, 2006

### lizzyb

It's in my book; "The direction of the angular momentum vector in [a figure] is parallel to the z axis, in the direction of increasing z. The magnitude of this vector is given by $$l = r m v \sin \theta$$."

6. Oct 28, 2006

### Staff: Mentor

That angle theta is not the angle theta in your problem! That angle theta is the angle between the linear momentum (mv) and the radius vector (r). But in your problem, theta is the angle between linear momentum and the perpendicular to the radius (east is tangent to the earth's surface).

7. Oct 28, 2006

### lizzyb

so I should use $$90 - \theta$$?

8. Oct 28, 2006

### Staff: Mentor

Yep. ......

9. Oct 28, 2006

thanks!! :-)