posted by on .

A meteor is approaching. Assume that the meteor started very far away from the earth and just grazes the earth. You may ignore all other forces except gravitational force between earth and meteor. Assume that mass of the meteor is much smaller than mass of the earth.

With respect to which fixed point is the angular momentum of the meteor constant in time?

a)Initial point when meteor is very far away from earth.

b) The point where meteor just grazes the earth.

c)The center of the earth.

d)There is no point where the angular momentum is constant.

The gravitational force pulling the meteor toward the center of the earth exerts no moment (torque) on the meteor.
If there is no external moment on a system, its angular momentum remains constant.
Therefore I suspect that the angular momentum of the meteor wrt the center of gravity of the earth remains constant.

For example the force toward the center of the earth does not line up with the point on the surface of earth where the meteor grazes and therefore the force exerts a moment about that point and the angular momentum about that point can change.

Hi, Damon
you mean c is the answer?

Yes :)

but understanding why is more important than the answer.

This is from
http://en.wikipedia.org/wiki/Angular_momentum
".............
The conservation of angular momentum is used extensively in analyzing what is called central force motion. If the net force on some body is directed always toward some fixed point, the center, then there is no torque on the body with respect to the center, and so the angular momentum of the body about the center is constant. Constant angular momentum is extremely useful when dealing with the orbits of planets and satellites, and also when analyzing the Bohr model of the atom.
............... "

Thank you