Is it possible to use L= r x p to find angular momentum here

[Warlic]

Homework Statement

[/B]

Homework Equations

L = I ω
L= r x p

The Attempt at a Solution

For b)

My proffesor found the moment of inertia at the point at which the wheel touches the ground, and used the formula L = I ω

What I don't understand is why can't one use L= r x p to solve the problem.
Wouldn't L then simply be L=RMV? [/B]

 

[gneill]

What I don't understand is why can't one use L= r x p to solve the problem.
Wouldn't L then simply be L=RMV?

If the wheel were sliding without friction along the road then that would be true. Then the wheel could be modeled as a point mass M moving past the point of interest at a perpendicular distance R.

But the wheel is rotating (about the instantaneous contact point on the road). So the wheel's moment of inertia and the parallel axis theorem is called for.

 

[Warlic]

If the wheel were sliding without friction along the road then that would be true. Then the wheel could be modeled as a point mass M moving past the point of interest at a perpendicular distance R.

But the wheel is rotating (about the instantaneous contact point on the road). So the wheel's moment of inertia and the parallel axis theorem is called for.

I see, thank you. So if the object is not rotating, I can use L=MRV, where R is the distance to the center of mass of the object?

 

[gneill]

I see, thank you. So if the object is not rotating, I can use L=MRV, where R is the distance to the center of mass of the object?

Yes. Then it's just a non-rotating rigid body undergoing linear motion.