# Homework Help: Is it possible to use L= r x p to find angular momentum here

1. Dec 3, 2015

### Warlic

1. The problem statement, all variables and given/known data

2. Relevant equations

L = I ω
L= r x p

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

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2. Dec 3, 2015

### Staff: Mentor

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.

3. Dec 3, 2015

### Warlic

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?

4. Dec 3, 2015

### Staff: Mentor

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