# Homework Help: Find angular momentum

1. Dec 25, 2016

### Neon32

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

2. Relevant equations
I= sum m r2
L= r p
or
L=I W

3. The attempt at a solution
I= m1 r12 + m2 r22
I= 5.20 (0.9)2+ 2.20(0.9)2= 5.994 kg.m2

Then I used the second equation of second momentum
L(Angular momentum) = I W
L= 5.994 x 4.60

In the solutions sheet, he used the first rule: L= r p and he got a different answer than mine: What did I do wrong?

2. Dec 25, 2016

### Staff: Mentor

"4.60" is the linear velocity. You need to use an angular velocity for W.

3. Dec 25, 2016

### Neon32

How do I know if it's linear or angular velocity? He didn't mention if it's linear or angular.

4. Dec 25, 2016

### Staff: Mentor

What are the units given for v? What are the units of linear velocity? How about angular velocity?

5. Dec 25, 2016

### Neon32

Linear velocity has unit m/s

I got it :D. Thanks!!

6. Dec 25, 2016

### Neon32

In the second equation L = rxp >> Is cross product between vectors so if we want the magnitude, we should use rxf sin(angle). Why he didn't do that?

7. Dec 25, 2016

### Staff: Mentor

L = r x p is a vector expression. The magnitude of L is given by L = |r||p|sin(θ).

In this instance the angle happens to be θ = 90° . Knowing that sin(90°) = 1 he wrote the simplified expression for the magnitude. Granted, to be technically correct he should have pointed this out in some fashion, but it's a common enough simplification that it shouldn't cause problems interpreting the solution.

8. Dec 25, 2016

### Neon32

Can you tell me why the angle between vector r and vecor p is 90? isn't the angle between them =0? since they are in same direction

9. Dec 25, 2016

### Staff: Mentor

p is a linear momentum of one of the particles. It would be co-linear with the velocity vector of that particle (p = mv). Since the particles are moving in a circle and thus velocities are tangential, the angle between the radius vector and the velocity must be 90°.