# Pendulum swing conserves what?

1. Nov 30, 2009

### Kaxa2000

Does a pendulum swing demonstrate angular momentum, linear, both, or neither?

2. Nov 30, 2009

### rock.freak667

What do you think it is and why?

3. Nov 30, 2009

### Kaxa2000

I believe its both because during the swing the bob has a linear momentum vector with the swing. This creates an angular momentum vector out of the plane of the pendulum swing. Once it reaches its max height during the swing the force of gravity causes it to fall back the other way creating a torque vector perpendicular to plane of swing.

I'm not sure if this is completely right...I ask for confirmation

4. Nov 30, 2009

### rock.freak667

then yes, it has both linear and angular momentum.

5. Nov 30, 2009

### Kaxa2000

Is that the same thing as saying a pendulum swing conserves angular and linear momentum??

6. Nov 30, 2009

### ideasrule

Let's see. At the top of the pendulum's swing (when v=0), what's the linear & angular momentum of the bob? At the bottom of the swing, what's the linear & angular momentum of the bob? Are the two the same?

7. Nov 30, 2009

### Kaxa2000

No

Angular momentum : L = r x p

At top of swing when v = 0

p = 0
&
L = r

At bottom of swing
p = mv

L = r x p

So they are not conserved??

Last edited: Nov 30, 2009
8. Dec 1, 2009

### ideasrule

Well, angular momentum at the top of the swing is actually 0 because p=0 and L=r x p. The conclusion is correct, however: neither is conserved.

9. Dec 1, 2009

### rock.freak667

Your question asked whether it has angular momentum and/or linear momentum, not whether it was conserved or not.

10. Dec 1, 2009

### Kaxa2000

x is the cross product in this case.

Is r cross product 0 = 0?

Sorry I kind of forgot cross product ...so I'm not sure

rockfreak, sorry maybe I shouldve restated it in the actual post but the topic title is "pendulum swing conserves what?"

11. Dec 1, 2009

### Kaxa2000

How does it show that neither are conserved? Is it because the result is not the same at the top of the swing and the bottom of the swing in both cases?