# Homework Help: What causes a 'simple pendulum' to stop?

1. Sep 5, 2009

### student321

1. The problem statement, all variables and given/known data
I need all the variables that causes the pendulum to stop.

2. Relevant equations

3. The attempt at a solution

I know friction causes the pendulum to stop but what type of friction is it and what from?

thanks

2. Sep 5, 2009

### Hootenanny

Staff Emeritus
Welcome to Physics Forums.

A simple pendulum will never stop once set into motion.

3. Sep 5, 2009

### student321

doesn't it stop due to air resistance etc.

4. Sep 5, 2009

### Hootenanny

Staff Emeritus
By definition a simple pendulum ignores air resistance, friction, etc.

5. Sep 5, 2009

### student321

is that even possible ignoring air resistance?

6. Sep 5, 2009

### Hootenanny

Staff Emeritus
I don't quite understand what you mean.

7. Sep 5, 2009

### student321

But real pendulums are subject to friction and air drag, so the amplitude of their swings declines.

8. Sep 5, 2009

9. Sep 5, 2009

### Hootenanny

Staff Emeritus
Is a simple pendulum the same a real pendulum?

10. Sep 5, 2009

### student321

OK, what causes a real pendulum to stop?

11. Sep 5, 2009

### Hootenanny

Staff Emeritus
Well, you've already given one cause:
Another cause could be friction from the pivot.

12. Sep 5, 2009

### student321

how does the pivot cause friction?

Thanks Hootenanny.

13. Sep 5, 2009

Energy is also transferred from the pendulum via the support system.If the support is firmly fixed in position the pendulum will swing for a longer time.

14. Sep 5, 2009

### Hootenanny

Staff Emeritus
The usual way is friction between axle and the bearing. Take the simple case of a piece of string tied round a pin. If the knot is loose then as the pendulum oscillates the string will 'rub' against the pin.

15. Sep 5, 2009

### noblegas

SHM is just a mathematical picture right? It would not exist in the real world unless it were place in a vacuous region of the universe

16. Sep 5, 2009

### Hootenanny

Staff Emeritus
You are indeed correct. We would also presumably need to invent frictionless bearings or ideal springs to practically realise SHM. We would also need to find massless strings and point particles.

17. Sep 5, 2009

### student321

is 'SHM' what a pendulum creates?

18. Sep 6, 2009

A simple pendulum is just one example of a system that vibrates with SHM.If we were to be fussy we would say that the motion is only exactly simple harmonic if the string were massless the bob had zero size and if the amplitude of swing was negligible,in other words,a real pendulum does not move with SHM.I suspect,however,that this fussiness is confusing you and that at your current level you do not need to be fussy.Provided that the size of the bob is small,the string is light and the amplitude of swing is small the motion is approximately SHM.

Last edited: Sep 6, 2009
19. Sep 6, 2009

### ideasrule

A pendulum can actually approximate SHM extremely well. The string doesn't need to be massless, nor does the bob need to be a point mass; extended pendulums still swing with SHM provided the amplitude of its oscillations is small.

20. Sep 6, 2009