# Balance CM

1. Jun 18, 2006

### Don Carnage

Hi ppl.

Why is it, the "centre of mass" determines how good the balance
of an object is ? I’ve tried this experimentally, on when I place the centre of mass below the object, its nearly impossible to push over, Or well... It IS IMPOSSIBLE!! Could someone please tell me which equations tells me about this ?

Thx

Peter:tongue:

2. Jun 18, 2006

### DM

The centre of mass is where gravity acts on a body.

It can be further studied if you introduce moments.

Clockwise moments = Anti-clockwise moments

F x d = F x d

3. Jun 18, 2006

### Don Carnage

Do you mean angular moments and torque ? I still dont get it.. dont you know an equation ?

4. Jun 18, 2006

### vinter

'angular moments'!
LOL!

5. Jun 19, 2006

### Galileo

It depends a bit upon the context, but suppose you are tightrope walking. If you are slightly out of balance (ie, your center of mass is not precisely above the rope), gravity will exert a nonzero net torque to make you fall over to the side completely.
But imagine you are hanging from below the rope (with your arms or something). Now if you swing sideways gravity exerts a torque to restore you to the equilibrium position right beneath the rope (much like a pendulum). The first is an example of unstable equilibrium, the second of stable equilibrium.
For a rigid body in a uniform gravitational field, gravity can be assumed to act on the center (centre?) of mass. If the center of mass is above the turning point (the contact point on the rope in this case) you have an unstable equilibrium, if the center of mass is below the turning point, you have stable equilibrium. You can easily convince yourself of this if you draw a picture and analyze the torques.

This is why tightrope walkers often practice with a long pole to which two long strings with masses are attached, so the the center of mass lies below the rope.

Last edited: Jun 19, 2006
6. Jun 19, 2006

### FredGarvin

I think you need to revisit what the term "balance" means. Something can be in balance and be easily nocked out of balance by a small force.