# Question on equilibrium

1. Dec 8, 2011

### rishch

Can three concurrent, non co planar forces be in equilibrium? also,

If g is a constant then why don't gas particles get accelerated towards the Earth?

2. Dec 8, 2011

### mikeph

I would say no, but it's the sort of question that demands a proof accompanying the answer... I would suggest that three vectors which are not all coplanar must also be orthogonal, and therefore there is no way that they can sum to zero.

To answer your second question, gas particles are subject to the force of gravity but there are other forces which stop all the molecules from falling onto the surface (collisions).

3. Dec 8, 2011

### torquil

They don't have to be orthogonal. But as you say, they cannot sum to zero. If they did, they would be linearly dependent. And then, any linear combination of the three could be reduced to a linear combination of two of them. Therefore, they would only span a plane, or less, which is against the assumption of not being co-planar.

4. Dec 8, 2011

### mikeph

A much better way of saying it! Thanks, orthogonal was the wrong word.