# Momentum in a mass resting on the Earth's surface

1. Jul 17, 2011

### starz_above

Hi all,

I'm trying to understand how the forces can modelled in a mass which is resting on the Earth's surface. Please see attached diagram.

It can be assumed that the mass is fixed to the bar and that bar is free to move where it is resting on the Earth's surface.

According to my thinking the mass at the top of the bar has higher momentum due to a higher velocity and mass than that which is at the bottom of the bar, hence this should cause the bar to topple over once any stabilising force is removed.

Am I correct in thinking this?

Thanks for any help.

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2. Jul 17, 2011

### Drakkith

Staff Emeritus
The reason for the bar to topple over isn't momentum due to the rotation of the earth, it is because of instability and gravity.

3. Jul 17, 2011

### starz_above

What would cause the instability if initially the bar was balanced on the surface of the earth?

4. Jul 17, 2011

### Drakkith

Staff Emeritus
The bar not being flat on the end, small motions in the air resulting in net force on the bar, not putting it on a level surface, and a thousand other little reasons.

5. Jul 17, 2011

### questioner121

OK, so if the bar was balanced in a vacuum and the surface was level and all other little reasons were accounted for then would that mean that the bar would not fall over due to the rotation of the Earth?

I was thinking that because there was higher angular momentum in the mass compared to the base of the bar it would cause it to fall over.

6. Jul 17, 2011

### Staff: Mentor

Momentum is not force. Check out Newton's first law and second law.

7. Jul 17, 2011

### Staff: Mentor

A mass sitting on a bar will not necessarily topple over.

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