# Torque or center of gravity

1. May 1, 2013

On the diagram in the thumbnail,You could see A,B,C,D
I read from somewhere that forces acting on the two sides should be equal to be in equilibrium
But a person moving closer does not change the forces i.e the forces on the two sides is still equal and the center of gravity still lies on the same place
Still the seesaw leans
How is it?
I know that the torque will be decreased but this does not make any sense to me .Please explain it without torque

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2. May 1, 2013

### Staff: Mentor

The torques must be equal, not the forces.

3. May 1, 2013

Then please explain the thing without torques

4. May 1, 2013

### Staff: Mentor

You need torque to explain it. Your statement that the force on each side must be equal is incorrect.

Perhaps you are thinking that the net force (on the plank, say) must be equal for it to be in equilibrium. That's certainly true, but insufficient.

5. May 1, 2013

If the forces are not balance will not one side accelerate
As F=ma

6. May 1, 2013

### Staff: Mentor

If the plank is in equilibrium, the net force on it must be zero. That does not mean that the forces pushing down on the plank on either side must be equal. Realize that there is a support force from the pivot point.

7. May 1, 2013

Thanks Doc Al.That means my teacher was wrong in explaining

8. May 1, 2013

### Staff: Mentor

I'm not sure I would jump to that conclusion: your questions imply to me that you misunderstood something.

9. May 1, 2013

### Staff: Mentor

Since you brought up center of gravity, you should get that concept straight. If person on one side moves closer to the pivot, that would indeed change the center of gravity of the system. For the system to be in equilibrium, the center of gravity must remain over the pivot. Note that this is equivalent to saying that the net torque must be zero.

10. May 2, 2013

but at the center of gravity,the forces on two sides should be equal?
whether the person moves closer to the pivot or not,the forces on two sides still should remain same

11. May 2, 2013

### Staff: Mentor

No, the torques on each side should be equal.

When a person moves, the force doesn't change but the torque exerted about the pivot does change. And thus the center of gravity changes.

Say you had two persons, one with a mass of 50 kg, the other with a mass of 100 kg. Where is the center of mass between them? Is it possible for them to balance the seesaw? How?

12. May 2, 2013