Why does a seesaw still lean even when the forces are equal on both sides?

[adjacent]

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

 

[Doc Al]

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

The torques must be equal, not the forces.

 

[adjacent]

The torques must be equal, not the forces.

Then please explain the thing without torques

 

[Doc Al]

Then please explain the thing without torques

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.

 

[adjacent]

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

 

[Doc Al]

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

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.

 

[adjacent]

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

 

[russ_watters]

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

 

[Doc Al]

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

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.

 

[adjacent]

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

 

[Doc Al]

but at the center of gravity,the forces on two sides should be equal?

No, the torques on each side should be equal.

whether the person moves closer to the pivot or not,the forces on two sides still should remain same

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?

 

[adjacent]

So I misunderstood the torque and forces.Thank you Doc Al for explaining.