Seesaw vs Beam balance

Classical Mechanics Level 1

Why is a seesaw always tilted to one side, while an empty beam balance is always balanced horizontally?

A. The center of mass of a seesaw is slightly above the hinge point, while that of a beam balance is slightly below the hinge point.

B. The center of mass of a seesaw is slightly below the hinge point, while that of a beam balance is slightly above the hinge point.

A B

5 solutions

Hdjdms Shsnd
May 27, 2018

Relevant wiki: Torque - Equilibrium

The center of mass of the seesaw is slightly above the hinge due to the rungs, and the center of mass of the scale is below the hinge due to the weights hanging downwards. You can imagine that balancing a seesaw is like balancing a pencil on a finger, whereas balancing a scale is like holding a pencil from the top like a pendulum. Where you contact the pencil is the hinge, and when the center of mass is below the hinge, the pencil is stable pointing vertically. When the center of mass is above the hinge, it is unstable.

Can you please elaborate?

Thennavan southee - 3 years ago

sure, on which part?

Hdjdms Shsnd - 3 years ago

Can you explain it in terms of torque?

Sripriya Karnati - 3 years ago

Sure. In the case of the seesaw and balance beam both, if it is perfectly balanced, in a theoretical world, there will be no torque. Now imagine that each one is perturbed by a small bit (the center of mass is shifted by a small bit by rotating the hinge slightly). If you draw a diagram (or imagine the pencil example), gravity creates a small torque that pulls the center of mass downwards. In the case of the scale, this brings the center of mass back down towards where it was originally was. Therefore, it is a stable equilibrium, and the scale naturally balances. For the seesaw, however, the center of mass is being pulled downwards, and that pulls it further and further away from its original position above the hinge. Therefore, this is an unstable point and the seesaw doesn't stay balanced (because small perturbations will force it further and further away from the balanced position)

Hdjdms Shsnd - 3 years ago

Second sentence: there will be no NET torque. Torques are present - but they are equal.

Jesse Otis - 3 years ago

For the seesaw, the COM is above the fulcrum (hinge) even without any rungs; it is due to just the seesaw board because it itself is above the fulcrum. However, if rungs (or anything) are/is attached to the top of the board then the COM is higher still.

Jesse Otis - 3 years ago

No, a perfectly symmetric beam balance is not always balanced horizontally. Infact its center of mass does not move to one side when the balance is rotated but rests exactly vertically below the the hinge point. This equilibrium is not stable but NEUTRAL.

Mario Pingitore - 3 years ago

this is a good point. Because the beam balance doesn't rotate rigidly, my analogy was not perfect

Hdjdms Shsnd - 3 years ago

Inksa Inkeroinen
May 27, 2018

I took a screenshot and marked the approximately places of the center of mass with a little black point:

Utkarsh Kulshrestha
May 28, 2018

The torque in the case of weighing machine is balanced counter forces , but that's not the case with see-saw

Gursimran Singh
Jun 1, 2018

COM of see saw is just above the hinge point suggesting its unstable equilibrium condition whereas in other case COM being below hinge point suggests stable equilibrium. As restoring torque works on second case towards lowest position

Tam Huynh
Jun 3, 2018

Just imagine that you have a pendulum with a metal rod/stick attached to a stand, and if left alone, it will stay hanging below the attached point, but if you try to raise the centre of mass above the attached point and balance it to stay like that, it would probably swing back down and stay there.

Seesaw vs Beam balance

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