Principle of moments

The principle of moments says that:

If an object is in equilibrium (it’s balanced and isn’t moving) the sum of its anticlockwise moments is equal to the sum of its clockwise moments.

That basically means that, if an object isn’t rotating, the moments on either side of the pivot must be the same, so that the force rotating it clockwise is the same as the force rotating it anticlockwise.

Seesaw

You might not have heard of the principle of moments before, but you’ll have seen it in practice.

When two people sit on a see-saw, one person is exerting a moment clockwise and one person is exerting a moment anticlockwise.

The distance that you sit away from the pivot changes the moment. That’s why people who weigh more (larger force) need to sit closer to the pivot, to minimise their moment in order to get the seesaw to balance.

That’s the principle of moments!

Questions

If an object with a clockwise moment of 8Nm is in equilibrium, what is its anticlockwise moment?

• Clockwise moment = anticlockwise moment
• So anticlockwsie moment = 8Nm

A force of 8N acts clockwise 2m away from the pivot. Find the anticlickwise moment.

• M_{clockwise}=8\times2=16Nm
• Anticlockwise moment = clockwise moment
• Anticlockwise moment = 16Nm

A parent of weight 600N sits on a seesaw with their child of weight 200N. How many times further should the child sit from the pivot, compared to the parent, so that the seesaw balances?

• The moments need to match, so:
  • M_1=M_2
  • 600\times d_p = 200\times d_c
  • 3d_p=d_c
• That shows us that the child needs to sit 3 times further from the pivot in order for the seesaw to balance!

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