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The Principle of
Moments: for a body in equilibrium
(balanced!) the sum of the clockwise moments is equal to the sum of the
anticlockwise moments.
 clockwise moments = anticlockwise
moments
Garfield has considerably more weight than Odie - so for them to 'teeter-totter' Odie has to sit a bigger distance away from the fulcrum. To play 'see-saw' their moments have to be equal (according to the Princlple of Moments):
F1d1 = F2d2
As Odie has less weight (force) he needs more distance!
How to tackle questions:
- Draw a diagram and mark on all of the information you have been given and allocate the unknown a symbol (usually F or d) - in an examination annotate the one on the paper.
- Identify the 'pivot point', 'turning point' or fulcrum - the point around which the whole system turns.
- Identify all of the forces acting.
- If they act through the support you can ignore them (as the support will produce a reaction force that will cancel them out!)
- You may have to calculate some of the forces as they may not be given to you - you may be given 'mass' instead of weight, for example.
- Calculate the perpendicular distance from each of the forces to the turning point - in advanced level questions you are rarely given the correct distance!
- Work out all of the clockwise moments and add them together
- Work out all of the anticlockwise moments and add them together
- State the Principle of Moments
- Equate the clockwise and anticlockwise moments
- Find the unknown
- Check that this unknown is actually the value you are asked for in the question - sometimes they ask you for a distance that requires this information before you can do the last step!
- Check that you have included the correct unit in your answer and that it is to the correct number of significant figures.
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