How to calculate moments with balanced objects
If an object is balanced, the total clockwise A turning effect of a force. about a A point around which something can rotate or turn. is equal to the total anticlockwise moment about that pivot. This is called ‘the principle of moments’.
If the object is balanced: total clockwise moment = total anticlockwise moment. The diagrams show two examples of balanced objects where there is no The fully circular movement of an object about a point. .
A ball at the bottom of a trough
A balanced see-saw
An object in In physics, an object in equilibrium will not turn or accelerate - there is no overall (resultant) force and the clockwise moments are equal to the anticlockwise moments. will not turn or accelerate - there is no overall (resultant) force and the clockwise moments are equal to the anticlockwise moments.
For a balanced object, you can calculate:
- the size of a force, or
- the perpendicular distance of a force from the pivot
Example
A parent and child are at opposite ends of a playground see-saw. The parent weighs 750 N and the child weighs 250 N. The child sits 2.4 m from the pivot. Calculate the distance the parent must sit from the pivot for the see-saw to be balanced.
child's moment = force × distance
250 N × 2.4 m = 600 Nm
Parent's moment = child's moment
Rearrange \(M = F \: d\) to find d for the parent:
\(d = \frac{M}{F}\)
Then calculate using the values:
\(d = \frac{600~Nm}{750~N}\)
\(d = 0.8~m\)
