Moments and 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.
The diagrams show two examples of balanced objects where there is no rotation.
A ball at the bottom of a trough
A balanced see-saw
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 Ă distance
250 N Ă 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:
\(d = \frac{600~Nm}{750~N}\)
\(d = 0.8~m\)