Two children sit on a see-saw. The see-saw is balanced at its centre (the pivot) and its own weight acts through the pivot.
Child X weighs 300 N and sits 1.6 m to the left of the pivot. Child Y weighs 400 N.
Calculate the distance $d$ from the pivot at which child Y must sit for the see-saw to balance. [4]
- moment = force × (perpendicular) distance (from pivot) or clockwise moments = anticlockwise moments(1)
- moment of X $= 300 \times 1.6 = 480$ (N m)(1)
- $400 \times d = 480$(1)
- (d =) 1.2 m(1)
- Answer: 1.2 m
- Examiner tip: Write the principle of moments as an equation first; the heavier child must sit closer to the pivot.