A uniform diving board is held by two fixed rods at points P and Q. A person stands at end R of the diving board, as shown.
The forces exerted by the rods on the board are vertical. The board remains in equilibrium as the person slowly moves towards point Q from end R.
Which row describes the changes to the magnitudes of the forces exerted by the rods on the board?
ANSWER: A
The Physics Behind
- With reference to the conditions of equilibrium,
- the upward force at Q cancels the sum of downward forces [i.e. force at P and weight] , and
- the clockwise moment due to weight cancels the counterclockwise moment at P.
- When the person moves towards point Q [pivot],
- the moment of weight decreases because the perpendicular distance decreases as the person moves closer to pivot,
- the force at P decreases as it needs to produce a turning effect that cancels the decreasing moment of weight, and
- since the force at P decreases, then the force at Q which is equal in magnitude to the sum of 2 downward forces also decreases.
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