Vertical Circle — RodHardQuestion 30637
Question
A ball on a rigid rod (length r) in a vertical circle. At the top, the rod exerts a thrust (push) when speed v_top satisfies:
- Av_top < √(gr)Correct
- Bv_top > √(gr)
- Cv_top = √(gr)
- Dv_top = 0 only
Correct answer
v_top < √(gr)
Explanation
At top: mg − F_rod = mv²/r. F_rod = mg − mv²/r. Rod pushes (thrust, positive outward) when F_rod < 0: v² < gr. Wait: if we define F_rod as pulling inward when positive: T = mv²/r − mg. When T < 0, rod pushes → v_top < √(gr).