Ah yaar, that minimum speed at the top is √(gR), when tension just becomes zero. But this √(5gR) you're asking about? That comes from the *minimum speed at the *bottom* of the vertical circle needed to just complete the full loop, using energy conservation from bottom to top. Bas, they combine the top condition with energy equations. Right now, I know this only because my tuition wale bhaiya made me derive it five times yesterday, each time over a fresh cup of cutting chai he made me pay for.
#middle-distance#5000m
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