1) First make three groups of nine coins. balace two of them.
2) Again make three groups of three.Balance two of them.
3) Now there only three coins left to check out and that's easy. :biggrin:
A tunnel is dug through the centre of the earth.A particle is projected into it with a velocity =(gR)^1/2.Find the time required to to cover the length of the tunnel.
The force of attraction is F=GMmx/R^3 where x=distance from centre
Then the time period is...