A mass of 60kg is on the edge of a plank. The Plank is 25 m long. There is a wedge 10 m from the 60 kg. Where would you position a 40kg so the plank would not rotate? If you placed the 40 kg at the end, where would you put a 10 kg mass to keep it balanced?

Question

A mass of 60kg is on the edge of a plank.  The Plank is 25 m long.  There is a wedge 10 m from the 60 kg.  Where would you position a 40kg so the plank would not rotate?  If you placed the 40 kg at the end, where would you put a 10 kg mass to keep it balanced?

ajayb · Answer

The moments of the gravitational forces should balance each other to prevent plank's rotation(about the wedge).
a)Taking moments about the wedge position:
40g*X = 60g*10
X = 15m from the wedge

b) You can't balance with a 10Kg mass.
40g*10 = 10g*X
X= 40m - beyond the length of the plank!