A uniform plank of length 4m and weighing 50kg is supported at two knife edges each at 1m from the ends of the plank. How close to the end of the plank can a man weighing 70kg walk before it tips.

Explanation

Given L=4m, W=50kg wt, M=70 kg wt
Taking moment about E, we get
`R_1bar(DE)+Mbar(EF)=Wbar(CE)` (moment of `R_2` about `E=0`)
`R_1(2)+70(x)=50(1)`
`2R_1=50-70x`
`R_1=(50-70x)/2=25-35x`

For `R_1=0`, gives the condition that the man can
walk a maximum distance 'x' from 'E' before the plank tips
`:. 25-35x=0`
i.e `5-7x=0`
x=0.714m from E
and `(1-0.741)=0.286m from end 'B'