Question

find the displacement of plank 20 kg if a man of 60kg is walking on it

Answer 1

solution

given data

mass of man (m)=60kg

mass of plank (M)=20 kg

assumption

length of plank is L meter and man starts to move one end of it to another end .

there are no friction between plank and surface.

so there are no any external force so center of mass of system will remain unchanged.

if plank move x meter distance then man will move (L-x) distance from one end to another.

com will not move because there are no any external force

$m(L-x)=Mx$

putting the value

$60(L-x)=20x$

$80x=60L$

$x=\frac{60L}{80}=0.75L$

so plank will move 0.75L meter.