Question

A 62kg person lies flat on uniform plank of mass 15kg.The plank, with the person lying on it,is placed on a brick at the head and a bathroom scale at the foot,both under the plank.The persons toe to head dist is 1.56m.The length of the plank is also 1.56m.

a)the reading on the bathroom scale is 30kg.Use this information to determine how far the centre of gravity of the person is from the toes.

Answer 1

Let X be the distance of the brick from the center of mass of the person.

Set the total moment about the brick equal to zero and solve for the unknown X.

30kg*1.56 m - 15kg*0.78 m - X*62kg = 0

(0.78 m is the location of the center of mass of the uniform board)

Answer 2

0.994m