Question

A 9-m long plank of weight 80 N is supported by two vertical posts at the two ends. If a 25-N person stands at 6 m from the right end of the plank.

a) The force on the left post is _____ N.

b) The force on the right post is ____ N.

Suggestion: Draw a sketch to determine forces and torques

Answer 1

25 N 6 m 4.5m 80 N 9 m

now , let us consider the torque about right post.

torque is given by F * perpendicular distance.

let the force on left plank be F_L

so , net torque = 0

so . F_L * 9 + 80* 4.5 + 25 * 6 = 0

=> F_L * 9 = -510

F_L = -56.66 N ( negative sign shows upward force and it is a normal force)

b) now lets take torque about left post

so . F_R * 9 + 80* 4.5 + 25 *(9- 6 )= 0

=> F_R * 9 = -435

=> F_R =-48.33 N