Question

Question Part Points Submissions Used Two kids create a makeshift seesaw by setting a 4-m long uniform plank on a saw horse. The saw horse is 0.5 m to the left of the center of mass of the plank. The child of mass m1 = 42 kg sits at the left end of the plank. The child of mass m2 = 31 kg sits 1 m to the right of the center of mass of the plank. What is the mass of the plank?

Answer 1

here

we can sum torques around the saw horse; each force generates a torque equal to force x distance from saw horse

so m1 is 1.5 m from the saw horse, and generates a counterclockwise torque of
42kg*9.8m/s/s*1.5m = 617.4Nm

the weight of the board produces a clockwise torque of mg*0.5 where m is the mass of the board

mass m2 produces a clockwise torque of 31kg*9.8m/s/s*1.5m=455.7Nm (the child is 1.5 m from the saw horse)

so we have

617.4=0.5m*9.8 +455.7 =>m=33kg