Question

At the local playground, a 21-kg child sits on the right end of a horizontal teeter-totter, 1.8 m from the pivot point. On the left side of the pivot an adult pushes straight down on the teeter-totter with a force of 151 N.

Part A

In which direction does the teeter-totter rotate if the adult applies the force at a distance of 3.0 m from the pivot? (clockwise/ counterclockwise)

Part B

In which direction does the teeter-totter rotate if the adult applies the force at a distance of 2.5 m from the pivot?
(clockwise/counterclockwise)

Part C

In which direction does the teeter-totter rotate if the adult applies the force at a distance of 2.0 m from the pivot?
(clockwise/counterclockwise)

Please show all steps, Thank you!

Answer 1

by convention a negative torque leads to clockwise rotation and a positive torque leads to counterclockwise rotation.

here weight of the child =21kgx9.8m/s² = 205.8N

the torque exerted by the child T_c = - (1.8)(205.8) = -370.44N-m ,negative sign is inserted because this torque is clockwise and is therefore negative by convention.

torque exerted by adult T_a = 3(151) = 453N , counterclockwise torque.

net torque T_net = -370.44+453 =82.56N , which is positive means counterclockwise rotation.

b) T_a = 2.5x151 = 377.5N-m

T_net = -370.44+377.5 = 7.06N-m , positive ,counterclockwise rotation.

c)T_a = 2x151 = 302N-m

T_net = -370.44+302 = -68.44N-m, negative,clockwise rotation.