© 2004 World Hovercraft Organization


       Torque is rotational mechanical force. Screws, bolts, engines, electric motors, flywheels, fans, and propellers all turn or rotate. The amount of force applied at a point on a rotating object, multiplied by the distance from the point of rotation determines the amount of torque. Torque forces are classified as causing either clockwise (cw) or counterclockwise (ccw) , sometimes called anticlockwise, rotation, determined by observing the rotation of the object as it faces you.

Torque = Force · Length of lever arm

       “Length of lever arm” is a way of saying the distance between where the force is applied and the point of rotation. In order to tighten or loosen a bolt, you have to exert a torque on it. Using a wrench, the torque applied is equal to the amount of force used to pull the wrench multiplied by the distance between your hand and the center of the bolt. If you’ve ever used a wrench you probably know that it’s easier to tighten a bolt when holding the wrench at its end and harder to pull when you move your hand closer to the bolt. This is because the further your hand is from the bolt, the longer the lever arm, and the larger the torque.

       You and a friend are on a see-saw trying to balance on either side. Your friend weighs 200 lbs [889.6 N] and is sitting 3 ft [0.914 m] from the see-saw’s point of rotation. You weigh 150 lbs [667.2 N]. How far from the point of rotation do you have to sit in order to balance the two torques on the see-saw?

      In order for the see-saw to be balanced, the torque exerted by you has to be equal to the torque exerted by your friend. We must first calculate the torque produced by your friend, then use that to calculate the distance you have to sit from the center of the see-saw.

Imperial Units
Friend’s Torque = Length of lever arm · Force
Friend’s Torque = (3 ft) · (200 lbs)
Friend’s torque = 600 ft lbs
SI Units
Friend’s Torque = Force · Length of lever arm
Friend’s Torque = (889.6 N) · (0.914 m)
Friend’s Torque = 813.1 N m

We know that the torque you exert must be equal to 600 ft lbs [813.1 N m] and we know your weight, so we can plug those values into the torque formula and calculate the length of the lever arm.

Torque = Distance to center · Weight
Distance to center = Torque ÷ Weight
Distance to center = (600 ft lbs) ÷ (150 lbs)

Distance to center = 4 ft
Torque = Weight · Distance to center
Distance to center = Torque ÷ Weight
Distance to center =(813.1 N m) ÷ (667.2 N)

Distance to center = 1.22 m

Since you weigh less than your friend, you must sit further away from the see-saw’s point of rotation in order to produce the same torque as your friend. This will keep the see-saw balanced.

Continue to Experiment 9