Key Terms
Static equilibrium
Motionless; net force = 0, net torque = 0
Dynamic equilibrium
Constant velocity; net force = 0, net torque = 0
Convention
Counterclockwise (ccw) torques are positive; clockwise (cw) torques are negative.
SI unit
Newton-meters (N-m)
Opening a door
Push hard, push far from the hinges, push perpendicular to the door. All three factors working together.
Counterclockwise torque
Positive Clockwise torque: negative
Example
Marble on a flat horizontal surface. Move it anywhere; it stays there.
Center of gravity (cg)
Point where total weight effectively acts
Nail puller / crowbar type
Pivot at one end, load near the pivot, input force at the far end. MA greater than 1.
Wheelbarrow type
Pivot (wheel axle) at one end, load between pivot and hands. MA greater than 1.
Shovel type
Input force between pivot (far end of handle) and load. Input lever arm shorter than output lever arm.
Inclined plane
Spreads the work of lifting over a longer distance. Less force required than lifting straight up; same total work done.
Crank
Lever rotating about its pivot. MA = ratio of crank radius to axle radius.
Single fixed pulley
MA = 1. Changes direction of force only, not magnitude.
Compound pulley
MA equals the number of rope segments pulling directly on the load. Two cables pulling up = MA of approximately 2.