5.6 Simple Machines: the pulley by Benjamin Crowell, Light and Matter licensed under the Creative Commons Attribution-ShareAlike license.
5.6 Simple Machines: the pulley
Even the most complex machines, such as cars or pianos, are built out of certain basic units called simple machines. The following are some of the main functions of simple machines:
- transmitting a force: The chain on a bicycle transmits a force from the crank set to the rear wheel.
- changing the direction of a force: If you push down on a seesaw, the other end goes up.
- changing the speed and precision of motion: When you make the “come here” motion, your biceps only moves a couple of centimeters where it attaches to your forearm, but your arm moves much farther and more rapidly.
- changing the amount of force: A lever or pulley can be used to increase or decrease the amount of force.
You are now prepared to understand one-dimensional simple machines, of which the pulley is the main example.
Example 10: A pulley
`=>` Farmer Bill says this pulley arrangement doubles the force of his tractor. Is he just a dumb hayseed, or does he know what he's doing?
`=>`To use Newton's first law, we need to pick an object and consider the sum of the forces on it. Since our goal is to relate the tension in the part of the cable attached to the stump to the tension in the part attached to the tractor, we should pick an object to which both those cables are attached, i.e., the pulley itself. The tension in a string or cable remains approximately constant as it passes around an idealized pulley. There are therefore two leftward forces acting on the pulley, each equal to the force exerted by the tractor. Since the acceleration of the pulley is essentially zero, the forces on it must be canceling out, so the rightward force of the pulley-stump cable on the pulley must be double the force exerted by the tractor. Yes, Farmer Bill knows what he's talking about.
5.6 Simple Machines: the pulley by Benjamin Crowell, Light and Matter licensed under the Creative Commons Attribution-ShareAlike license.