Pulleys are an excellent example of how our work efforts can be increased. We’ve learned that the 6 common simple machines are the pulley, lever, wheel and axle, screw, inclined plane and a wedge. Through the generous help of a Skyline PTSA grant (HUGE thanks to Skyline’s PTSA!), we had Christian, a local engineer in Salt Lake City, propose the problem of how to get our teacher up to the rim of a basketball hoop and dunk a ball, using the 7th graders’ minds and muscles. We considered time, money, science and safety to make our decisions and to create a plan. We learned that each pulley increased our effort, or provided more mechanical advantage. With one pulley, the force we used to lift our teacher wasn’t increased, but it changed the direction of our force. To lift our teacher, we’d need to lift up, but now we could pull down. Because our teacher weighs 215 lbs., we couldn’t lift him. When we set up a two pulley system, our teacher’s weight was divided in half (divided by 2) but only Christian could lift him because we don’t weigh more than 107.5 lbs. so even if we jumped into the air to lift our teacher, we couldn’t produce enough force. With 3 fixed pulleys we started to have success and our teacher’s weight was now divided by 3 and we only needed to pull with 71.7 lbs. pounds of force. Success!
We had fun using the concept and dividing our weight by 2 and 3, to see how much force our classmates needed to use to lift us!
The 8th grade Physical Science class added to the concepts of learning by figuring out the force distance tradeoff. Simple machines don’t do work, they just make work easier, or more efficient, for us. To lift a load a certain distance, that distance must be multiplied by the number of pulleys and moved by whatever force is lifting the load. For example, if we lift our 215 lb. teacher 4.25 feet to the rim of an official NBA basketball hoop (keeping in mind that he’s 6 feet tall and where the harness connects to the pulleys is 9 inches from his head) using a 3 pulley system;
distance to move the load x # of pulleys = distance whatever is applying the force must move
4.25 ft. x 3 = 12.75 ft. the puller must move in order to lift our teacher 4.25 ft.
Jarrod weighs 250 lbs. and would like to be lifted by a pulley system into the back of a pickup truck. If the top of the tailgate is 48 inches high;
1. How much force is needed to lift Jarrod with a 3 pulley system?
2. If a 4 pulley system is set up, how far would the person pulling the rope have to move?
A cubic foot of coal weighs 84 lbs. If this load of coal needs to be lifted into a truck at a height of 5 feet,
3. How much force is needed to lift the coal with a 3 pulley system?
4. If a 4 pulley system is set up, how far would the person pulling the rope have to move?