In the Classroom: Mechanical Advantage

Heave! Students in Renee Blacken’s physics course spent their class this morning learning about mechanical advantage in order to move a sports utility vehicle up the main driveway.

A large maple tree sits in the main circle in front of McLane, the main academic building. In the last few days two sap buckets were hung on it’s trunk by the Farm and Forest Program for their annual sap boil. In a few weeks, its branches will hold faculty children (and White Mountain School students, too) as they climb up into the secret jungle world just feet from the primary school walkway.
But today, our favorite sugar maple tree was also host to a long rope and a few different pulleys. It was time to learn mechanical advantage. Working against the downhill slope of the driveway, the class would learn how they could simply and easily pull a Toyota Sequoia up the hill with their hands. Ted Teegarden, Director of the Outdoor Education Department and Brady Callahan, an intern in the OED, were on hand to help the class and explain other real-world applications for understanding and applying mechanical advantage.

Renee says, “It’s important to actually experience the things we talk about in the classroom. I enjoy watching students work together as they process what they’re learning and help each other out. I like to do the car-pull, because it’s a great way to get students out of the classroom and have them gain understanding by actually feeling the reduction in effort force and observing the decreased distance a load moves as the mechanical advantage of the system increases. When the students actually have to set-up the system, they are able to clarify for themselves how mechanical advantage systems are constructed and how they function. They also get a sense of some of the real-world considerations and applications of these types of systems.”

The class talked about the friction that would be added each time the rope was redirected. They talked about adding a pulley and changing the direction they were pulling from. They even discussed how they could use gravity to their advantage.
The class also experimented with different numbers of mechanical advantage, a means of multiplying effort force, as they added pulleys to their system, tested the angle from which they were pulling, slipped a couple times on the melting snow and coped with a rope that started to stretch as it got wet. They learned that as their mechanical advantage increased, they could use less force to move the load, but the load moved a proportionately smaller distance. For example, when pulling the SUV with mechanical advantage of 2, it required less force (ideally half as much) to move as simply trying to pull the truck directly. However, when the students pulled on the rope and walked a given distance, the SUV would only move (at best) half as far. Experiencing this first-hand reinforced the ideas that mechanical advantage multiplies effort force, but that mechanical work, or force times distance, cannot be multiplied even by an ideal machine. Energy must be conserved!
After having seen and learned the systems themselves, the class went back into the Steele Science Center to do some follow up work in their lab. As Renee began to look over the previous night’s homework assignments, she challenged the class to build their own new mechanical advantage system in order to easily lift her water bottle from the floor to the ceiling. As students started thinking about how they might tackle her challenge, she reminded them to talk through their thinking and communicate their ideas to one another. The group devised a “block and tackle system” of two pulleys with a rope threaded between them to lift the water bottle to the ceiling of the classroom before the period ended.  

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