When teaching the six simple machines to elementary school students, the pulley is often the most difficult to explain due to its complexity. One way to surmount this obstacle is with the help of a simple model pulley system that you can build from readily available materials to demonstrate a variety of modes of pulley operation. The lessons derived from this simple pulley system (only one string/rope running through it) can be generalised to all pulley systems and also help with students' general understanding of the concept of mechanical advantage.
- Skill level:
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Things you need
- 2 or more simple pulleys (available online or at hardware stores)
- 2 stable chairs
- Several heavy books
- 6-inch segment of string
- 3-foot segment of string
- Several weights (0.454kg., 0.907kg.)
Place the two chairs about two feet apart in a clear area with their backs toward each other. Place an approximately equal weight of books on the seat of each chair.
Place the meter-stick on top of the backs of the two chairs so that it goes from one to the other.
Loop the 6-inch segment of string through the top of one of the pulleys (not around the wheel) and loop it around the meter-stick before tying the ends together so that the pulley is suspended from the meter-stick.
Tie the end of the 3-foot segment of string to the weight and run the other end around the pulley. This is the simplest type of pulley system, wherein the force required to lift the weight is simply redirected downward but its magnitude is unchanged.
Leave the single pulley suspended from the meter-stick alone, but remove the 3-foot string and untie it from the weight.
Attach the weight to another pulley with an S-hook.
Tie one end of the string to the bottom of the pulley suspended from the meter-stick. Run the other end first through the pulley from which the weight is suspended and then around the upper pulley. The pulley with the weight attached should hang below the one tied to the meter-stick, and the weight is raised by pulling down on the free end of the string.
Tips and warnings
- To illustrate a one pulley system in which the force required to lift the weight is reduced (by half), disassemble the single pulley and tie one end of the 3-foot string to the meter-stick. Attach a weight to the pulley with an S-hook, and run the free end of the string through the pulley so that the pulley hangs suspended by the string (one end of which is held by you, the other by the meter-stick). Pulling upward on the loose end of the string raises the weight, but with a required force of only half the weight (half of the force suspending the weight is supplied by the end of the string tied to the meter-stick)
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