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Engineering science fair project:
Build a variation of a teeter-totter for one person




Science Fair Project Information
Title: Build a variation of a teeter-totter (seesaw) for one person
Subject: Engineering
Subcategory: Mechanics
Grade level: Elementary School - Grades 4-6
Academic Level: Ordinary
Project Type: Engineering
Cost: Low
Awards: 2nd place, Canada Wide Virtual Science Fair ($20)
Affiliation: Canada Wide Virtual Science Fair (VSF)
Year: 2014
Materials: Wood plank, 6 pulleys, hinge, string, Barbie doll
Description: The goal of this project is to create a compound machine using pulleys and levers that would be able to lift a barbie up and down by pulling a string by the player. A common problem on the playground is that you sometimes can't find another person to go on the teeter totter with you. We thought that if created a structure that allowed children to "teeter-totter" on their own, it could revolutionize solo play!
Link: http://www.virtualsciencefair.org/2014/park14a
Short Background

Pulley Hoisting Systems

A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.

The pulleys are assembled together to form blocks and then blocks are paired so that one is fixed and one moves with the load. The rope is threaded, or reeved, through the pulleys to provide mechanical advantage that amplifies that force applied to the rope.

A block is a set of pulleys or "sheaves" mounted on a single axle. The assembly of two blocks with a rope threaded through the pulleys is called tackle. A block and tackle system amplifies the tension force in the rope to lift heavy loads. They are common on boats and sailing ships, where tasks are often performed manually.

A differential pulley, also called sometimes "chain hoist", is used to manually lift very heavy objects like car engines. It is operated by pulling upon the slack section of a continuous chain that wraps around pulleys. The relative size of two connected pulleys determines the maximum weight that can be lifted by hand.

A differential pulley can lift very large masses a short distance. It consists of two fixed pulleys of unequal radii that are attached to each other and rotate together, a single pulley bearing the load, and an endless rope looped around the pulleys. To avoid slippage, the rope is usually replaced by a chain, and the connected pulleys by sprockets (a wheel with teeth or cogs that mesh with a chain, track or other perforated or indented material).

The two sections of chain carrying the single pulley exert opposing and unequal torques on the connected pulleys, such that only the difference of these torques has to be compensated manually by pulling the loose part of the chain. This leads to a mechanical advantage: the force needed to lift a load is only a fraction of the load's weight. At the same time, the distance the load is lifted is smaller than the length of chain pulled by the same factor. This factor (the mechanical advantage MA) depends on the relative difference of the radii r and R of the connected pulleys.

The difference in radii can be made very small, making the mechanical advantage of this pulley system very large. In the extreme case of zero difference in radii, MA becomes infinite, thus no force (besides friction) is needed to move the chain, but moving the chain will no longer lift the load.

See also:
http://en.wikipedia.org/wiki/Block_and_tackle
http://en.wikipedia.org/wiki/Differential_pulley

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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