Pulley-a wheel with a grooved rim around which a cord passes. It acts to change the direction of a force applied to the cord and is chiefly used (typically in combination) to raise heavy weights.

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbingultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation). The common name given to the atmospheric gases used in breathing andphotosynthesis is air. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen,[1]0.93% argon, 0.039% carbon dioxide, and small amounts of other gases.

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Watch this short video of how a pulley works.