How does a pulley makes work easy?

Answer 1

If you just tie a rope around something and try to lift it, every inch that your hands move causes the load to move an inch as well, so you're lifting all of the weight. Throw that rope over a pulley and the situation doesn't change. You may be pulling down instead of up, but an inch of hand movement is still an inch of load movement, so a pulley by itself does NOT make work easier (unless its easier to pull down than up).

What you are thinking of is called a block and tackle. It is a little hard to describe without pictures, so I suggest going to Wikipedia. It makes work easier by allowing you to use less force over a longer period of time. Every extra loop of rope between the block and the tackle reduces the distance that the load is lifted when you pull on the end of the rope. With just one extra loop, when you pull the rope three inches, the load is only moved one inch. You might not think this is helpful because to lift 300 pounds 10 feet in the air, you'd have to pull the rope 30 feet, and it would take 3 times as long. The trick is, the total amount of work you're doing (300 pounds times 10 feet lifted = 3,000 foot-pounds) is always the same, so if the rope moves 30 feet, you only have to pull it with 100 pounds of force (100 pounds times 30 feet pulled = 3,000 foot-pounds). Use a block and tackle with four extra loops and you'll have to move the rope 90 feet to lift the load 10 feet, but it only takes around 34 pounds of force to do it.