"how can pulleys be useful in reducing the effort to lift"?

Answer 1

A pulley is a chain, belt or rope wrapped around a wheel. The mechanical advantage of a pulley system is approximately equal to the amount of supporting ropes or strands.

Therefor, if you had a mass of 60kg and wanted to lift it using two supporting ropes, you would have mechanical advantage (MA) of 2. The mass will feel like one half of what it really is. When lifted with the help of the pulley system your 60kg would only feel like 30kg. Thus the effort force equals 30kg.

Here are a couple Of websites that explain this more.

http://www.technologystudent.com/gears1/pulley7.htm

http://www.swe.org/iac/lp/pulley_03.html

Answer 2

If you suspend a 90 pound weight with a single rope, the "pull" on the rope is 90 pounds. If you use two ropes they share the load at 45 pounds each. Use three ropes and each supports 30 pounds of the load. A pulley system allows you to use a single rope to exert multiple "pulls" on the load and to move the load too. The tension (pull) on the rope is the same along its entire length. Therefore, divide the weight of the load by the number of ropes actually supporting the load and that is the force you must exert on the loose end of the rope to life the weight. Nothing is for free. If you must pull with a force equal to haff the load, you must pull twice as much rope (twice as far) than if you lifted the weight without a pulley. Of course, pulleys can be used to pull loads horizontally. Using a more complicated block and tackle works the same way.

Answer 3

google "mechanical advantage" and "velocity ratio" in physics.