A block and tackle pulley system uses two or more pulleys together to lift heavy objects more easily. It works by distributing the weight of the object equally between all pulleys, thereby decreasing the amount of force required to lift the object
Using a single pulley to lift an object does not change the amount of force required to lift it. For example, to lift a 400 pound object vertically up, 400 pounds of force need to be applied to the rope. The only advantage of using a single pulley is that the direction of the force is opposite to the direction in which the weight is lifted.
The force required to lift an object can be reduced by introducing more pulleys. If two pulleys are used, instead of just one, the weight of the load is distributed between the two pulleys, so only half the force is required to lift the object. For example, to lift a 400 pound object with a two-pulley block and tackle system, only 200 pounds of force is required. However, the amount of work done in lifting the box needs to be conserved.
The trade off for decreasing the force required to lift the object is the distance by which the rope has to be pulled. If the 400 pound object needs to be lifted 50 feet in the air, then 100 feet of rope has to be pulled with a 200 pound force. Either way, 20000 joules of work has to be done, but using a block and tackle pulley system decreases the amount of force required to do the same work, thereby making it easier to achieve.