You want to prove the law of conservation of energy to a friend. for your demonstration you show that you can use a block and pulley machine to lift 100n with only 20n of input force. what would you say to your friend to explain how is this possible?

Pulleys help reduce the force needed by changing its direction; however, the direction of the movement is also changed. Therefore, the pulley's rope must be pulled further than the actual displacement of the block. Say if the block is raised by 1 meters, then the rope would have to be pulled 5 meters, thus conserving energy.

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.