A 20-car train standing on the siding is started in motion by the train’s engine. 2 cm slack is between each of the cars, which are 11 m long. The engine is tightly connected to the first car and moves at a constant speed of 21 cm/s. How much time is required for the pulse to travel the length of the train? Answer in units of s.

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.

What even is this. where is the question

The objects are acted upon by only one force. gravity. mass times the acceleration of gravity (-9.81meters/second squared).