Explain in terms of Newton's first law why you should never step off a bus or train before
it has completely stopped.

Newton's 1st law demonstrates that unbalanced forces impact the motion of yourself when you get off the train. You would most likely lose your balance due to the difference in motion between the train and the ground.

Explanation:

Newton's first law states:

An object in motion stays in motion and an object at rest stays at rest. However, this changes if the object is acted upon by an unbalanced force I.E. You are on the train, meaning that you are in motion with the train and are moving at the same velocity. If you get of the train while its moving, you are still moving at the speed of thr train. Therefore, your momentum is forced to stop when you hit the other ground; causing you to lose your balance.

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.