The downward acceleration of a falling body on earth is 9.81m/s2. on the moon the same quantity is 1.62m/s2. an astronaut in a space suit has a mass of 145kg. (a) what is the astronaut’s weight on earth? (b) on the moon? (c) what is the astronaut’s mass on the moon?

The astronaut's mass doesn't change.  It's the same wherever he goes,
because it doesn't depend on what else is around him.

His weight depends on what else is near him, so it changes, depending
on where he is.

         Weight  =  (mass) x (gravity)

On Earth,  Weight = (145 kg) x (9.81 m/s²)  =  1,422.5 newtons.
                                                               (about 320 pounds)

On the moon, Weight = (145 kg) x (1.62 m/s²)  =  234.9 newtons.
                                                                   (about 53 pounds)

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.