An astronaut lands on a new planet. the new planet’s mass is half that of the
earth and the new planet’s radius is half that of the earth. if he weighs 500 n on
earth, what will the force of gravity be on him on the new planet?

Ah hah !  Very nice problem.

-- Since the mass is 1/2 of the Earth's mass, the gravitational forces between him
and the new planet would be 1/2 of what they are on Earth.

-- Since the distance between him and the center of the new planet is 1/2 of the
distance between him and the Earth's center, the gravitational forces between him
and the new planet would be (2)² = 4 times what they are on Earth.

-- These conditions combine to make his weight  (1/2) x (4) = 2 times his earth weight.

-- He weighs 1,000 N (about 224.8 pounds) on the new planet.

(Maybe that's why they selected an astronaut for this mission who only weighs
about 112.5 pounds on Earth.)

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.