How can the motion of an object that is already moving change?
an object that is already moving can . .

answer:

an opposing force

explanation:

answer: imagine you were throwing a ball to your friend. the ball wouldn't just keep on going wherever you threw it, no. gravity would pull it down and stop it from flying so you friend can catch it. that's one way the motion   of an object can change if it is already moving. another way is if you were rolling a bowling ball and your friend blocked it from going anywhere.