According to newton's 1st law of motion, what is required to make an object slow down?

Newton’s first law state that an object in motion stays in motion unless acted upon force so the answer would be RESISTANCE

Hello,

 that there must be a cause—which is a net external force—for there to be any change in velocity, either a change in magnitude or direction.

So your answer is Force.

Force is required

Hope this helps

C: plants and animals

High-energy and unstable.