Texas Science Teacher Resource Hub

A rocket does not need the ground to push against. It works the same way in outer space, where there is nothing to push off of. The engine fires hot gas downward at high speed, and by Newton's Third Law, the gas pushes the rocket upward with an equal and opposite force. The rocket pushes the gas, and the gas pushes the rocket. That's the real action-reaction pair.

This trips up a lot of students. By Newton's Third Law, the forces in a collision are equal in size. If a truck hits a bike, the truck pushes on the bike with the same size force that the bike pushes on the truck. The reason the bike gets wrecked and the truck doesn't isn't the force. It's the mass. The same force produces a huge acceleration on the small mass (the bike) and a tiny acceleration on the large mass (the truck). That's Newton's Second Law working on top of the Third Law.

A ball doesn't carry a supply of force inside it. Once it leaves the player's hand, foot, or bat, the only forces acting on it are gravity, air resistance, and whatever it hits. The ball slows down because friction and air resistance are pushing back on it. Newton's First Law says the ball would keep moving forever if nothing slowed it down, but in the real world, something almost always does.

Seatbelts are a Newton's First Law story. When a car stops suddenly, the car gets pushed backward by the crash, but your body keeps moving forward at the same speed because of inertia. A seatbelt applies a backward force on your body, slowing you down with the car so you don't keep moving into the windshield. The belt isn't holding you to the car. It's supplying the force that overcomes your inertia.