Three Laws Working Together

By the end of this lesson, you will be able to describe how all three of Newton's laws of motion work at the same time in a single event, like a car stopping suddenly.

Picture yourself riding in a pickup truck on a Texas highway at 60 miles per hour. You are moving just as fast as the truck is. When the driver slams on the brakes, your body keeps moving forward until the seat belt stops you. That forward lurch is Newton's first law, which says an object keeps doing what it is doing unless an unbalanced force acts on it. This tendency to keep moving is called inertia, the resistance of an object to any change in its motion.

Newton's second law is working in that same moment. It says the acceleration of an object depends on the force applied and the object's mass, or the amount of matter in it. A hard, fast stop means a large change in motion in a short time, so the seat belt must push on you with a large force. A gentle stop spreads that change over more time, so the force on your body is much smaller. That is why airbags and crumple zones are designed to make the stop last a few extra fractions of a second.

Newton's third law is happening too. It says that for every action force there is an equal and opposite reaction force. Your body pushes forward on the seat belt, and at the very same instant the seat belt pushes backward on your body with equal strength. Neither force comes first; they are a pair.

All three laws were true during that one braking event, which is how it works in football tackles, roller coasters, shifting tectonic plates, and rocket launches too.

Try it: When a rocket pushes hot gas downward, which law explains why the rocket moves upward?