Why things move the way they do
Isaac Newton’s three laws of motion, published in 1687, are still the foundation for almost everything we know about how objects move — from a dropped pencil to a rocket launch.
First Law: Objects like to keep doing what they’re already doing
An object at rest stays at rest, and an object in motion stays in motion at a constant speed and direction, unless a force acts on it. This is also called the law of inertia. A hockey puck sliding on frictionless ice would never stop — it’s friction and air resistance, not some “natural tendency to slow down,” that brings it to a halt.
Second Law: Force equals mass times acceleration
F = ma
The harder you push something (more force), the faster it speeds up (more acceleration) — but heavier objects (more mass) are harder to accelerate. This one equation explains why it’s easier to push an empty shopping cart than a full one.
Third Law: Every action has an equal and opposite reaction
When you push on something, it pushes back on you just as hard. This is why rockets work: the engine pushes exhaust gas backward, and the gas pushes the rocket forward with equal force.
Try this
Sit on a rolling chair and throw a heavy backpack away from you. You’ll roll backward — that’s Newton’s third law in action.
Check your understanding
3 practice questions in the style of AP Physics. Pick an answer to see if you got it, plus a short explanation. Where numbers are needed, use g = 9.81 m/s². These questions were written for this site and are not College Board questions.
1. A 5 kg object is pushed by a net force of 20 N. What is its acceleration?
2. A heavy truck collides head-on with a small car. How does the force of the truck on the car compare with the force of the car on the truck?
3. Which statement correctly describes an object moving at a constant velocity?
Want the real thing? The College Board posts freepast AP Physics 1 exam questions ↗.