Forces & Motion: Newton's Three Laws
Three laws that explain all movement
Three laws that explain all movement
An object at rest stays at rest. An object in motion stays in motion at the same speed and direction. Unless a force acts on it. Click to give the box a push. Toggle friction to see the difference.
No friction: the box will slide forever. Add friction to see it slow down.
Force equals mass times acceleration. Apply the same force to different masses and watch the heavier object accelerate slower. Double the force and the acceleration doubles.
When two objects interact, they push on each other with equal and opposite forces. Click to launch the ball at the wall. Watch both objects respond.
Two balls collide. Equal force, opposite direction. Lighter ball bounces faster.
Every physical event involves all three laws simultaneously. A car crash: the car stops (Law 1: passengers keep moving due to inertia), the crumple zone absorbs force over time (Law 2: same impulse, less acceleration), and the ground pushes back on the tires (Law 3: friction is a reaction force).
A soccer ball stays still until kicked (Law 1). Harder kick = more speed (Law 2). Your foot feels the impact too (Law 3).
A rocket in space keeps moving without fuel (Law 1). More thrust = more acceleration (Law 2). Exhaust goes down, rocket goes up (Law 3).
You push the ground backward with your foot (action). The ground pushes you forward (reaction). Without friction (ice), you cannot walk.
Newton published his three laws in 1687 in the Principia Mathematica. He was 44. The same book also contained the law of universal gravitation. It is widely considered the most important scientific book ever written.
You've experienced Newton's three laws firsthand. Every moving object in the universe, from atoms to galaxies, follows these same three rules. They are the foundation of all mechanics.
An object at rest stays at rest. An object in motion stays in motion. Unless a force acts on it. This is why you lurch forward when a car brakes.
Force equals mass times acceleration. More force = more acceleration. More mass = less acceleration for the same force. This is why trucks accelerate slower than sports cars.
Every force has an equal and opposite reaction. Push a wall and the wall pushes back. A rocket pushes exhaust down, the exhaust pushes the rocket up.
Without friction, everything would slide forever. Friction opposes motion, converting kinetic energy to heat. Ice has low friction, sandpaper has high.
Put your new knowledge into practice!