Newton's Three Laws of Motion
In 1687, Isaac Newton published the Principia Mathematica, containing three laws that explain how everything moves.
First Law: Inertia
"An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by an external force."
This is the law of inertia. Objects resist changes to their motion.
- A hockey puck slides until friction stops it
- You lurch forward when a car brakes (your body wants to keep moving)
- Earth keeps orbiting because space has no friction
Without forces, motion doesn't change. Forces cause acceleration.
Second Law: F = ma
"Force equals mass times acceleration."
This is the most famous equation in mechanics: F = ma
- Force (F): Push or pull, measured in Newtons
- Mass (m): Amount of matter, measured in kilograms
- Acceleration (a): Rate of velocity change, measured in m/s²
- More force = more acceleration
- More mass = less acceleration (for same force)
- A truck needs more force than a bicycle to accelerate equally
This law lets engineers calculate exactly how objects respond to forces.
Third Law: Action-Reaction
"For every action, there is an equal and opposite reaction."
Forces always come in pairs.
- You push the ground backward; it pushes you forward (walking)
- A rocket pushes exhaust down; exhaust pushes rocket up
- A gun fires a bullet forward; recoil pushes the gun backward
The forces are equal in magnitude but opposite in direction, acting on different objects.
Why Newton's Laws Matter
- How cars, planes, and rockets move
- Why seatbelts save lives
- How planets orbit stars
- Engineering of bridges, buildings, machines
For 200+ years, Newton's laws were considered complete. Einstein later showed they're approximations that break down at extreme speeds or gravity — but for everyday physics, they're still perfectly accurate.
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