In 1687, Isaac Newton published three laws that changed physics forever. Nearly 340 years later, engineers use these same laws to design cars, bridges, spacecraft, and sports equipment. Here’s what they mean in plain language.
First Law: The Law of Inertia
“An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by a net external force.”
In plain English: Things don’t change what they’re doing unless something makes them.
- A ball sitting on a table won’t move until you push it.
- A hockey puck sliding on ice keeps sliding until friction or a wall stops it.
- Astronauts in space drift in a straight line forever unless they fire thrusters.
Why seatbelts matter: When a car brakes suddenly, your body wants to keep moving forward (inertia). The seatbelt applies the external force needed to stop you with the car.
Second Law: F = ma
“The acceleration of an object equals the net force acting on it divided by its mass.”
$$F = ma$$
Where:
- F = force (Newtons)
- m = mass (kilograms)
- a = acceleration (m/s²)
What this tells us:
- A greater force → greater acceleration
- A greater mass → less acceleration for the same force
- Push a shopping cart vs. push a truck with the same force — the cart accelerates much more
Example: A 1,000 kg car accelerated at 3 m/s² requires a net force of 3,000 N.
Third Law: Action & Reaction
“For every action, there is an equal and opposite reaction.”
Forces always come in pairs. When you push against something, it pushes back with equal force in the opposite direction.
- Rockets: Exhaust gases are expelled downward → rocket is pushed upward
- Swimming: You push water backward → water pushes you forward
- Walking: Your foot pushes backward on the ground → ground pushes you forward
- Recoil: A gun pushes a bullet forward → the bullet pushes the gun backward
Note: equal and opposite forces act on different objects. They don’t cancel each other out.
Putting It All Together
These three laws work together. A rocket launch involves all three:
- (1st Law) The rocket stays on the pad until thrust exceeds gravity.
- (2nd Law)
F = madetermines the acceleration based on thrust and the rocket’s mass (which decreases as fuel burns). - (3rd Law) Exhaust propels the rocket upward.
Explore Further
Test motion concepts with our Physics Motion Simulator and look up the formulas with the Physics Formula Sheet. For advanced problems, try the Dimensional Analysis tool to keep your units straight.
Newton’s laws are the foundation of classical mechanics — master them and you’ll have the tools to understand almost everything that moves.