Newton's Laws of Motion
Explore the three fundamental laws that govern the motion of everything in the universe, from apples to galaxies.
1st Law
"Objects keep doing what they're doing."
2nd Law
"F = ma. Force causes change in motion."
3rd Law
"Forces always come in pairs."
Key Physics Vocabulary
The Code of the Cosmos
In 1687, Sir Isaac Newton published a book that changed the world forever: the Principia. Inside, he outlined three simple rules that explain why apples fall, why moons orbit planets, and why rockets fly. These rely on the concept of Force—a push or a pull that changes motion.
Inertia is Laziness
Think of mass as a measure of "laziness." The more mass an object has, the more it resists changing its state. A freight train has huge inertia—it's hard to start moving, and once moving, it's very hard to stop.
Interaction Pairs
Forces never exist alone. They are always an interaction. If you punch a wall, your hand hurts. Why? Because while you hit the wall (Action), the wall hit your hand back (Reaction) with the exact same force!
Common Misconceptions (Exam Traps!)
Physics exams often test if you truly understand the laws or just memorized them. Watch out for these traps:
"If an object is moving, there must be a force pushing it."
False! According to the 1st Law, objects keep moving FOREVER without any force. Force is only needed to change speed or direction (acceleration).
"Action and Reaction cancel out."
False! They act on DIFFERENT objects. If a horse pulls a cart, the Action is on the Cart. The Reaction is on the Horse. They don't cancel because they target different things.
"Heavier objects fall faster."
False! Gravity pulls harder on heavy objects (More Force), but heavy objects also resist moving more (More Inertia). The ratio $F/m$ stays the same ($g$), so they fall at the same rate!
The Math of Motion
The 2nd Law ($F=ma$) connects dynamics (Force) to kinematics (Motion).
"Acceleration is proportional to Net Force and inversely proportional to Mass."
Frequently Asked Questions
What is Newton's First Law (Inertia)?
It states that an object will not change its motion unless a force acts on it. If it is still, it stays still. If it is moving, it keeps moving at the same speed and direction. This resistance to change is called Inertia.
What does F = ma mean?
This is the Second Law. It means Force equals Mass times Acceleration. It tells us that heavier objects need more force to move (High Mass), and the harder you push, the faster it speeds up (High Force).
Do Action and Reaction forces cancel each other?
NO! This is a common mistake. They are equal and opposite, BUT they act on DIFFERENT objects. If you push a wall, the wall pushes YOU. The forces don't cancel because one force is on the wall, and the other is on your hand.
Who was Isaac Newton?
Sir Isaac Newton (1643–1727) was an English physicist and mathematician. He published these laws in his famous book "Philosophiæ Naturalis Principia Mathematica" in 1687, laying the groundwork for classical mechanics.
Does the 1st Law apply in space?
Yes! In deep space, there is almost no friction or gravity. If you throw a ball, it will travel in a straight line forever at a constant speed, perfectly demonstrating the First Law.
What is the unit of Force?
The Newton (N). One Newton is defined as the force needed to accelerate 1 kg of mass at 1 meter per second squared ($1 N = 1 kg \cdot m/s^2$).
Is Gravity a force?
In classical mechanics (Newton), yes, gravity is a force pulling objects down. In Einstein's General Relativity, gravity is actually the curvature of spacetime, not a force. But for school physics, treat it as a force ($F_g = mg$).
What is "Normal Force"?
The Normal Force ($F_N$) is the support force exerted upon an object that is in contact with another stable object. For example, if a book rests on a table, the table pushes UP on the book with a normal force to balance gravity.
Why do I lurch forward when a car brakes?
Because of the 1st Law (Inertia). Your body was moving at the car's speed. When the car stops, your body wants to KEEP moving forward at that speed. The seatbelt provides the unbalanced force to stop you.
Can an object exert force on itself?
No. You cannot lift yourself up by pulling your own hair. Forces always arise from interactions between TWO different objects (3st Law).