Newton's Laws of Motion, Explained Step by Step
Updated 2026-07-12
Newton's three laws describe how forces change motion. First: an object keeps its velocity unless a net force acts on it. Second: net force equals mass times acceleration (F = ma). Third: every force has an equal and opposite reaction force on the other object.
Newton's laws are the foundation of mechanics, but most students memorize the three sentences without seeing how they fit together. The trick is that they answer three different questions: what happens with no force, what happens with a force, and where forces come from.
First law: inertia
An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless a net force acts on it. The key word is net. Forces can be present and cancel, and the motion still does not change.
This is why a hockey puck slides for a long time on ice: friction is small, so the net force is close to zero, so the velocity barely changes.
Second law: F = ma
When there is a net force, it produces acceleration in the same direction, and the amount is force divided by mass. Written as an equation, net force F equals mass m times acceleration a.
Read it as a cause and effect: force is the cause, acceleration is the effect, and mass is how much the object resists. The same push gives a shopping cart a big acceleration and a truck a tiny one.
Third law: action and reaction
If object A pushes on object B, then B pushes back on A with equal size and opposite direction. The two forces act on different objects, which is why they do not cancel.
A rocket works this way: it pushes gas down, the gas pushes the rocket up.
Worked example
A 2 kg box is pushed with a net force of 10 N. What is its acceleration?
- Start from the second law: F = ma.
- Solve for acceleration: a = F / m.
- Substitute the numbers: a = 10 N / 2 kg.
Answer: a = 5 m/s^2, in the direction of the push.
The common mistake
Wrong: Rearranging F = ma the wrong way, writing a = F times m.
Fix: Acceleration is force divided by mass, a = F / m. Heavier objects accelerate less for the same force, so mass must be on the bottom.
Wrong: Thinking the action and reaction forces cancel, so nothing moves.
Fix: They act on different objects, so they never cancel on one object. The box and the hand feel opposite forces, not the same object.
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Frequently asked questions
What are Newton's three laws of motion in simple terms?
First, things keep doing what they are doing unless a force changes it. Second, a net force makes an object accelerate, by an amount equal to force divided by mass. Third, every push produces an equal and opposite push back.
What is the formula for Newton's second law?
Net force equals mass times acceleration, written F = ma. To find acceleration, rearrange to a = F / m.