Why doesn't a book on a table violate Newton's first law?

It doesn't — the book is in equilibrium. Gravity pulls it down, and the normal force from the table pushes it up with exactly equal magnitude. The net force is zero, so the book remains at rest. Law 1 applies to net force, not individual forces.

What is the difference between mass and weight?

Mass (kg) is the amount of matter in an object and is constant everywhere. Weight (N) is the gravitational force acting on that mass — W = mg. On the Moon, your mass stays the same but your weight is about 1/6 of what it is on Earth because g is smaller.

If action and reaction forces are equal and opposite, why do things accelerate?

Because action and reaction forces act on different objects. When you kick a ball, your foot exerts force on the ball (ball accelerates) and the ball exerts force back on your foot (your leg decelerates slightly). The net force on each object is calculated separately.

Do Newton's laws work everywhere?

Newton's laws are an excellent approximation for everyday speeds and scales. They break down at speeds approaching the speed of light (where special relativity takes over) and at atomic scales (where quantum mechanics applies). For most engineering and everyday physics, they are exact enough.

Newton's Laws of Motion — Interactive Simulator

Newton's three laws of motion are the foundation of classical mechanics. Published in 1687 in the Principia Mathematica, they describe how objects behave when forces act on them — and they remain the most useful framework for understanding everyday motion, from cars braking to rockets launching.

The three laws work together: Law 1 defines what "no force" looks like, Law 2 quantifies how force produces acceleration, and Law 3 ensures forces always come in pairs. These laws are the foundation for understanding everything from projectile motion to oscillating springs.

Launched at 10 m/s, it covers 60 m in 6 s with nothing to slow it; with μ = 0.30 it covers 17.2 m.

Position against time on a flat surface, with kinetic friction μ·g slowing the block until it stops. The solid curve is the same block with no friction at all.

Law

Law 1 — Inertia

An object in motion stays in motion unless acted on by an external force. Friction is an external force: without it, the object never stops.

Initial velocity10 m/s

Friction coefficient (μ)0.30

The Three Laws

Law 1 — 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.

Inertia is resistance to change in motion. A heavier object has more inertia. Without friction, a sliding block would never stop — there is no force to change its velocity.

Law 2 — F = ma

The net force on an object equals its mass times its acceleration: F = ma.

Quantity	Symbol	SI Unit
Net force	F	Newton (N)
Mass	m	kilogram (kg)
Acceleration	a	m/s²

This law is quantitative: doubling the force doubles the acceleration. Doubling the mass halves the acceleration for the same force.

Law 3 — Action and Reaction

For every action there is an equal and opposite reaction.

The forces in Law 3 act on different objects. When you push a wall, the wall pushes back on you with the same force. When a rocket expels gas downward, the gas pushes the rocket upward.

Worked Examples

Worked example

Example 1 — Braking car

A car of mass 1200 kg brakes with a net force of 6000 N. What is its deceleration?

Using F = ma: $a = \frac{F}{m} = \frac{6000}{1200} = 5 \text{ m/s}^2$

The car decelerates at 5 m/s² (i.e. loses 5 m/s of speed every second).

If it was travelling at 20 m/s, it takes: $t = \frac{v}{a} = \frac{20}{5} = 4 \text{ s}$

to come to a complete stop.

Worked example

Example 2 — Rocket thrust and reaction

A rocket engine expels exhaust gas at 3000 m/s and burns 10 kg of fuel per second. What thrust force does the rocket experience?

By Newton's 3rd Law, the thrust equals the reaction force to expelling the exhaust:

$F = \dot{m} \times v_e = 10 \times 3000 = 30{,}000 \text{ N}$

The rocket experiences 30 kN of upward thrust, while the exhaust experiences the same force downward.

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