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Understanding Newton's 3 Laws of Motion

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  1. First law — the law of inertia

    Inertia is the tendency of an object at rest to remain at rest, and an object in motion to keep moving in a straight line at the same speed, as long as no external force acts on it. The reason your body lurches backward when a bus suddenly starts moving, and forward when it suddenly stops, is also because your body tends to maintain its original state of motion — inertia.

  2. Second law — the law of acceleration (F=ma)

    When a force is applied to an object, it produces acceleration, and the magnitude of that acceleration is proportional to the magnitude of the force and inversely proportional to the object's mass. Expressed as a formula, this is F=ma (force = mass × acceleration), which also explains why, when the same force is applied, a light ball accelerates quickly while a heavy rock accelerates relatively slowly.

  3. Third law — the law of action and reaction

    When one object exerts a force (an action) on another object, that second object simultaneously exerts a force of equal magnitude and opposite direction (a reaction) back on the first. A rocket rising as a reaction to the force of expelling fuel downward, and a boat moving forward when an oar pushes water backward, are both examples of action and reaction.

  4. When Newton published these laws

    Isaac Newton systematically laid out these three laws of motion, along with the law of universal gravitation, in his 1687 book Mathematical Principles of Natural Philosophy (commonly known as the Principia). This book went on to form the foundation of classical mechanics and stood as the standard theory in physics for more than 200 years.

  5. A real-life example of the law of inertia

    A car's seatbelt is a device that prevents your body, due to inertia, from continuing to move forward during a sudden stop, and the magic trick where a tablecloth is yanked quickly out from under dishes while they remain in place happens because the dishes' inertia means they receive almost no significant force during that very short span of time.

  6. A real-life example of the law of acceleration

    When pushing a shopping cart with the same amount of force, an empty cart accelerates quickly, but a heavy cart loaded with goods accelerates much more slowly. This is a direct illustration of the formula F=ma, which shows that the greater the mass, the smaller the acceleration for the same applied force.

Why Newton's laws of motion matter

Newton's three laws of motion form the backbone of classical mechanics, explaining why and how objects move. From the movement of a car to the principles behind a rocket launch, these three laws alone can explain a significant portion of nearly every motion phenomenon around us.

Motion created by force eventually leads to energy

Applying force to an object to set it in motion is also, in effect, a process of transferring energy. See our guide to understanding the law of conservation of energy to learn how potential and kinetic energy convert into one another.

Frequently Asked Questions

Isn't the first law just a special case of the second law?

That's correct. In F=ma, if the force (F) is 0, the acceleration (a) is also 0, meaning the object maintains a constant velocity (including being at rest) — this is exactly what the first law (the law of inertia) states. That said, Newton specifically stated the concept that an object's state of motion is maintained even without force as a separate, standalone law.

Doesn't action and reaction mean the forces cancel each other out?

No. The action force and the reaction force are equal in magnitude and opposite in direction, but because they act on two different objects, they don't cancel out. For example, when a person pushes against a wall, the wall pushes back on the person with an equal force, but the two forces act on two different objects — the wall and the person, respectively.