Newton’s 3 Laws of Motion

Laws of Motion Explained for Kids

Newton proposed three laws of motion that explain interactions between solid objects, describing force, inertia, and reaction forces.

Newton’s three laws of motion were the first quantitative and predictive laws of mechanics. For over two hundred years, physicists were unable to produce any experiment that invalidated any one of these laws, and even today they function as close approximations for the vast majority of real-world problems, which is why engineers still use them for calculations.

First Law of Motion: Mass & Inertia

Pbjects at rest tend to stay at rest and that objects that are in motion tend to stay in motion

Newton’s first law is simply a restatement of Galileo’s observations, and in his publication, Newton gave credit to Galileo for the discovery. The first law states that objects at rest tend to stay at rest and that objects that are in motion tend to stay in motion. This law, while not validated by everyday experience, was easy to predict by adding friction to virtually every calculation of motion. This law was also the first mention in the history of the idea of frames of reference in which laws were valid, a concept that would later become the basis for the theory of relativity.

The Second Law: Force, Mass & Acceleration

Any force applied to a body produces acceleration and that the product of the acceleration and the mass of the object is the force applied

The second law states that any force applied to a body produces acceleration and that the product of the acceleration and the mass of the object is the force applied. This means that an object that is twice as heavy as another requires twice the force to accelerate it to a certain speed. The law also states that the acceleration is in the same direction as the force, which is an intuitive idea.

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Third Law: Action & Reaction

For every action there is an equal and opposite reaction

The third law states that for every action there is an equal and opposite reaction. If some object exerts a force on another object, then the object exerting the force is subject to the same force in an opposite direction. As a result of this law, we get the fact that energy is always conserved in mechanical systems. Although this means that in every exertion of force, the magnitudes of the forces are equal, the accelerations need not be so. If a massive object strikes a small object, then the small object will accelerate a lot faster than the large one.

An object’s mass is a constant inherent property that measures the amount of matter. An object’s mass is related to its inertia and gravitational force.

History and Relevance

Newton proposed the first two laws in a paper titled Principia Mathematica, and the third shortly thereafter. When combined with the universal law of gravitation and the invention of calculus, Newton’s laws were the first laws that provided a complete explanation for universal phenomena, which lasted over two hundred years until the discovery of light speed and relativistic mechanics.

Further Reading

Scheck, Florian. From Newton’s Laws to Deterministic Chaos, Springer, 2009.

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