Newton’s 3 Laws of Motion Explained

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February 5, 2021
4:07 am

The universe is a constant flurry of activity. At any given instant, invisible electrons buzz within atoms, vehicles start and stop amid crowded intersections, planes jet across the skies, and the most massive of celestial bodies whirl about each other. All things, seen and unseen, alive and inanimate, are in a state of constant motion.

What are Newton’s 3 Laws of Motion?

Newton’s three laws of motion describe how forces govern the movement of objects:

Newton’s First Law (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion in a straight line, unless acted on by a net external force.
Newton’s Second Law (F = ma): The net force acting on an object equals its mass multiplied by its acceleration.
Newton’s Third Law (Action-Reaction): For every action force, there is an equal and opposite reaction force acting on a different object.

These laws are a core part of the O-Level Physics syllabus in Singapore, and understanding them well lays the groundwork for dynamics, force diagrams, and beyond. Students preparing for their O-Level examinations will encounter all three laws across multiple question types.

However, precisely because movement is an ever-present part of our existence, we often take for granted how these motions arise and the laws that govern them.

Forces, defined as a push or a pull, are what ultimately determine how an object will move. The study of Newtonian Mechanics in physics provides a detailed mathematical framework of how forces affect an object. Fundamental to the entirety of Newtonian Mechanics are Newton’s 3 laws of motion.

Though forces are often invisible, they shape every object’s path through space and time. Newton’s three laws explain the mechanics behind them.

A Brief History of Newton’s Laws

Newton’s 3 Laws of Motion have their roots in the work of Galileo Galilei and his basic principle of relativity. In Galileo’s 1632 book, “Dialogue concerning the Two Chief World Systems”, he describes that a person on a ship moving at a fixed speed in a single direction in smooth water would have no sense of movement by himself. Cannonballs dropped from a mast would thus land at its foot and not land backwards nearer the stern as was common intuition.

This basic idea that a system moving at a constant speed in a straight line is indistinguishable from one that is at rest provided the basis for Newton to formulate his 3 laws of motion.

Newton presented his 3 Laws of Motion in his ground-breaking work, ‘Principia Mathematica Philosophiae Naturalis’, in 1686. He was motivated in large part by an astronomical puzzle: why do the planets orbit the Sun in ellipses rather than perfect circles? Together with Kepler’s Laws, Newton’s three laws of motion provided the answer. It has since become an important piece of scientific literature that forms the basis of much of classical physics.

Let us now take a closer look at each of the three laws.

Newton’s 1st Law: The Law of Inertia

‘An object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force.’

Newton’s First Law asserts that all objects tend to resist changes in their present state of motion. Simply put, without the interference of an external force, an object at rest will stay at rest, and an object in motion will continue to move.

This inherent laziness is also referred to as “inertia”. The greater an object’s mass, the greater its inertia thus requiring a larger force to deviate from its current state of motion.

Though it may be unintuitive at times, here are a few real-life examples to help you wrap your head around it.

Generally, inertia can be broken down into 3 categories:

Inertia of Rest

An object at rest will remain at rest unless acted on by an external force.

The famous parlour trick where a tablecloth is pulled quickly from under a table full of dishes perfectly demonstrates the inertia of rest.

In this case, the dishes wanting to remain at rest will only move when an external force is applied, and in this case, this force is provided by the friction of the tablecloth. Provided that the cloth is pulled out from underneath the table quickly enough, the friction is unable to provide a force large enough to pull the dishes along, leaving the crockery undamaged on the table sans tablecloth.

Inertia of Motion

A moving object will continue to move unless acted on by an external force.

Believe it or not, we experience the effects of this type of inertia daily. Most of us, being no strangers to public transport, would understand the daily struggle of balancing ourselves while standing on a crowded bus or train carriage.

It is second nature to us to brace ourselves when the vehicle comes to a stop. As our bodies are moving at the same speed as the bus or the train, upon an abrupt deceleration, our bodies move forward faster than the vehicle itself resulting in us being propelled forward if we are not properly secured to our seats. The reverse is true when accelerating as well, with our bodies initially at rest, the vehicle moves forward from under our feet, resulting in our bodies being left behind.

Be it during acceleration or deceleration, just remember to hold on tight whenever you are travelling on the bus or MRT!

Inertia of Direction

An object moving in one direction will continue to move in the same direction unless acted on by an external force.

Referencing the example from above, the inertia of direction is directly observable when a bus turns a corner. If you are a passenger in that bus, inertia causes your body to resist the change in direction. Your body thus travels in a straight line while the rest of the bus moves along the curved path. To the other (secured) passengers, you will appear to be flung out towards the walls as your path deviates from that of the bus.

There are plenty more phenomena involving inertia in the world around us. Take your time to observe your surroundings and you will be certain to catch inertia in action.

Why Newton’s First Law Matters

While it is easy to take the law of inertia for granted, a deep understanding of inertia and its effects has led to various discoveries and applications. Vehicular safety in the form of seat belts, airbags, and car seats is one such example. Beyond that, the law of inertia also forms the underlying mechanism behind centrifuges, which are used to separate mixtures in chemistry and biology. For O-Level and JC Physics students, understanding inertia is particularly useful for force diagram questions, where identifying which forces are acting on an object — and which are absent — is the first step to solving the problem correctly.

Newton’s 2nd Law of Motion: F = ma

‘Newton’s Second Law states that the net force (F) acting on an object equals its mass (m) multiplied by its acceleration (a).’: F=ma’

In its simplest form, Newton’s Second Law of motion states that the net force acting on an object is equal to the product of the object’s mass and acceleration.

The second law provides a clear quantitative relationship between the forces applied, the inertia and the object’s change in motion.

Suppose your car broke down in the middle of the road and in order to prevent the obstruction of traffic, you proceed to push it out of the way after enlisting the help of several other helpful motorists. After the force applied has overcome the frictional and resistive forces, the remaining force would be used to increase the speed of the car from rest; in other words, accelerate the car. Newton’s Second Law says that the car will speed up at a rate equal to the amount of net force divided by its mass.

Force and Acceleration

As the acceleration of the car is proportional to the net force applied, suppose you enlisted the help of more people. With your combined strength, this would produce a greater net force, resulting in a corresponding increase in the acceleration.

Force and Mass

If, however, you replaced the car with a motorcycle and applied the same net force, the motorcycle, being much less massive, would produce a much larger acceleration. On the other hand, a broken-down bus would accelerate at a much slower rate due to the larger mass.

Worked example: A net force of 500 N acts on an object with a mass of 50 kg. Using F = ma, the acceleration is calculated as: a = F ÷ m = 500 ÷ 50 = 10 m/s². This is the kind of direct substitution question that appears regularly in O-Level Physics papers — know the formula, identify the known values, and solve.

Why Newton’s Second Law Matters

Newton’s second law is a straightforward presentation of the relationship between forces and the motion of objects. In the scientific world, it is used to calculate all kinds of scenarios involving forces and motion. With that knowledge, we can predict certain scenarios with precision, such as the trajectory of an aeroplane and even the acceleration of a rocket. Evidently, it is an indispensable knowledge that has enabled mankind to create inventions that are literally out of this world.

Newton’s 3rd Law of Motion: Action and Reaction

‘For every action force exerted by one object on another, there is an equal and opposite reaction force exerted by the second object on the first.’

A common misconception among students is that action and reaction forces cancel each other out. They do not. Action and reaction forces act on different objects, which is why they cannot be added together. If object A pushes on object B (action), object B pushes back on object A (reaction). However, these forces are each affecting a separate object. They are equal in magnitude and opposite in direction, but they never cancel.

Newton’s third law postulates that for every action, there is an equal and opposite reaction.

Newton was a keen observer of his surroundings. Through his observations he realised the existence of action-reaction force pairs.

He theorised that for every interaction between two objects, there exists a pair of forces: action and reaction acting on both interacting objects. Furthermore, this pair of forces always oppose each other and are equal in magnitude.

In nature, Newton’s third law can be observed in the movement of animals. For example, a bird taking flight flaps its wings; this action pushes the air particles underneath its wings down. A reaction takes place simultaneously in which the air particles push the bird up, resulting in a lifting force.

Another subtle example of Newton’s third law happens even when we are walking. When we walk we are applying a force on the ground, simultaneously, the ground applies an equal force on our foot which pushes us forward.

Why Newton’s Third Law Matters

As a result of this discovery, scientists learned how to work with naturally occurring forces to cross frontiers, including building rockets to the moon and beyond. Newton’s third law was used as the foundation for building rocket engines that could create enough thrust for take-off.

Within the combustion chamber of a rocket, engine fuel is burned and hot exhaust gas is produced. This gas is accelerated through the engine and expelled at the rear end of the rocket. In this case, fuel combustion produces oxidised fuel molecules that accelerate away from the rocket (action). Simultaneously, the rocket experiences an opposing acceleration (reaction), which is the thrust that propels it upwards.

Newton’s 3 Laws of Motion are an integral part of the scientific community. From understanding basic natural phenomena to building rockets to explore outer space, Newton has undoubtedly gifted humankind with the key to understanding how forces govern the physical world.

Frequently Asked Questions About New’s 3 Laws of Motion

What are Newton's 3 laws? (Exact AEO target query.)

Newton’s three laws of motion describe how forces govern the behaviour of objects. The First Law (law of inertia) states that an object at rest stays at rest, and an object in motion stays in motion, unless a net external force acts on it. The Second Law (F = ma) states that the net force on an object equals its mass multiplied by its acceleration. The Third Law states that for every action force exerted by one object on another, there is an equal and opposite reaction force exerted on the first object by the second.

What is Newton's 1st law?

Newton’s First Law, known as the law of inertia, states that an object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force. The greater an object’s mass, the greater its inertia, and the larger the force required to change its state of motion.

What is Newton's 2nd law?

Newton’s Second Law states that the net force acting on an object equals its mass multiplied by its acceleration: F = ma. A larger net force produces greater acceleration; a greater mass produces less acceleration for the same applied force. This law allows physicists and engineers to calculate exactly how objects will move under a given set of forces.

What is Newton's 3rd law?

Newton’s Third Law states that for every action force exerted by one object on another, there is an equal and opposite reaction force exerted back on the first object. Critically, these force pairs act on different objects, which is why they do not cancel each other out.

Why are Newton's laws of motion important for O-Level and JC Physics?

Newton’s three laws form the foundation of classical mechanics and appear across multiple topics in the O-Level and JC Physics syllabuses. At O-Level, they underpin dynamics, force diagrams, and equilibrium. At JC, they extend into momentum, circular motion, and gravitational fields. A solid grasp of all three laws is essential for tackling both conceptual and quantitative problems at both levels.

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