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CSEC Physics: Newton's Laws of Motion

Isaac Newton, probably best known for his discoveries in relation to gravity, also discovered a few laws relating to motion now considered axiomatic. In this post, we will discuss each of Newton's three laws of motion.

First Law: The Law of Inertia

Newton's first law states that an object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an external unbalanced force. This basically means that the velocity of an object will remain constant if no forces act on it.

This quality of objects is known as inertia, or the resistance to change. As you know, a force is an external influence/interaction that causes a change in an object (whether speed, direction, shape, etc). So, inertia is that resistance to the change made by a force.

For example, if you have ever been in a car or bus while it is moving, you know how it feels when that vehicle comes to a stop. You feel yourself being pulled forward as the vehicle brakes, and that is due to inertia. You see, your body was being accelerated by the force being applied by the car. However, when a car brakes, it uses friction (a force opposite to the direction of movement of the car) to stop. Your body was moving at a certain velocity when the car stopped, meaning that it will resist a change in that velocity. So, you continue to move forward even though the car is stopping (always wear a seat belt, kids).

Inertia is why things don't just randomly move without influence. In order for a stable pen on a table to move, an external unbalanced force must act on it.

Second Law: The Force Law

Newton's second law states that the net force acting on an object is directly proportional to the acceleration of the object given that the mass of the object is constant. More simply stated, it tells us how much an object will accelerate for a certain net force. This can be written like this:

F = m × a


F = force

m = mass of the object

a = acceleration of the object

For example: John is pushing a supermarket trolley while shopping. After leaving it still to go pick up some green beans, he pushes the trolley with a force of 120 N. The trolley has a mass of 22 kg. What is its acceleration?

F = m × a

120 N = 22 kg × a

120 N/22kg = a

a = 5.45 m/s^2

Third Law: Law of Action-Reaction

This law states that every action has an equal and opposite reaction. So, if you push a door open with your hand, your hand is also pushed upon by the door. If you lift a table, the table's weight also pushes back down on your hands.

This law tells us that forces occur in pairs, an action and a reaction, that are equal in magnitude, and act in opposite directions.

We discussed this concept in a previous post on reactionary forces. Essentially, if you were to push a book on a table, the book would experience 2 pairs of forces, the force of gravity (or weight), the reactionary force (also called the normal force) from the table, the force of your hand pushing on it and the reaction force of friction acting in the opposite direction.

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