The Laws of Motion

Isaac Newton’s Laws of Motion are made up of three essential laws in classical physics. His discovery of the laws of motion came upon after watching an apple fall when studying the effects of gravity. It is assumed that this incident and his curiosity as to why the planets above us do not fall to the ground, led him to establish the laws of motion. In 1686 he published his three laws of motion.

Isaac Newton’s first Law of Motion is also referred to as the ‘Law of Inertia.’ This law is usually stated as, “an object at rest stays at rest, while an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.” There are two sections to this statement. The first being the one that predicts the behaviour of moving objects and the other is the prediction of the behaviour of stationery objects.

The behaviour of all objects can be explained by stating that objects tend to “keep on doing what they’re doing,” unless they are acted upon by an unbalanced force. Isaac Newton discovered this first law of motion by realising that a mysterious force made an object in motion stop. He identified that the mysterious force was friction. Friction makes moving things slow down and stop, without friction moving objects would keep going forever.

For example; a person playing golf.

A golf ball is on the ground. It cannot miraculously move into the hole unless some kind of force will act on it. The person using the golf club and hitting the ball is the force. The balls will now move, proving the first part of Isaac Newton’s first law of motion. The law also states that the ball will stay in a state of constant velocity in the absence of an unbalanced force. This means that there is no frictional or any other force and the ball will never stop. Although, when we throw a ball we always see it stop…there is a reason behind this. If the ball is hit into the air and then drops to the ground, the ground is the force of friction making the ball slow down and eventually stop moving. This is the unbalanced force.

Isaac Newton’s second law of motion:

Isaac Newton’s second law of motion states that “acceleration is produced when a force acts on a mass. The greater the mass of the object being accelerated the greater the amount of force needed to accelerate the object.”

Everybody unintentionally knows Newton’s second law of motion. If you want to move something heavier the same distance as a lighter object…the heavier object is going to require more force.

For example; a heavy rock needs to be moved.

A young child wants to move a rock by pushing it. The rock moves slightly. If an adults pushes the rock, the rock will move a greater distance. This proves Newton’s second law of motion. The child applies less force so less acceleration is produced. The adult applies more force and more acceleration is produced. Therefore, force is equivalent to acceleration.

An object with a smaller mass means that the amount of applied force needed to move it is less, whereas something with a greater mass requires more force to move it. This force is called a Net Force which is the overall force acting on an object.

The amount of force needed to push an object is equal to the mass of an object and the force is also equal to the acceleration needed to push an object. This means that force equals the product of mass and acceleration.

Force = Mass                                                             Force = Acceleration

This law can be expressed in a mathematical equation:

F = M A

FORCE = MASS times ACCELERATION

The standard metric unit of force is given in Newtons, (N).

Quick examples:

Net Force (N)	Mass (kg)	Acceleration (m/s/s)
10	2	5
20	2	10
20	4	5

Newton’s third law of motion:

Newton’s third and final law of motion states that “for every action there is an equal and opposite reaction.” This law relates to the ability of force. A force is a push or a pull that acts upon an object to form an interaction with another object.

This statement means that there is always a pair of forces acting on two interacting objects. The size of the forces on both objects are equal but the direction of the forces are opposite. E.g., an upward force pairs up with a downward force.

According to Newton’s third law of motion, whenever two objects interact with each other they exert forces upon each other.

For example; sitting on a chair…

When we sit on a chair our body exerts and downward force on the chair, the chair also exerts an upward force on the body. This relates to the forces being exerted upon both objects are equal and the direction of the forces are opposite. If the chair would not have exerted a force, the chair would collapse and we would not be able to sit on a chair. 

Isaac Newton’s law of motion also helps birds to fly. The bird’s wings exert a downward force on the air and the air exerts and equal and opposite force which pushes the bird upwards. This action-reaction combination helps birds to fly in the air. 

                                                                    Downward force

                                                               Upward force