One of the most important topics you will be learning about in physics is motion. In this topic, you will learn how to describe the movement of certain objects. Although it may seem simple in theory, there is a lot of physics in action when something is moving!

Sir Isaac Newton was a scientist from the 17^th century – apart from discovering gravity, he made significant contributions to the field of motion. More specifically, he came up with three laws that describe the relationship between moving objects and the forces that act upon it. These laws revolutionised the people’s understanding of the physical world and laid the foundation for Newtonian Mechanics.

It is important that you understand these laws well enough so you can apply them to any situation - this article will take you through them in detail.  

Newton’s First Law

Have you ever wondered why you jolt forward when someone rapidly hits the breaks in a car? Or perhaps you’ve wondered what causes something to move? 

These phenomena can be explained by Newton’s First Law, which states:

An object at rest will remain at rest, and an object in motion remains in motion (at constant speed) and in a straight line unless an unbalanced force acts upon it.

Essentially, an object will not change its motion unless it experiences a net force. If a soccer ball is stationary on the ground, it is not going to move unless someone kicks it (applied force). Remember, a net force is the vector sum of all the forces that act upon an object.

Commonly, this law is referred to as ‘Newton’s Law of Inertia.’ An objects inertia is its tendency to resist any changes in motion.

If a car very suddenly comes to a stop, the passengers in the car will continue to jolt forward, due to their inertia. This is why cars are equipped with seatbelts and airbags – these are what stops the forward movement of the person (external force). Otherwise, there will be dangerous consequences if a person moves forward and hits the windshield.  

Newton’s Second Law

There’s a reason why race cars are designed to be as light as possible, and why you struggle to push heavier objects compared to light ones.  

Newton’s Second Law expresses that:

The acceleration of an object is directly proportional to the applied net force, and inversely proportion to its mass.

This is governed by the following formula:

F_net =ma

Where:

F_net: Net Force (N)

m=Mass of object (kg)

a=Acceleration of object (m/s²)

The lighter the object and larger the applied net force, the greater the acceleration will be. If a body is not accelerating, you can conclude that there is not net force acting upon it.

You can also rearrange this formula to find the mass, or acceleration, and just note that this relationship only holds for objects with a constant mass.

 Newton’s Third Law

Newton’s Third Law is one that is widely applicable to lots of everyday scenarios, and it comes from the idea of momentum conservation.

This law explains that:

For every action, there is an equal, but opposite reaction.

What does this mean actually? Well, let’s say we have someone pushing a wall. This person is applying a force to the wall. Simultaneously, the wall is pushing back on the person. These two forces are identical in magnitude, but opposite in direction.

To summarise, if A exerts a force on B, B also exerts a force back on A. All forces result from direct interactions.

These two forces are called action-reaction pairs.

One common misconception that students have is thinking that the weight of an object and its normal force is an action-reaction pair. This is untrue – can you think of a reason why? The weight force and normal force acts on the same object. Action-reaction forces occur on different objects (i.e., in the example above, one force acts on the wall and one force acts on the person). Additionally, the weight and normal force are not always equal (for example, on an elevator), which doesn’t agree with Newton’s Third Law.

Tips and Tricks

·      The questions you get for Newton’s first and third law will be quite theory based, so it is important that you get familiar with the correct terminology used to answer such questions.

·      The questions you get for Newton’s second law will involve calculations with the formula. Make sure you practise applying the formula to a wide range of scenarios.

·      Make sure you know how to calculate a net force and how to specify a direction for Newton’s Second Law.

·      To enhance your understanding of these laws, observe what’s around you and think about the laws. What law is in play to make your textbook sit on your desk?

Hopefully this article help clarify any doubts you may have had on Newton’s Laws. These laws lay the foundation for all your studies in motion, so make sure you gain a good understanding of them over the year. 

If you'd like to learn more, you can download detailed notes on Newton's Laws here.