Isaac Newton was a renowned scientist and inventor of the 17th century. He devised a number of laws to explain how objects move as they usually do. They are known as the three laws of motion. The laws reflected scientists’s view of the physical universe for the next three centuries. In this basic level tutorial we look at the theory and application of Newton’s first law of motion.

Newton’s first law is also known as the law of inertia. The law of inertia is defined as:

An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This means that an object in the stationary or moving state will continue in the same state unless an external force acts on it. The law can be broken down into two parts.

1. The first part tells you about the behavior of stationary objects.
2. The second part is about the behavior of the moving objects.

To sum up all objects continue in their present state unless an external force is applied – and they resist change in their current state of motion. You may like to brush up your Physics fundamentals with this course.

## What is Balanced Force?

Let’s take an example of a book placed on a table. Here two forces act upon the book which is in the state of rest.  One force is the gravity of the earth which pulls the book down.  While the other force is the push of the table on the book in the upward direction.  As these forces are exactly in the opposite direction and possess equal magnitude, they are termed as balanced forces. Since there is no unbalanced force to act on the book, it’s said to be at the state of rest.

## What is Unbalanced Force?

Now try pushing the book on the table to the right. The book starts sliding down. This rightward motion is resisted by the friction between the table and the book surfaces. The gravitational force and the upward force exerted by the table on the book balance each other. However, the frictional force is not equal in magnitude to the force that causes the book’s motion. The result is that the book will slow down due to the frictional force. This is an example of an unbalanced force which causes the motion of the book to slow down.

Lets understand the concept of the Law of Inertia with some simple examples.

## Real Life Application of the Law of Inertia

Example 1: Walking with a bowl of water filled to the brim

Try to do this exercise. Fill a bowl to the rim with water. Walk around an athletic track with this bowl, trying to complete a lap in minimum time. You will notice that the water in the bowl spills at certain locations. There are three situations when the water was likely to spill.

• When the bowl was at rest and you started walking.
• You were moving and you tried to stop walking.
• You change the direction of your movement.

Example 2: Why coffee spills when you start the car from the state of inertia

Have you ever wonder about the behavior of a coffee cup filled to the brim while traveling in a car? When you start the car the road acts as an unbalanced force to move the car forward. However, the coffee in the cup, which was in a state of rest wants to remain at rest.  As the car moves forward from under the coffee cup, the coffee spills on your lap. And when you apply brakes while the car is in the motion, the car stops. However, the coffee in the cup continues to forward with the same speed and direction of car before the brakes were applied – and hence spills over. Yes, even coffee has a mind of it’s own! Check out this coffee crash course to get a peek into it’s mind.

Example 3: Purpose of seat belt in a car

Have you thought how a seat belt actually protects you? Say you are travelling in a car as a passenger without wearing your seat belt. When brakes are applied to the car at motion, the car comes to a standstill. However, your body continues to move forward with the same direction and speed which the car had before the driver applied the brakes. This is dangerous for your safety. Meaning your head will continue in motion and go hit the dashboard.  The seat belt effectively pulls you back, preventing your body from continuing it’s motion right into the dashboard.

It’s better to be safe than sorry. Follow the rules and drive safely. If you live in areas which have cold snowy winters, you may even want to take this course to learn winter driving safety tips – especially how Newton’s laws can help you avoid slipping on an icy road.

## Day to Day Occurrences of Newton’s First Law of Motion

• Say you are riding a skate board. If you hit an obstacle which suddenly stops the motion of the skating board, your body moves forward. The reason is your body wants to continue in its previous state of motion. This is an example of body wanting to resist any change in its state of motion. Have you tried the run up to a ramp, and then sailing through? That’s another example. Wait, you do know how to skate board? Well, try out this course to help you skateboarding in no time!
• Law of inertia works in the kitchen too. How do you get the tomato sauce from the very bottom of the bottle? Turn the bottle upside down and shake it downwards with great force. When you stop this action abruptly, you will see that the sauce falls on the saucer placed on the table. This is because the sauce at the bottom of the bottle is moving downwards at high speed and continues to do that even when you abruptly halt the action. Yes, we see the laws of physics at work everywhere, and don’t even know it.
• How do dust the rugs in your home? The rug is given a sudden jerk. This causes the rug move in a particular direction. However, the dust particles in the rug which were in a state of rest will resist the movement and not move. This results in the dust to fall off the rugs.
• Have you tried this trick? Place a coin on a plate and cover it with a tumbler. If you suddenly slide of the plate, you will notice that the coin has fallen into the tumbler. This is because, the coin was at rest. Even though the plate on which it was resting moved, the coin wanted to retain its previous resting state. Hence, it did not move along with the plate.

It is clear that all material bodies try to preserve their state of rest or of uniform motion in a straight line. Then how to change this? Well, just apply an external force on the body. Newton’s First Law thus gives the definition of force. Force is an external variable which changes the state of rest or the uniform motion of an object. One also realizes that inertia is a fundamental property of all matter. This is why Newton’s First Law of Motion is also known as the “Law of Inertia”.  Newton’s laws form the basis of modern physics as we know it – even laying down the groundwork for Quantum Physics. Don’t believe it? Then take this introductory course to Quantum Physics and see for yourself!

Page Last Updated: February 2020