# Newton's First Law: inertia

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## Introduction

Isaac Newton was one of the most influential scientists in the field of physics during the 17th century. His theories about mechanics, optics (the study of light) and mathematics are, in many cases, theories we still use today. Newton's Three Laws of Motion are commonly used in the field of mechanics to explain the motion of objects.

## Newton's First Law of Motion

Newton's First Law of Motion states that an object at rest tends to stay at rest and that an object in motion tends to stay in motion. In order for a change to happen to an object's motion, a force must be applied to it. A force is an action that can change an object's motion or shape. It can be described as a push, pull or twist. A force can change an object's motion in a number of ways - it can cause it to speed up, slow down, or change direction.

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Consider a ball resting on a table. In order to cause the ball to move, you would have to apply a force to it by, for example, pushing it with your hand. The ball will continue rolling until it is stopped by another force, such as stopping it with your other hand. Another force that could stop the ball is the force of friction. (Refer Chapter 6)

Newton's First Law was created with the help of Galileo Galilei's theories about mechanics. Galileo showed that objects, when forced into motion, have a natural tendency to keep moving. He called this tendency inertia. This was the main predecessor to Newton's First Law.

## Inertia

Inertia is something that we feel on a regular basis. An example can be found when riding in a motor vehicle such as a car or bus. When a vehicle accelerates, you feel as if your body is pulled back into the seat. This is because your body's inertia tends to keep it at rest, but it is being pulled forward by the seat you are sitting in, which is moving with the rest of the vehicle. When the vehicle has finished accelerating, however, you do not feel yourself being pulled into your seat because you and the vehicle are travelling at the same velocity. If the vehicle stops suddenly, you would feel yourself being pulled forward because your body wants to keep moving while the car comes to a stop. When the vehicle turns a corner, you move to the side of the car because your body wants to keep travelling in a straight line.

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Inertia is also used to keep people safe in cars. Seatbelts are devices that stop inertia from keeping your body in motion when the car comes to a sudden stop (as it would in a collision). If you did not wear a seatbelt in the car, your body would move forward very quickly, possibly smashing into the dashboard or through the windscreen. A seatbelt forces your body to stop with the car.

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