Explain Newton's First Law

In 1665, at the age of 23, Newton developed his famous Laws of Motion. They are commonly known as Newton's first, second and third law. Aside from the catchy names, these laws are important because they are the first realistic attempt to fully describe how objects move. They represent a turning point in our understanding of the physical world. To summarize: until Newton came around we were clueless.

Long ago, an ancient Greek named Aristotle believed that objects needed an external force in order to move. All motion, according to Aristotle, resulted from a push a pull or the nature of the object itself. If an object was in its proper place it would cease to move.

Galileo turned it all upside down by stating that in the absence of force an object will keep moving in the same direction. In other words, objects don't like to change the way that they move. This tendency to resist change is called an object's inertia.

The Law of Inertia - Every object continues in its state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it.

Even though we like to attribute Law #1 to Newton (because it sounds better) Newton's first law is pretty much the same as Galileo's principle.

Every object in the world possesses inertia. The amount of inertia depends on the amount of matter present. Inertia is proportional to mass.

Example 1 - The magic trick of pulling the tablecloth out from under the dishes depends upon the inertia of the dishes which keeps them from moving in the same direction as the tablecloth. This inertia results from the force of friction between the tablecloth and the dishes. The dishes tend to stay at rest and fall straight down onto the table.

Example 2 - A semi-tractor trailer moving along the highway at 29 m/s (65 mph) hits a tumbleweed in the road. Does the truck stop? Of course not. It would take a lot more than a tumbleweed to stop such a heavy, fast moving truck which tends to stay in motion.