 Welcome to Newton's first law of motion, the law of inertia. Sir Isaac Newton was an English mathematician, astronomer, physicist, and philosopher. He lived during the last days of the Renaissance period, and his work in physics laid the foundation for the modern age. His book, Mathematical Principles of Natural Philosophy, first published in 1687, laid the foundation for classical mechanics. In the book, Newton outlined his understanding of motion. His understanding became the driving force for how all scientists understand the physical universe today. For the past three hundred years, Newton's helped scientists understand how comets move through the night sky, how waves and tides collide with the shore, and how planets move through the universe. Even today, if you're trying to explain how forces affect any object, including an apple falling from a tree, it lying on the ground, or that same apple being thrown across the orchard, you're going to want to use Newton's laws. His first law of motion sets the stage for how everything in the universe is related. An object at rest stays at rest. An object in motion stays in motion at a constant speed in a straight line unless acted on by an external, unbalanced force. Newton's first law is all about inertia. It describes why an object has the tendency to keep doing what it's doing. Throughout the universe, moving objects like to keep moving and resting objects like to rest. Basically, if you want to change an object's inertia, you need to use force. An object at rest stays at rest. The fallen apple stays on the ground. Newton's first law explains that the apple will keep doing what it's doing. Every object resists a change in its state of motion. The apple stays where it is. That is, until your little brother picks up the apple and throws it across the orchard. Then we're on to the second part of Newton's first law. An object at rest stays at rest unless an external, unbalanced force acts on it. I'm not saying your little brother is unbalanced, but he's using an external, unbalanced force to launch the apple across the orchard. He's using the strength in his arm to pick up and throw the apple, which is greater than the resting force currently acting upon the apple. In this case, the apple's ability to remain at rest has been overpowered by an external unbalanced force. It's now flying across the orchard. Next we get to, an object in motion stays in motion at constant speed in a straight line unless an external unbalanced force acts on it. The apple now continues in a straight line across the orchard until it falls back to the earth after hitting a tree, hitting a fence, or encountering enough resistance in the air to stop its motion. Any of these act as another external unbalanced force that changes the apple's state of motion again and causes it to return to a state of rest. Unless of course your little brother runs after the apple, picks it up and starts the process all over again. In summary, Newton's first law states an object at rest stays at rest. An object in motion stays in motion at a constant speed in a straight line unless acted on by an external unbalanced force. That apple could be anywhere by the time you're ready to take him home. Thanks, Newton. Newton's first law of motion, the law of inertia.