Have you ever wondered why a ball rolls so far when you push it but the wall of your house doesn't move no matter how hard you push on it? Or maybe you've wondered why a feather falls slowly but a book falls straight to the ground? There are answers to those questions, and they start with a man named Isaac Newton. Sir Isaac Newton was a mathematician and scientist in the 17th century. In 1687, he published Philosophiae Naturalis Principia Mathematica. Though more than 300 years have gone by, Newton's book is still considered one of the most important scientific works ever published. In it, he describes three basic principles that explain how forces affect objects. These principles have collectively become known as Newton's laws of motion.

Newton's First Law

Newton's first law of motion states, "An object at rest stays at rest and an object in motion stays in motion, unless acted on by an external force." The easiest way to think about this law is to imagine a ball sitting on a table. The ball sits on the table without moving until you apply a force to it, such as hitting it. Once it's hit, the ball will roll until another force makes it stop or change direction.

Newton's Second Law

Newton's second law of motion says, "The force acting on an object is equal to the mass of that object times its acceleration." Newton's second law is expressed as the equation F = ma, where "F" is the force acting on the object, "m" is the mass of the object, and "a" is the object's acceleration. Imagine that ball sitting on a table is a basketball or soccer ball. Pushing the ball to get it rolling should be easy, since it does not have a lot of mass. Now, imagine trying to push a bowling ball. You would need to use more force because the bowling ball has more mass. Newton's second law tells us something very important about the relationship between an object and its motion: A force exerted on an object causes it to accelerate, and the more mass an object has, the more force is necessary to slow the object down, speed it up, or make it turn.

Newton's Third Law

Newton's third law states, "For every action, there is an equal and opposite reaction." When we push a ball, it rolls, but the ball also pushes a little bit against your hand. However, while the amount of force in both directions is equal, you have a lot more mass than the ball does, so the ball moves more than your hand does. Now, think about pushing against a wall of your house. Hopefully, the wall doesn't move, but your hand may flatten against it depending on how hard you push. This happens because while you are pushing against the wall, the wall is pushing against you, too.

What Happens in an Elevator?

When you step into an elevator, you feel like you're the same weight you were when you were waiting in the hallway. When you go up in an elevator, you feel slightly heavier, and when you go down, you feel slightly lighter, if only for a few seconds. If you were to stand on a scale in an elevator, your weight would read slightly different going up, staying stationary, and going down. When you are stationary, your acceleration is 0, so the scale will only read your apparent weight. When the elevator is going up, though, you are accelerating, which adds more force to the scale and increases your apparent weight. When the elevator is going down, the same is true, but the acceleration is negative, subtracting force from the scale and decreasing your apparent weight.

Resources for Practice

Newton revolutionized science and math with his three laws of motion. Much of what we know about our universe, including how planets move, is based on his work. Some of the concepts may seem strange at first, but keep at it. Understanding physics requires dedication, a desire to learn, and practice.

Important Links & Resources