What Are Newton's Three Laws of Motion?

Newton's Laws of Motion help us to understand how objects behave when they are standing still; when they are moving, and when forces act upon them. There are three laws of motion. Here is a description of Sir Isaac Newton's Laws of Motion and a summary of what they mean.

Newton's First Law of Motion states that an object in motion tends to stay in motion unless an external force acts upon it. Similarly, if the object is at rest, it will remain at rest unless an unbalanced force acts upon it. Newton's First Law of Motion is also known as the Law of Inertia.

Basically, what Newton's First Law is saying is that objects behave predictably. If a ball is sitting on your table, it isn't going to start rolling or fall off the table unless a force acts upon it to cause it to do so. Moving objects don't change their direction unless a force causes them to move from their path.

As you know, if you slide a block across a table, it eventually stops rather than continuing on forever. This is because the frictional force opposes the continued movement. If you threw a ball out in space, there is much less resistance, so the ball would continue onward for a much greater distance.

Newton's Second Law of Motion states that when a force acts on an object, it will cause the object to accelerate. The larger the mass of the object, the greater the force will need to be to cause it to accelerate. This Law may be written as force = mass x acceleration or:

F = m * a

Another way to state the Second Law is to say it takes more force to move a heavy object than it does to move a light object. Simple, right? The law also explains deceleration or slowing down. You can think of deceleration as acceleration with a negative sign on it. For example, a ball rolling down a hill moves faster or accelerates as gravity acts on it in the same direction as the motion (acceleration is positive). If a ball is rolled up a hill, the force of gravity acts on it in the opposite direction of the motion (acceleration is negative or the ball decelerates).

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction.

What this means is that pushing on an object causes that object to push back against you, the exact same amount, but in the opposite direction. For example, when you are standing on the ground, you are pushing down on the Earth with the same magnitude of force that it is pushing back up at you.

Sir Isaac Newton introduced the three laws of motion in 1687 in his book entitled "Philosophiae Naturalis Principia Mathematica" (or simply "The Principia"). The same book also discussed the theory of gravity. This one volume described the main rules still used in classical mechanics today.