Newton’s First Law, originally published in his seminal work Philosophiæ Naturalis Principia Mathematica (Principles of Natural Philosophy), was the first of three which changed our fundamental understanding of the universe by explaining mathematically how the motion of objects works.
Newton’s First Law or The Law of Inertia
Any object which is in a state of uniform motion will remain in this state unless a force is applied to it.
The first application of this is intuitively obvious but is nonetheless an important fact to note. If something is stationary (equivalent to a state of uniform motion, as there is no motion), it will not move unless something makes it move – that “something” is called a force. A force could be applied directly (for example a kick to move a football), or more indirectly (such as tipping a table, thus causing gravity to pull a vase on it downwards).
Uniform Motion vs Acceleration
The second application requires a little more understanding. An object in a state of uniform motion is defined as an object that is not accelerating. Since acceleration is a change in velocity, any object moving at a constant speed is defined as having uniform motion. Think of an astronaut in space, which is a vacuum and so as good as frictionless; if the astronaut pushes himself away from his spacecraft (applying a force) he will continue to move away from it without having to constantly push himself. To stop himself moving, he will have to apply another force (as a burst from his manoeuvering “jet pack”).
The Difference Between Velocity and Speed
Note the difference between velocity and speed: velocity is a vector quantity, having both magnitude and direction, while speed is a scalar quantity which only has magnitude. A simple example would be that by saying “the car moves at 20 mph” you describe its speed, but by saying “the car moves at 20mph towards the south west” you describe its velocity. Something can change velocity without changing speed, as well; something spinning in a circle at a constant speed is still changing velocity, because its direction of motion is constantly changing as it spins.
Reference
"Newton's Laws of Motion," online at NASA's Glenn Research Centre; Beginner's Guides Grc.nasa.gov, accessed May 2010.
