by Tenebris

Created on: November 17, 2009

In 1687, Sir Isaac Newton published the Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). It was quickly recognised as one of the most important scientific works ever written. For the first time in human history, the behaviour of moving objects had been reliably and predictably explained.

The three laws of motion first introduced in this book form the foundations of classical mechanics. Even in today's quantum universe, these three laws of motion still hold valid in our everyday world.

First Law of Motion

A body at rest stays at rest and a body in motion stays in motion unless acted upon by an external force.

Items with mass do not start to move unless they are subjected to a force. Bodies which are already moving will continue to move in the same direction and with the same speed unless subjected to a force which changes either their speed or direction. (Friction counts as a force.) For this reason, Newton's first law of motion is sometimes called the law of inertia.

The common version of this law simplifies the vector sum of forces into a single external force. If all active forces completely balance each other out, it is the same as if no force were acting. In this case, we say that the vector sum of forces equals zero. In physics, a vector combines magnitude with direction. Two forces acting on an object will not cancel each other out unless they are pushing or pulling equally hard in opposite directions.

Second Law of Motion

A body acted upon by a force will accelerate proportionally to its mass and the force applied.

Mathematically, this is expressed as F = ma, where F represents the applied force, m represents the mass of the object, and a represents the resulting acceleration of the object. Actually, by introducing acceleration, this becomes a derivative equation examining force relative to the time derivative of momentum m(dv/dt), but we don't need to drag calculus into this to understand its point.

Basically, it means that the more force you apply to an object with constant mass, the greater the acceleration. A larger mass subjected to the same force will not accelerate as rapidly as a smaller mass. A train starts up much more slowly than a car, and also takes much longer to come to a complete stop.

Note that in physics, acceleration refers to any change in velocity: slowing down, speeding up, or changing direction. This is because velocity is not just speed, but a combination of speed and direction. Similarly,

• 1
• 2
• Next Page >>