We have all heard the story of the apple falling on Newton's head while he was sitting against the tree, thus
giving him the sudden idea for the Universal Law of Gravity. This, as I am sure most of us know, is not really
what happened. A more accurate version would probably be, Newton observed an apple falling from a tree
and started contemplating the physics of the apple's fall. The acceleration experienced by the apple, which
started at zero when hanging in the tree, then increased as the apple fell. This as Newton reasoned from his
second law of motion, means there must be a force acting upon the apple in order to cause the acceleration,
this force we will call gravity. Thus giving us the acceleration due to this force as "acceleration due to gravity,"
a term most of us have probably heard. This idea is known as Newton's Universal Law of Gravity. This idea
is basically Newton's second law of motion which we will look at more in depth. We will also look at Newton's
other two laws of motion.
Newton's 1st Law
For us in everyday life it is slightly hard for us to comprehend Newton's fist law of motion. This is due to the fact
in real life there is not frictionless surface or even space for that matter. However if we imagine for the moment
that there is no friction or air resistance or anything else of the sort we can then go on and look at Newton's 1st
law of motion.
An object at rest remains at rest and an object in motion
continues with a constant velocity in a straight line
unless an external force is applied to either object.
One way for us to see how this works is to take a look at an air track. This is a device that essentially gives us the
ideal friction's surface we want to be able to observe Newton's 1st law.
Newton's 1st Law allows us to see if a force is acting upon an object. From this we are able to tell that
if an object is either accelerating or decelerating than an external force must be acting on it even if it is
simply the force of gravity, air resistance, or friction.
Newton's 2nd Law
This is probably the most known of all three of Newton's Laws of motion.
The acceleration of an object is directly proportional to the net force
acting on it, while being inversely proportional to its mass.
More often though we see this law written as;
Fnet = ma
Where "F" equals the net force on the object in Newtons, "m" equals the mass in kilograms and
"a" equals the acceleration of the object in meters/second^2. Since force is a vector this can be
rewritten in three separate equations to take into account direction;
Fx = ma, Fy = ma, Fz = ma
Fnet = Fx + FY+ Fz
We must also remember that acceleration as well is a vector and therefor has three components just
like the force. When learning how to use Newton's 2nd Law, however, one usually only uses examples