Who was Isaac Newton?
Isaac Newton was a
physicist and
mathematician who
developed the principles
of modern physics,
including the laws of
motion. He was born on
January 4, 1643 and died
on March 31, 1727.
Newton’s 3 Laws of Motion
Watch this video and see how it applies to Newton’s 3 Laws of Motion.
Newton’s first law is commonly stated as:
An object at rest stays at rest and an object in motion stays in motion.
However, this is missing an important element related to forces. We could expand it by stating:
An object at rest stays at rest and an object in motion stays in motion at a constant speed and direction unless acted upon by an unbalanced force.
By the time Newton came along, the prevailing theory of motion—formulated by Aristotle—was nearly two thousand years old. It stated that if an object is moving, some sort of force is required to keep it moving. Unless that moving thing is being pushed or pulled, it will simply slow down or stop. Right?
This, of course, is not true. In the absence of any forces, no force is required to keep an object moving. An object (such as a ball) tossed in the earth’s atmosphere slows down because of air resistance (a force). An object’s velocity will only remain constant in the absence of any forces or if the forces that act on it cancel each other out, i.e. the net force adds up to zero. This is often referred to as equilibrium. The falling ball will reach a terminal velocity (that stays constant) once the force of air resistance equals the force of gravity.
Newton’s Second Law
Newton's second law of motion pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. As the force acting upon an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased.
Force equals mass times acceleration.
Or:
\vec{F} = M\vec{A}F=MAF, with, vector, on top, equals, M, A, with, vector, on top
Why is this the most important law for us? Well, let’s write it a different way.
\vec{A} = \vec{F}/MA=F/MA, with, vector, on top, equals, F, with, vector, on top, slash, M
Acceleration is directly proportional to force and inversely proportional to mass. This means that if you get pushed, the harder you are pushed, the faster you’ll move (accelerate). The bigger you are, the slower you’ll move.
Weight vs. Mass
The mass of an object is a measure of the amount of matter in the object (measured in kilograms).
Weight, though often mistaken for mass, is technically the force of gravity on an object. From Newton’s second law, we can calculate it as mass times the acceleration of gravity (w = m * g). Weight is measured in newtons.
Density is defined as the amount of mass per unit of volume (grams per cubic centimeter, for example).
Newton’s Third Law
This law is often stated as:
For every action there is an equal and opposite reaction.
This law frequently causes some confusion in the way that it is stated. For one, it sounds like one force causes another. Yes, if you push someone, that someone may actively decide to push you back. But this is not the action and reaction we are talking about with Newton’s third law.
Let’s say you push against a wall. The wall doesn’t actively decide to push back on you. There is no “origin” force. Your push simply includes both forces, referred to as an “action/reaction pair.”
A better way of stating the law might be:
Forces always occur in pairs. The two forces are of equal strength, but in opposite directions.
Now let’s discover how the famous American artist used the laws of motion in his art. Go to this link and follow the directions.