Newtonian Mechanics

Corpus omne persevare in status suo quiescendi vel movendi uniformiter in directum nisi quantenus illud a viribus impressis cogitur statum suum mutare.

Newton’s First Law Every body continues in its state of rest or uniform motion in a straight line unless compelled by an external force to change that state.

Galileo and the Law of Inertia Galileo looked at motion differently. If you push an object across a surface, the smoothness or lubrication on the surface affects how hard you have to push the object to keep the object moving. As the surface gets slicker and slicker, the force required gets smaller. Eventually, the force required to keep the object moving would decrease to zero. This leads to:

Galileo’s Inertia

The Law of Inertia The Law of Inertia states that a body once set in motion and thereafter undisturbed will continue in uniform motion forever, all by itself. This can be simplified to, “An object continues doing what it is already doing.”

Newton add to the Law of Inertia Newton used Galileo’s Law of Inertia as the basis for his first law that was published in his 1687 work, Principia Mathematical Principles of Natural Philosphy. Newton extended this law with other laws of motion.

Newton’s Life Born December 25, 1642 Gregorian calendar January 4, 1643 but it was not adopted until 1752. Started Trinity College June 1661 and wanted to be a lawyer. The plague closed Cambridge for two years starting in 1665. Newton did work at home. Appointed to the Lucasian Chair in 1669.

More on Newton 1687 published Philosophiae Naturalis Principia Mathematica 1693 had a nervous breakdown 1696 appointed Warden of the Royal Mint 1699 appointed Master of the Royal Mint 1703 elected president of the Royal Society 1708 Knighted 1727 died

Inertia and Mass So strong is our belief in the Law of Inertia, we try to use inertia reference frames. Inertial reference frames are frames of reference where the Law of Inertia does work. The rotating frame of reference is an example of a not inertial reference frame. Our minds start to create forces to explain apparent deviations from the expected motions.

A Word About Forces Forces are defined as a push or pull on an object. Forces are vector quantities. In the Metric System of measurement, forces are measured in Newtons (N). 1 N = 1 kg·m/s²

The Fundamental Forces There are four fundamental forces in Nature. All other forces are really combinations of the four fundamental forces. The four fundamental forces are: Gravity Electric-Magnetic Force Strong Nuclear Force Weak Nuclear Force

Force of Gravity The force of attraction between any two masses. This force is proportional to the product of the masses and is inversely proportional to the square of the distance between the masses.

Electric Force The electric and the magnetic force are two different aspects of the same force. The electric force can be an attractive or a repulsive force and depends on the charges involved and the distance between the charges.

Strong Nuclear Force The strong nuclear force is the attractive force between two nucleons (neutrons or protons). The strong nuclear force is responsible for holding the nucleus of an atom together.

Weak Nuclear Force The weak nuclear force can be either an attractive or repulsive force and is responsible for radioactivity.

“Common” Forces Normal Force The force a surface exerts on an object. The normal force is perpendicular to the surface. Tension The force that is exerted by a rope, string, cable etc. on an object. Remember, you can’t push with a rope. Elastic Force The force exerted by stretching or compressing an elastic object.

More “Common” Forces Friction The force between two surfaces acting parallel to the surface. Buoyancy The force a fluid applies to an object due to differences in density.

Units of Force In the SI system, forces are measured in Newtons (N). A Newton is equivalent to a kilogram·metre/second². 1 N = 1 kgm/s²

System Diagram

Free-Body Diagram FN FA FF Fg