Until now we have only talked about the description of motion "as it happens". We have not talked about what can give rise to these different types of motion. The motion of macroscopic bodies at speeds well below the speed of light is described by the three laws of Newtonian mechanics.
Isaac Newton discovered his laws of motion at the young age of 24 when he was living in the British country side away from Cambridge University to escape the plague of 1665-1667. Newtons is considered one of the greatest Mathematical geniuses ever. No modern era scientists can match the achievements of Newton. Alone in the country side and as young as you, Newton discovered three laws of physics which predict motion from the swinging of a pendulum to the motion of planets and galaxies. His achievement is even more impressive because Newton had to invent differential analysis along the way. His formulation of the laws governing mechanics are deceptively simple but nonetheless had eluded mankind until Newton's work. Surprising to physicists today who live under the edict ''Publish or Perish'' Newton did not publish his findings until twenty years later when he became aware that others were close to making similar discoveries. His book called "Philosophiae Naturalis Principia Mathematica'' may be the greatest and most influential scientific work ever written. In 1692 Newton suffered a nervous breakdown which forced him to retire for several years and although he fully recovered from the illness i it did mark the end of his scientific career. He retired from Cambridge at the age of 59 and went on to work for several years in public administration.
The basis for Newtons theory of mechanics are the following three laws.
Newton I: The velocity of a body on which no net forces act cannot change
In everyday life friction is almost universally present so that to achieve motion we need to apply a force which compensates for friction ie. which cancels it out. For 2000 years this experience perpetuated the view of Aristoteles that a driving force is necessary to induce motion. Perhaps if civilization had developed further from the equator where lakes freeze in the winter this notion would have been challenged sooner. It took Galilei to realize that constant motion is the "natural" state of matter and that force only is required to change the velocity of a body. Newton provided the mathematical framework for this observation.
Acceleration requires a force.
From the equation we see that the unit for force is
Newtons second law is the
first time the mass of a body enters our description of physics.
In kinematics we only dealt with distance and time:
From the pendulum to the dime and feather
mass did not enter our formulae. Newtons second
law introduces mass as a scaler which determines how much force is
required to accelerate, to change the velocity of, a body. The greater
the mass the smaller the acceleration for a given
force.
As you have already seen in homework problems due this week
Newtons second law is a vector relation. It can be written in terms of
its projection on three orthogonal directions :
in words : acceleration along a direction in space is caused by a force
acting along that direction and is independent of forces acting in
orthogonal directions.
We will see lots of experiments which illustrate Newtons second law here and in the lab course. We have a qualitative and a quantitative experiment today both designed to illustrate that larger mass gives smaller acceleration for given force.