6.1. The Big Idea http://www.ck12.org
6.1 The Big Idea
In the previous chapters, we studied the behavior of accelerating objects in one and two dimensions. We did not,
however, address the issue ofwherethe acceleration comes from: in other words, why, in certain situations, do the
velocities of objects change? It might make sense that a cart moves if I push it, but what about a dropped object: is
it accelerating for a different reason, or for the same one? Is there something common toallaccelerating objects?
Building on the insights of scientists before him, Isaac Newton created a mathematical analysis of moving and
accelerating objects; the rules he discovered are now known as Newton’s Laws of Motion. Newton is a legendary
figure to physicists, and it’s hard to underestimate his influence on the field. Actually, the substance of his Laws had
been summarized by scientists before him. Still, the mathematical framework for their interpretation that Newton
created was a revolutionary achievement, since it unified the existing knowledge of mechanics in a consistent system
and cemented math as the accepted method of interpreting physical phenomena.
Here are Newton’s Laws, in modern English:
Newton’s First Law
Every body continues in its state of rest, or of uniform motion in a right (straight) line, unless it is compelled
to change that state by forces impressed upon it.Newton’s Second Law
The change of motion is proportional to the motive force impressed; and is made in the direction of the right
(straight) line in which that force is impressed.
Newton’s Third Law
To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each
other are always equal, and directed to contrary parts.(Taken from the Principia in modern English, Isaac Newton, University of California Press, 1934).