Newton's laws

Newton, Philosophiae naturalis principia mathematica (1687) Translated by Andrew Motte (1729)

Newton's 3 laws of motion:

1.  Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.


2.  The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.


3.  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.


In simpler language:

1.  A body will continue doing what it is doing unless there is REASON for it to do otherwise.  It will continue in a straight line at a constant velocity, unless something changes that motion.  This idea is often referred to as INERTIA.

2.  The second law is trickier:

An unbalanced force (F) causes a mass (m) to accelerate (a).  Recalling that acceleration means how rapidly a body changes its speed (in meters per second per second, or m/s/s):

F = m a

There is a new unit here:  the kg m/s/s - this is called a newton (N)

Note that a larger force gives a larger acceleration.  However, with a constant force - the larger the mass is the smaller the acceleration.  Imagine pushing me on a skateboard vs. pushing a small child with the same force - who would accelerate more rapidly?

3.  To every action there is always opposed an equal reaction.

You move forward by pushing backward on the Earth - the Earth, in turn, pushes YOU forward.

A rocket engine pushes hot gases backward - the gases, in turn, push the rocket forward.

If you fire a rifle or pistol, the firearm "kicks" back on you.