MadSci Network: Physics

Re: Can you explain to me Newton's 1'st, 2'nd and 3'rd laws?

Area: Physics
Posted By: Tom Cull, MadSci Admin
Date: Thu May 15 15:51:44 1997
Area of science: Physics
ID: 863503413.Ph
Newton's Three Laws of Motion and my interpretation.

FIRST LAW: Consider a "body" on which no net force acts.  If the body
is at rest; it will remain at rest.  If the body is moving with a constant
velocity, it will continue to to do so.

MEANING:  In some way of looking at the body there is no acceleration.  The
body will continue to do what it is already doing (sitting still, sliding...).
A force is needed to change the velocity of the body.  In everday life, friction
usually makes the FIRST LAW hard to observe.  If I slide a book along a table, the 
book will eventually stop.  A rolling ball is a better example.  The ball will
roll along in a given direction until either it hits something (like a wall or a bump)
or until it gets to a slope of some sort and changes direction.  The change of
direction at a slope results from gravity acting upon the ball.

SECOND LAW: Force = mass * acceleration.

MEANING: Just the specifics of the FIRST LAW.  Mass (or inertia) is the body's
ability to resist change.

THIRD LAW: Usually stated something like:  For every action there is an equal
but opposite reaction.

MEANING: If I am standing on the ground, my body pushes on the earth.  The earth
pushes back upon me with the same force that I apply but in the opposite direction.
My force is my weight in the downward direction.  The earth pushes up with a force
of the same amount.

I hope this clears up some of the confusion.

Tom "Physics Attorney" Cull

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