Newton's 3rd Law: Any single force actually exists in conjunction with another force.  These  two forces are equal in magnitude and opposite in direction.

HUH?

The idea isn't too bad if you look at it this way...
 

  • A bat hits a ball and sends it flying.
  • The bat acts on the ball, obviously becuse the ball goes flying.
  • The ball acts on the bat too, because you feel the jolt in your hands when you hit it.
  • If the forces are equal, then how can there be any motion at all?
  • The force that acts on the ball is enough to accelerate its mass out into the air (F=+ma).
  • The force that acts on the bat doesn't make it accelerate as much (F=-ma)  because the mass of the bat is much greater.  (I added the minus sign to show that the bat is actually slowed down a little, or accelerated in the opposite direction)


    • Put more simply...

    Bat hits ball with force "F"
    Ball hits bat with force "-F"

    Ball has small mass "m" and gets a big acceleration "+a"
    Bat has big mass "m" and gets a small acceleration "-a"

    REALIZE THAT WHEN AN OBJECT MOVES, THE FORCES IN QUESTION ARE BEING APPLIED TO DIFFERENT OBJECTS, in this case the BALL and the BAT