### Re: Describe throwing a frisbee as an example of Newton's third law.

Date: Fri Mar 5 17:26:09 1999
Posted By: Graham Beckhorn, Undergraduate, Physics, residence
Area of science: Physics
ID: 920423692.Ph
Message:

Trying to identify the initial action and reaction in this situation can be
confusing.  The major effect of the forces involved in the problem is to
move the frisbee through the air, that comes from the initial action.  When
someone throws a frisbee he first holds it by curling his fingers around
the "lip" - the part where the edges come down.  When he throws it he is
actually pushing on that lip with his fingers.  His muscles give the energy
to the arm to move through the air, and the fingers push on the inside of
the frisbee and move it also.  The frisbee exerts an equal force against
the fingers, the reaction.  The fact that the reaction is applied to the
fingers rather than the frisbee is the reason why the frisbee is able to
move instead of remaining motionless because of equal forces in opposite
directions.  The reason the fingers and arm are able to continue moving
forward to follow through with the throw rather than being stopped by the
reaction force coming from the frisbee involves the standing position of
the thrower.  The reaction force goes through the body and its effect is to
make the feet push against the ground, thereby allowing the hand and
fingers to continue pushing against the frisbee without making the person
move backwards.
Even if the person was in space when he threw the frisbee, with
nothing to brace his feet on and prevent him from moving backwards, the
amount of his movement backwards would be so small that he would still be
able to throw the frisbee.  The force exerted on an object is actually its
mass X the acceleration produced.  If a .1kilogram frisbee was accelerated
to 5meters/second then the force applied would have been .5Newtons.  That
means that .5Newtons would also be exerted on the thrower by the reaction.
If Force = Mass X Acceleration, then Acceleration = Force / Mass.  When
.5Newtons of force are applied to a 130kg person the acceleration produced
is only .0038meters/second backwards which is not very significant.  This
explains why the effects of the initial action (moving the frisbee forward)
appear so large compared to the equal reaction.  You can see it go very
fast for many meters on a good throw because the .5Newtons will produce a
large acceleration on such a small object whereas that same .5Newtons will
only accelerate a more massive object like the thrower a very small amount.
In short, the action of throwing a frisbee is the force exerted on
the frisbee by the fingers which push it, and the reaction is the force
exerted by the frisbee back on the fingers.

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