MadSci Network: Physics

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

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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