Re: Forces on Free Falling Objects

[Maurice Barnhill <mvb@UDEL.EDU>]

Scott,

   The reaction force required by Newton's Third Law must act on the
*other* body, so it cannot reduce the acceleration of a falling
boulder.  The Earth pulls on the boulder, causing it to accelerated
downward toward the Earth.  According to the Third Law, the boulder must
also pull on the Earth with the same strength but in the opposite
direction.  This force on the Earth makes the Earth accelerate upward
(the force on the Earth is opposite to the downward force on the
boulder), so the Earth accelerates toward the boulder.  The reaction
force therefore increases the rate at which the Earth and boulder
approach each other, so rather than overcoming gravity the force
enhances gravity's effects.

   To see how this works, put your hands a bit apart and clap them by
moving one hand.  Then do it again moving both hands at the same rate
you moved the one hand.  Which way takes less time?  Of course, the
Earth does not move anywhere near as fast as the boulder, but the
direction of the change of the time interval is the same as the hand
clapping.

   There are other ways to stop an object which is falling.  You must
exert some kind of force on the object that causes an upward
acceleration.  For a baseball, a fielder's glove will do nicely.  That
is one example in which gravity is "overcome" when the object was not
originally at rest.  Rockets can be accelerated upward by the upward
pressure of the exhaust gasses on the on the rocket before they leave
through the tail; the Harrier jet is pushed upward by the upward
pressure on the plane by the air the plane is pushing downward, another
Third-Law pair.

   I have glossed over a few things.  If you don't see how and where
now, you will in a month or two.  You will hear all about them in the
comments this posting will attract shortly, which is one of the reasons
you need a bit of a thick skin to post to PHYS-L.

jmanor wrote:
> I'm a senior in a high school physics class and were studying Newton's
> Third Law.  When reading one of the examples of acting and reacting
> forces in my text, I was baffled.  They gave an example of a boulder
> falling towards the earth with the earth's gravity pulling down on the
> boulder and the boulder pulling up on the earth.  I was thinking that if
> every falling object had a force within them that pulled up on the
> earth, than eventually a force could be found to overcome earth's
> gravity.  I know such examples of this exist such as rockets and harrier
> jump jets, but never have I seen a falling or free falling object have
> enough force to overcome gravity.  The examples I listed always start
> out at rest.  Will we ever find a force great enough to halt a falling
> object to a rest?  Pardon my elementary logic, but if I could get a
> reply it would be much appreciated.  Thank you
>
> Scott Beattie
> Mrs. Manor's Physics Class
> Orchard View High School

--
Maurice Barnhill, mvb@udel.edu
http://www.physics.udel.edu/~barnhill/
Physics Dept., University of Delaware, Newark, DE 19716