Ylli- You're right up to a point. The resultant (that is, the total)
force on the parachutist's body right after the parachute opens is
upwards. According to Newton's third law, the parachutist will
accelerate in the upwards direction. This does not mean he will move in
the upwards direction. The parachutist is moving rapidly downwards
before he opens the parachute, and acceleration upwards really is the
same thing as deceleration downwards -- he just slows down.
If the upwards acceleration continues unchanged for a long time,
the parachutist eventually will start moving upwards. This doesn't
happen in the case of the parachute, because force due to air friction
changes strength (although it still points up). The strength of the air
friction force depends on how rapidly the parachute is moving through
the air. As the parachute slows down, the upward air friction force
drops. It gradually gets very close to equal to the downward
gravitational force, leaving almost no net force and thus a new
terminal velocity, which is now less than the one with the parachute
closed. So although the acceleration is upwards, it never leads to an
upward velocity, but just slows the downward velocity to a new, smaller
value.
There's a key math idea behind this, and perhaps confusion about it
led to the confusion about the physics. You can add an infinite number
of positive numbers (say little upward changes in velocity) and still
never exceed some limit (say the initial downward velocity.) Think of
adding the numbers 1/2+1/4+1/8+1/16........ It's not hard to see that
no matter how far you go, you never get bigger than 1.
Mike W.
(published on 10/22/2007)
