1. Parachutes should work about as well or better at low altitudes
as they do at high altitudes (that is, if you are falling to the ground
with a parachute properly deployed, it will not somehow stop working as
you near the ground, thankfully!).
It is very important, however, to make sure you use the parachute
when you are falling from a large height rather than from a small one,
for two very important reasons:
a) Parachutes usually have ten or so feet of cords between the
parachute material and the parachutist. If you deploy the parachute
when you are less than a few tens of feet from the ground, the
parachute will just be beginning to stretch out when you hit the earth,
providing no slowing force. A fall from a few tens of feet can be
fatal. Even if the parachute cords are fully stretched, it will take
some distance falling in order to get the fabric of the parachute fully
taut and full of air.
b) If you are falling from a great height, say, from an airplane,
you can fall for a while before pulling on the rope that will deploy
the parachute. Skydivers use this time for doing outlandish stunts,
like somersaults, riding surfboards, shaking hands and taking pictures,
etc. If the parachutist waits too long before deploying the parachute,
it can be fatal. The parachute may open, fill with air, and be
functioning correctly, but there is still a limit to how much drag it
will exert on the parachutist (which is good, otherwise it can yank the
parachutist so hard when deployed as to be injurious). If the
parachutist has not been slowed down to the new terminal velocity with
the open parachute when he or she comes into contact with the ground,
he or she may hit the ground much harder than is necessary.
2. Please check the search engine on our site for "Magnus force" and you'll get a good description of why curveballs curve.
(republished on 07/30/06)