# Motion under gravity

by Gughanath
Tags: freefall, gravity, motion
 P: 118 I read a question about motion under gravity. It says that two parachut divers jump of a plane, and after a few seconds, they reach terminal velocity. Now, how is possible to have a constant velocity, if someone is falling under gravity (accelerating constantly)?
 Mentor P: 41,489 Because gravity is not the only force acting on the divers! The parachute allows the air to exert an upward force. When the divers reach terminal speed, the force of gravity (down) is exactly balanced by the air resistance (up). Thus: no net force, no acceleration.
 P: 4,777 The air acts as a retarding force. This force is dependent on velocity. At some point, the velocity will be so great that the force due to the air friction, or drag, will be equal and opposite to gravity. From then on the forces are balanced and they have reached terminal velocity. Darn Doc Al, you beat me to it by a split second
 Sci Advisor HW Helper PF Gold P: 12,016 Motion under gravity It's not possible; it is the increasing air resistance which eventually balances the force of gravity so that no NET force acts on the parachuter. So, it is definitely NOT a free-fall situation. EDIT: I'm getting old and feeble..not one preposter, but TWO!
 P: 118 So, first the divers falls under greavity, but eventually, the air resistance balances out the force of gravity, there the divers falls with constant velocity, since there is not net force acting?
Mentor
P: 22,305
 Quote by Gughanath So, first the divers falls under greavity, but eventually, the air resistance balances out the force of gravity, there the divers falls with constant velocity, since there is not net force acting?
Correct.

Now what would happen if you jumped out of a balloon at 100,000 feet where the atmosphere is so thin it may as well not be there (this has been done)?
 P: 1,544 Gughanath Another part of your question is – Are you weightless? A good way to demonstrate both of these yourself is come on out to a SkyDive Center and make a few jumps. If you use large plastic pop bottle vented but not so air will flow through it, and rig a spring stretched out by a weight when held vertical inside it. It can be a bit tricky holding it vertical while exiting against the plane’s prop wash but the spring will pull the weight all the way up the moment you exit the plane. That’s because you’ve started your Freefall – Gravity will get you going til you reach terminal velocity in about 10 to 15 seconds. In prone postion that is about 120 miles/hr when your weigth is balanced out by the air cushion or “Drag” . And you can show that you really do have weight at this speed by checking out the weight protected from the wind in your bottle, It has by now completely stretched out the spring again. And you can boost that speed by manuvering to a head down or feet down position! With the lower Drag you can get up to at least 180 Miles/hr but after the 5 seconds or so it takes to get there – the spring will again feel a full stretch by the weight. Althogh you may think ya ‘feel’ weigthless while in Freefall – this will prove you are not. (This also makes a great bar-bet as many a skydiver thinks they were wiegthless). Note: Don't forget your chut as your TERMINAL VELOCITY >> your TERMINAL IMPACT SPEED --- unlike ant's and most small insects where there TERMINAL VELOCITY >> insect TERMINAL IMPACT SPEED - lucky little guys get all the fun. OK not up for going out of an Airplane – do a search on Indoor Skydiving. Also – I have a First Cover on Joe Kittinger as first to Solo ballon accoss the Atlantic, and his info says he was also first man to go supersonic in a freefall by jumping from 102,800 feet. RandallB

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