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If an object is say thrown down (vertically) at an initial speed that is faster than its terminal velocity, what would happen to that objects speed? Would it slow down?

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What happens to a skydiver who has reached terminal velocity and then opens his chute, giving himself a lower terminal velocity? –  DJohnM Sep 24 '13 at 3:24
Home experiment: 1) Go get the free demo version of Kerbal Space Program (not affiliated), 2) Build a rocket, 3) Launch it straight up - don't even worry about getting an orbit, as long as you get 100 or so km up, 4) Watch the speed as it falls back down, especially when the light show starts. :) (Caveat: the physics of KSP is far from perfect, but it is good enough for this question... and to teach you basic orbital mechanics as well.) –  Michael Brown Sep 24 '13 at 3:46

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Yes, the object would slow down to its terminal velocity. To see why, notice that the net force on a falling object of mass $m$ near the surface of the earth is \begin{align} F = F_\mathrm{drag} - mg \end{align} where $F_\mathrm{drag}$ is the force due to air resistance, and here I have assigned "up" to be the positive direction. On the other hand, the terminal velocity occurs when the force of drag equals $mg$. When the force of drag is great than $mg$, then then net force $F$ will be positive, and by Newton's Second Law, its acceleration will therefore point upward and will slow down the falling speed. This will happen until the object attains terminal velocity at which point the drag force and weight will again balance, and the object will remain at terminal velocity.

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You got fancy and included the equation... –  tpg2114 Sep 24 '13 at 3:29
Wow! That's insane to think it will actually slow down!! –  user29730 Sep 24 '13 at 3:35
@user29730: that's exactly what happens to a spaceship when it re-enters Earth's atmosphere. –  John Rennie Sep 24 '13 at 6:41
@JohnRennie that will be gravity assist crash or slingshot without air. –  Waqar Ahmad Sep 24 '13 at 8:06
@WaqarAhmad: when the spaceshuttle re-entered the atmosphere it was primarily atmospheric drag that slowed it. –  John Rennie Sep 24 '13 at 8:15

Terminal velocity results from a force balance between the falling body and the drag force acting upon it to slow it down. In other words, the terminal velocity is the speed at which the gravitational force is equal to the drag force (assuming the body is unpowered).

So, if the initial velocity is larger than the terminal velocity, the drag force will be larger than the gravitational force. This results in a net force upwards, which results in an acceleration due to Newton's second law, and the body will slow down. Once it reaches terminal velocity, the forces are in balance and there is no more acceleration.

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