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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    $\begingroup$ What happens to a skydiver who has reached terminal velocity and then opens his chute, giving himself a lower terminal velocity? $\endgroup$ – DJohnM Sep 24 '13 at 3:24
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    $\begingroup$ 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.) $\endgroup$ – Michael Brown Sep 24 '13 at 3:46

Yes. The object will slow down to its terminal velocity if its speed starts higher than its terminal speed.

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, I have assigned "up" to be the positive direction, and $g$ is the magnitude of the acceleration due to gravity near the Earth's surface. When the drag force is greater than $mg$, the net force will be positive, and by Newton's Second Law, the object's acceleration will point upward and will slow the object down. This will reduce the drag force until the drag force and weight are equal, the object's acceleration is zero, and the object will have constant speed -- it's terminal speed.

  • $\begingroup$ You got fancy and included the equation... $\endgroup$ – tpg2114 Sep 24 '13 at 3:29
  • $\begingroup$ Wow! That's insane to think it will actually slow down!! $\endgroup$ – user29730 Sep 24 '13 at 3:35
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    $\begingroup$ @user29730: that's exactly what happens to a spaceship when it re-enters Earth's atmosphere. $\endgroup$ – John Rennie Sep 24 '13 at 6:41
  • $\begingroup$ @JohnRennie that will be gravity assist crash or slingshot without air. $\endgroup$ – user28737 Sep 24 '13 at 8:06
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    $\begingroup$ And its probably happens to a skydiver hundreds of times in each jump - when they shift position to a slightly less streamlined position they slow down, then with another tiny change of body position they will speed up again. $\endgroup$ – bdsl Oct 20 '15 at 23:54

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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