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I'll take the sky diver example. Right when a sky diver jumps out of the plane the net force of the diver is greatest. Then the diver begins to reach terminal velocity where the force of resistance = the force of gravity. So while the force of air resistance increases to equal the force of gravity is the sky diver decelerating?

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    $\begingroup$ Hi DataTx, and welcome to Physics Stack Exchange! Have you tried anything to figure this out yourself? What's your level of knowledge about Newtonian mechanics? $\endgroup$
    – David Z
    Nov 11 '15 at 5:38
  • $\begingroup$ Rate of change of velocity is positive while Rate of change of acceleration (jerk) is negative. $\frac{dv}{dt}>0$ $\frac{da}{dt}<0$ $\endgroup$
    – Sathyaram
    Nov 11 '15 at 6:30
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Does an object decelerate when reaching terminal velocity

No, it ceases to accellerate.

So while the force of air resistance increases to equal the force of gravity is the sky diver decelerating?

Consider $F = ma$. You say because $F_r = -F_g$, net $F$ is zero. What does that imply about $a$?

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God No. Here is why: Deceleration is a decrease in velocity, i.e dv/dt is negative. With terminal velocity,it just becomes zero. You can see the same thing if you plot a graph. For the skydiver's Velocity -time graph, the slope of the curve( acceleration) is NEVER negative. It becomes zero at terminal velocity.

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No, unless you shoot the object downwards with a cannon providing it with an initial velocity higher than the terminal one.

Regarding your example, sky divers do decelerate. In groups they use different (maybe unstable) positions in order to change their resistance to air and thus reaching speeds higher that the terminal one (in some stable position, like freefall) in order to join their friends to build nice patterns. They can do it easier with special suits called wingsuits.

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It depends.

Usually the skydiver starts from zero vertical speed, the resultant force is downwards and his/her speed increases (ie there is acceleration) until terminal velocity is reached.

At this time the drag force equals the skydiver's weight. However if the skydiver then opens the parachute the drag force suddenly increases so that the resultant force is upwards whereas the skydiver is still moving downwards. The skydiver's speed decreases (ie there is deceleration) until a new lower terminal velocity is reached.

Likewise a bullet dropped into water will accelerate to reach terminal velocity whereas one which is fired into water from a gun will decelerate to reach terminal velocity.

So there can be acceleration or deceleration towards terminal velocity depending on whether your initial velocity is below or above terminal velocity respectively.

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