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Part of the second law states that an object will accelerate in the direction of the unbalanced force.

However, I've been thinking about parachuting and skydiving a bit lately, and I know that when the skydiver opens their parachute, the air resistance becomes greater than the force of gravity.

How does the person not travel upwards, in that case?

Qmechanic
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Admin Voter
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  • I've looked around and this might be it: in theory the diver should accelerate up, however since the diver was already accelerating so fast due to gravity, the air resistance force is only sufficient enough to slow down the movement. I'm not 100% sure if that's correct though. – Admin Voter Jan 04 '15 at 19:37
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    @AdminVoter Yes, after deployment of the parachute, the drag will initially be very high (reducing speed), then lower until it equals gravity. From then on, the speed is roughly stable. – Řídící Jan 04 '15 at 19:42
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    When you slam on your brakes in a car, do you travel backwards, or accelerate backwards? – DJohnM Jan 04 '15 at 20:04
  • That's an insanely good analogy. – Admin Voter Jan 04 '15 at 21:29

1 Answers1

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Drag, as a force, is proportional to velocity squared. If you are not moving, there is no drag, and you are free to move again.

Yes, when the parachute first opens, the velocity of the person will generate a drag greater than the force of gravity. As the velocity decreases, the drag force decreases faster. Therefore, the person's velocity will decrease until the force of the drag equals the force from gravity. So unless there is an updraft, the person will never stop falling (zero velocity), but their velocity could be close to zero.

This is similar as 'terminal velocity'. The drag from the person prevents their velocity from increasing until they encounter the ground.

LDC3
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