If an object is moving at terminal velocity and then opens the parachute, the air resistance is higher than the weight so why doesnt he go upwards.

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    $\begingroup$ This misunderstanding is tightly coupled with that in your previous question. They are both resolved by understanding that force is related to acceleration and not to velocity. So, if you are not clear on the exact relationship between acceleration and velocity it would be worth your time to focus on that difference for a while and then re-visit these force questions after you have mastered the kinematics issue. $\endgroup$ – dmckee May 6 '18 at 17:48
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    $\begingroup$ You may have seen film or video in which a free-falling sky diver pulls the rip cord and suddenly seems to shoot up into the sky. But that's only because the one who pulled the rip cord suddenly was falling much more slowly than the skydiver who was holding the camera. From the camera's point of view, the one who pulled the rip cord seems to be going up, but from the point of view of a person standing on the ground, they're both still falling down. $\endgroup$ – Solomon Slow May 6 '18 at 18:06

Because force and motion aren't always in the same direction. Suppose you slam on the brakes in your car. Your car doesn't instantly go backwards. Instead, it accelerates backwards.

When a skydiver opens their parachute, they accelerate upwards. This is not the same as saying the move upwards.

Acceleration is a change in the velocity. So the amount of downward velocity the skydiver has gets smaller and smaller, until they reach a new terminal velocity. During this time, the change in their velocity is upwards.

If we illustrate the skydiver's velocity in red and force and acceleration in blue, we might get something like this:

enter image description here

Time goes from left to right. The skydiver opens their parachute and feels and upward force. It pushes upward on them, but isn't enough to completely stop them and make them move upward. Instead, it just slows them down a bit as they continue falling downward. They keep feeling an upward force (while moving downward) that slows them until they get to a new terminal velocity.

Note that this picture shows the velocity arrows next to each other in space; in reality, the skydiver gets lower and lower in space as we move to the right, but that was harder to draw.

  • $\begingroup$ In your diagram, why doesn't the acceleration decrease to zero as the velocity decreases? Instead, it seems to increase. $\endgroup$ – sammy gerbil May 7 '18 at 17:07
  • $\begingroup$ That's because it's just a rough sketch; it's not intended to show as much detail as you're asking about. $\endgroup$ – Mark Eichenlaub May 8 '18 at 1:01

Force can oppose motion. Parachutes provide upward acceleration but the acceleration downwards is much larger in the bigger picture. This is not the same as saying the move upwards; parachutists acceleration > acceleration due to parachutes.

Acceleration is a change in the velocity**

downward velocity eventually decreases to match air resistance and they reach a terminal velocity phase where both upward and downward forces are equal and the parachutist will fall at the same rate.

unless there's an upward force that's much larger than the downward force on a stationary object, the object will not move upwards.



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