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EDIT: guys, I am not a physicist and it is not my study field, So even if you see my question is silly to be answered, I hope you can give me a clue.

I hope you are doing well.

I have some questions, I hope you can answer me about.

in parachuting, when parachutist is moving vertically(up to downward), the forces affect him are the gravity and the drag (as I have learned), the parachute is designed to maximize the drag force, but where is the lift force, why it is ignored, I think there must be a lift force but on the places I have looked in, None mentioned it.

if it exists, should I add it to the forces that affect the parachute motion:

$$F = FG - (FD + FL)$$

and if the parachute moves horizontally and vertically at the same time:

the drag force affects the parachutist from down to up and from left to right(for example), and the lift force affects it from down to up.

if So, what are the forces equations in both (x and y axis).

I hope I have cleared my thoughts.

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  • $\begingroup$ Have you learned how lift and drag are defined? $\endgroup$
    – D. Halsey
    Commented May 8, 2023 at 19:15
  • $\begingroup$ Hi @D.Halsey I am looking to learn from you. $\endgroup$
    – f.alnayef
    Commented May 8, 2023 at 19:23
  • $\begingroup$ I already gave you a clue, but I guess you need a stronger one. How are the lift and drag directions related to each other? $\endgroup$
    – D. Halsey
    Commented May 8, 2023 at 19:58
  • $\begingroup$ Hi @D.Halsey again, thank you for replying, as I know the drag force is opposite to the direction of motion, and the lift force is perpendicular on the direction of motion, which means the lift force is only to up and the drag force could be any direction depending in the motion of the parachutist, I am sorry, but I hope you would correct me if am wrong $\endgroup$
    – f.alnayef
    Commented May 8, 2023 at 20:13
  • $\begingroup$ If the motion is downward, then the lift force would be horizontal, not up. $\endgroup$
    – D. Halsey
    Commented May 8, 2023 at 22:36

2 Answers 2

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With the traditional round parachute falling straight downward in the airmass, there is no separate lift force. The only force opposing gravity is the drag force from the downward motion.

In physics 101 problems, this simple model is usually what is assumed to allow the basics to be studied. Lift is ignored because it is not part of the model.

If you move on to other types like ram-air/parafoil chutes, those do generate significant lift from their forward motion through the air (and the drag forces become more complex). A full analysis of this motion would be closer to that of a glider than a round parachute. A reasonable model of such devices would not ignore lift.

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  • $\begingroup$ that's exactly what I was looking for!!, thanks a lot FRIEND $\endgroup$
    – f.alnayef
    Commented May 8, 2023 at 20:34
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You can think of drag force as a form of friction, it opposes direction of relative motion. Therefore, if there is no initial velocity in the x direction, there is no drag force acting that direction.

Usually we write drag force in the form :

$F_{drag} = -bv^2$
(where $v$ is a vector quantity)

I guess you can try to write the force equations yourself now :)

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  • $\begingroup$ thnx, but the main question is not about the specific formula, but it is about the lift force, why it is ignored why we do not add it to the affecting force (even if it is small) $\endgroup$
    – f.alnayef
    Commented May 8, 2023 at 19:42

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