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Homework Help: Fall velocity, strange answer.

  1. Jun 13, 2016 #1
    1. The problem statement, all variables and given/known data

    1. I'm trying to write a program to calculate newtons of impact force from a fall given the weight, general size, shape, and density of an object. I want to take air resistance into account. I also want to take the density of the thing the object falls on into account. I am having trouble figuring out why a 200 lb object falling 200 ft is going ~23 m/s at the point of impact. This is the loop I am using.

    2. For reference:
    3. accel = acceleration, which is set to 9.81 m/s^2 originally.
    4. densitya = density of air, set to 1.2754 kg/m^3
    5. drag = drag coefficient, set to .6 for the shape.
    6. area = area of the object, set at .58 m^3
    7. dragf = drag force in newtons
    8. weight = weight. Mass in kg * 9.81
    9. mass = mass in kg, 90.7185
    10. rheight = remaining height. This is in meters, and is set to 60.96

    11. while (rheight > 0)
    12. {
    13. time = time + 0.01;
    14. velocity = velocity + (accel*.01);
    15. dragf = ((densitya*drag*area)/2)*(velocity*velocity);
    16. accel = (weight - dragf)/mass;
    17. rheight = rheight - (velocity*.01);
    18. }
    19. cout << velocity << " " << time;

    20. Am I getting a formula wrong, is the code bugged, or is it something else?
    2. Relevant equations
    Drag Force = (p*Cd*A)/2*v^2
    Velocity = d/t
    Weight = m * 9.81
    Acceleration = W - Df

    3. The attempt at a solution
    Seen above.
    Last edited: Jun 13, 2016
  2. jcsd
  3. Jun 13, 2016 #2

    Charles Link

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    I think I found your error in line 19. It should read rheight=rheight-velocity*.01. A first test of the program would be to try setting the drag force equal to zero and see if you get a good answer.
  4. Jun 13, 2016 #3
    Thank you. That was causing error, but now I am having a different problem. I have no idea what to use for area. Equations for shapes are easy, but what about for a human in various positions free-falling? (It's a fall damage calculator for a game)

    The main ones I'd need to know is kneeling position, prone, and swan dive.
  5. Jun 13, 2016 #4

    Charles Link

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    Try reading about skydiving/parachuting. I think with arms spread out, etc., a free fall reaches a limiting velocity of about 100 m.p.h. , but if you google it, you might find the info you need.
  6. Jun 13, 2016 #5
    I tried that. I tried that a lot. The best I get is to find the cross-sectional area. I have no idea how to find the area on a freeform object. Would that just be looking from directly below?

    Could I use paint and pixels to feet as a proportion?
  7. Jun 13, 2016 #6

    Charles Link

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    Projected area would give a suitable estimate. I do think any kind of free fall estimates through the atmosphere is a rather inexact science.
  8. Jun 13, 2016 #7
    Estimates are fine as long as they're at least mostly realistic in this case. Thank you for your help. I got it from here.
  9. Jun 20, 2016 #8
    To convert impact speed to force you would need to know the spring constant of the landing surface.
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