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Fall velocity, strange answer.

  • Thread starter Kamiden
  • Start date
  • #1
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Homework Statement



  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?

Homework Equations


Drag Force = (p*Cd*A)/2*v^2
Velocity = d/t
Weight = m * 9.81
Acceleration = W - Df

The Attempt at a Solution


Seen above.
 
Last edited:

Answers and Replies

  • #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.
 
  • #3
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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.
 
  • #4
Charles Link
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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.
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.
 
  • #5
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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?
 
  • #6
Charles Link
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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?
Projected area would give a suitable estimate. I do think any kind of free fall estimates through the atmosphere is a rather inexact science.
 
  • #7
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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.
 
  • #8
801
173
To convert impact speed to force you would need to know the spring constant of the landing surface.
 

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