# Fall velocity, strange answer.

## 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

Seen above.

Last edited:

## Answers and Replies

Charles Link
Homework Helper
Gold Member
2020 Award
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.

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.

Charles Link
Homework Helper
Gold Member
2020 Award
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.

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?

Charles Link
Homework Helper
Gold Member
2020 Award
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.

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.

To convert impact speed to force you would need to know the spring constant of the landing surface.