Projectile motion + Air resistance equation?

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Discussion Overview

The discussion revolves around the equations and methods used to calculate the range of a projectile under the influence of air resistance, contrasting it with the simpler case of projectile motion without air resistance. Participants explore the complexities introduced by air resistance and the mathematical models that may be employed in simulations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant shares a link to a web program that calculates projectile motion without air resistance and seeks clarification on how it accounts for air resistance.
  • Another participant suggests that understanding the direction of the resistance force is crucial and recommends applying Newton's kinematic equations separately for the x and y directions, considering the effects of air resistance.
  • It is noted that air resistance is not constant and varies with projectile velocity, with some proposing that the drag force can be modeled as either proportional to velocity (F drag = C * v) or to the square of velocity (F drag = C * v^2), with the latter being a better fit according to experiments.
  • A participant explains that the nonlinear nature of air resistance necessitates numerical solutions to the differential equations governing projectile motion, involving iterative calculations of drag and acceleration until the projectile reaches the ground.
  • Discussion includes the choice of resistive force model (linear vs. quadratic) and references to external resources that provide differential equations for both cases.
  • One participant mentions that the web program likely uses either a linear or quadratic model for air resistance and discusses the implications of each model on analytical solutions.

Areas of Agreement / Disagreement

Participants express varying opinions on the appropriate model for air resistance, with no consensus reached on which specific equation or method the web program employs. The discussion remains unresolved regarding the exact nature of the air resistance used in the simulation.

Contextual Notes

Limitations include the dependence on the chosen model for air resistance and the potential for other forms of drag not discussed. The discussion also highlights the complexity of solving the equations of motion under varying conditions.

Thaer
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Take a look at this little web-progarm
http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/ProjectileMotion/jarapplet.html

I know that it uses the following equation to calculate the range of the projectile without air resistance
R = (v^2 sin2(theta))/g
R - Range
v - intial velocity

But I have no clue how it calculates range of the projectile with air resistance.
Does anyone know what equation(s) the program uses to calculate the range of the projectile with air resistance (air resistance enabled in the program)
 
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You just need to know the direction(s) of the resistance force.Assuming the projectile takes place in x-y plane and the air resistance force is given by:

[itex]F_r=F_x i + F_y j[/itex]

You need not remember the formulae, just apply Newton 'kinematic equations' in x direction and y direction separately , in this case taking into consideration the 'acceleration/decelleration due to air resistance force'

BJ
 
Thaer said:
But I have no clue how it calculates range of the projectile with air resistance.
Does anyone know what equation(s) the program uses to calculate the range of the projectile with air resistance (air resistance enabled in the program)

Air resistance does not fit into a simple equation. Force of air resistance is not a constant, it changes as a function of projectile velocity. You can assume

F drag = C * v or F drag = C * v^2

C is the coefficient of drag for an object. It depends on numerous variables.
Experiments show that the later relationship is a better curve fit. You can bet that the equation(s) they used are formulated on a combination of theoretical and experimental results. You might consider going to the source and asking them.
 
As Pete explained, air resistance is a nonlinear function of velocity, so it is time varying. The only way to solve the differential equations of the movement is numerically. You attribute an initial value to the velocity, calculate the drag and from that the accelerations in the horizontal and vertical directions. You give a small time increment and calculate the new value of the velocity. Work iteratively until you find that the height is zero. The horizontal distance at that instant is the range.
 
The model (differential equation) of the projectile motion will depend on the relationship between the resistive force and the velocity of the projectile that is chosen. Will it be f=k v, f=k v_squared? Wikipedia has some relevant comments in this area and the site at math.fullerton.edu/mathews/n2003/ProjectileMotionMod/ (continues beyond my page) has Mathematica programs which contain the differential equations for the f=k v case and the f=k v_squared case.
 
Hi Thaer,

The web-program you've mentioned very likely uses the air resistance as proportional to v or v^2, as has been mentioned by others. For the kv, you get a nice analytical solution in 2-d. Not so in the kv^2 case. But when the object is thrown vertically upward, both equations can be integrated quite easily in 1-d.

All the parameters like height, time etc are shown in the program. From this, after solving the kv or kv^2 case analytically, it will be possible to say which one matches your results when you put 90 deg in the program. Maybe the air resistance is not either of these in the program, but some other power or function of v.

-----------------------------------------------------

Hi Dr.Brain,

Don't we all wish that things were just always so simple...
 
Last edited:

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