Fundamentals of Iterative Prediction with Varying Force

It is rare to have a force which is perfectly constant, and iterative analysis of more realistic varying-force systems is substantially more complicated. A toy model demonstrates how programs may be written to analyze these systems.

Main Idea

The physics of iterative prediction with varying force is the same as for prediction with constant force, but it is necessary to generalize the mathematical expressions, which adds complexity to the code.

A Mathematical Model

To begin with, consider a one dimensional force, which may vary with both as a function of time and/or dependent variables such as position and velocity. Then we write this as [math]\displaystyle{ F(t,x,v) }[/math]. Now, using the momentum principle, we know that [math]\displaystyle{ F = \frac{\text{d}p}{\text{d}t} }[/math], which in discrete terms is [math]\displaystyle{ \Delta p = F\Delta t }[/math].

Just as with a constant force, this lets us write out for some iteration at [math]\displaystyle{ (t_0,x_0,v_0) }[/math] that

[math]\displaystyle{ p_{final} = p_{initial} + F(t_0,x_0,v_0)\Delta t }[/math]

Which we combine with kinematics to produce a new set of variables [math]\displaystyle{ (t_1,x_1,v_1) }[/math]. The difference we now have is that whereas before [math]\displaystyle{ F(t_0,x_0,v_0) = F(t_1,x_1,v_1) }[/math], we must now recalculate [math]\displaystyle{ F(t_1,x_1,v_1) }[/math] using the relevant formula. This will take the form of an extra step in each iteration. It is important to note that although we write the force as a function of all of these variables, in most cases it will only depend upon one of them. In a spring, for example, we will see that [math]\displaystyle{ F(t,x,v) = F(x) }[/math], meaning that only the position is necessary to compute the force.

A Computational Implementation

Code for an implementation of varying force iterative prediciton can be found in this Google colaboratory jupyter notebook. The following code snippet covers the physics calculation portions:

def calcmotion(self): #this is the method of the class which does the actual computation
   #First, we want to know how many steps we will calculate, and round this to an integer
   #Ideally, the user will make dur%tstep = 0, but we can't be sure, so we need to sanitize the input
   numsteps = int(self.dur/self.tstep) #the use of self. calls these quantities which are defined elsewhere
   
   #Now we make an array to hold all of our data. For more information on numpy arrays, see 
   #https://docs.scipy.org/doc/numpy/user/basics.types.html
   #It will have 5 columns and numsteps number of rows
   #The columns are, in order, time, position, momentum, velocity, and force
   #np.zeros takes a tuple for the shape of the array, and the optional argument dtype for the data type
   #The data type will default to float, which is what we need, so this argument is not passed
   self.data = np.zeros((numsteps,5),dtype=float)
   #Now we perform the actual work of iterating:
   for i in range(numsteps): 
     #Here we compute the time stamp of each step, but considering the iteration number and the timestep width
     self.data[i,0] = i*self.tstep
     #If this is the first step, we set our initial position and momentum
     if i ==0:
       self.data[i,1] = self.xinit
       self.data[i,2] = self.pinit
     #Otherwise, we use kinematics to compute a new position, and the momentum principle to compute a new momentum
     else:
       #xfinal = xinitial + v*delta t
       self.data[i,1] = self.data[i-1,1] + self.data[i-1,3]*self.tstep
       #pfinal = pinitial + F*delta t
       self.data[i,2] = self.data[i-1,2] + self.data[i-1,4]*self.tstep
     #We compute velocity using v = p/m
     self.data[i,3] = self.data[i,2]/self.mass  
       
     #Finally, we compute the force at our new position
     self.data[i,4] = self.forcefunc(self.data[i,0],self.data[i,1],self.data[i,2]) 
   return self.data

First, time data is computed by simply iterating over each time step, since we are using even steps. Next, if this is the first iteration, initial position and momentum data is input. If it is not the first iteration, then the momentum and position of the previous step are read to determine the new position, and the force and momentum of the previous step are read to determine the new force. Finally, the velocity column is filled out using the definition of momentum, and the new force is computed using the user defined force function.

The rest of the notebook consists of the code used to define these functions, and to view the results, both as plots and as animations.

Examples

Simple

Imagine a force defined by

[math]\displaystyle{ F=kx }[/math]

Where [math]\displaystyle{ k = 3 N/m }[/math]. Now, assume it has an initial position of [math]\displaystyle{ x_0 = 5 m }[/math], and an initial momentum of [math]\displaystyle{ p = 0 }[/math]. After a time step of [math]\displaystyle{ 0.2 s }[/math], what will be its new momentum according to our model?

Solution

We have the formula [math]\displaystyle{ \Delta p = F \Delta t }[/math]. We may compute [math]\displaystyle{ F = (3 N/m)(5m) = 15 N }[/math], so plugging this in gives [math]\displaystyle{ \Delta p = (15 N)(0.2 s) = 3 N\cdot s = 3 \frac{kg\cdot m}{s} }[/math]