Raindrops

Part 4: Modeling Small Raindrops

Here again is out initial value problem: Find v = v(t) so that

We have specific values for g and c, both obtained experimentally: g = 32.2 ft/sec² and c = 52.6 sec^-1.

We will use Euler's Method to calculate approximate values for the velocity v at n equally spaced points in a fixed time interval. We saw in the Limited Population module that the Euler procedure gives a better approximation to the exact solution if n is large rather than small. Thus, for convenience, we set n = 100. Our time interval will be 0 < t < 0.2 seconds -- the reason for this choice is suggested by the slope field in Part 2. Thus the distance between consecutive t values will be Delta-t = 0.2/n = 0.002 sec.

1. Enter the constants and starting values in your worksheet. Calculate v₁, v₂, and v₃ to make sure you understand how the steps start out.
2. Write down the numbers t₀, t₁, t₂, and t₃. Then write a general formula for t_k. Enter this formula in your worksheet.
3. Enter in your worksheeet a general formula for v_k in terms of v_k-1. Check to make sure your formula produces the same starting values as in Step 1.
4. Create and plot all the points (t_k, v_k) for k ranging from 1 to n = 100.
5. Check your results by overlaying the solution plot on the slope field from Part 2.
6. There is something different in this graph -- something that did not occur in the model without air resistance in Part 1. Describe the difference.
7. Estimate the limiting value of the velocity as time increases. This is called the terminal velocity. Express your answer in both feet/sec and miles/hour.
8. Compare your terminal velocity with what you obtained in Part 1 as the velocity when a raindrop hits the ground after falling 3000 feet. Which model seems more reasonable?
9. As t increases and velocity v approaches terminal velocity, what happens to the slope of the velocity versus time curve? What happens to the derivative dv/dt?
10. Using your answer to the preceding question, calculate the terminal velocity directly from the original differential equation, dv/dt = g - c v.
11. As you have seen, a drizzle drop approaches its terminal velocity quite rapidly. Estimate the time it takes the drop to fall to the ground from 3000 feet by assuming that the velocity is the constant terminal velocity during the whole duration of the fall. How does this time compare to your time-of-fall answer in Part 1, where no air resistance was assumed?

Last modified: October 14, 1997