# Chapter 4

## Falling Bodies In a Fluid or Gas Part B - Numerical solution of Differential Equations for Falling Bodies

A fundamental method for finding solutions to non-linear differential equations was provided by Vannevar Bush in:

The Differential Analyzer, V. Bush,

The Journal of the Franklin Institute, Vol. 212, No. 4, October 1931.

He began with a means for providing solutions to the linear form of an equation and then modified the linear form by entering variable coefficients, v and g, using Input Tables. For each input table, an operator turned a crank to keep an index on an applicable plotted curve as the solution proceeded.

For example, where gravity might previously be assumed to be constant through variations in height, it could instead be input as some chosen function of height by using an input table.

It is the Bush method, adapted for spreadsheet use, that is employed herein for finding solutions to non-linear differential equations.

For ease of discussion it will be assumed throughout this topic that falling bodies start from rest and then increase in velocity as influenced by the acceleration of gravity and an opposing drag force.

In the linear form the acceleration acting on the body will be g less some multiplier of velocity.

### The Linear Form

a = g - k * v = δv/δt    (1)

This form is derived from Newton's second law, the force on the body F = m * a = m * g less a factor due to the resistance presented by the medium. That factor is termed a drag force, Fd.  Dividing though by m, we find:  a = g - Fd/ m.

Stokes found for a body moving slowly, creeping, in a viscous fluid that the drag force could be taken as proportional to the velocity of the body.

As numerical methods are presumed, employ finite but small steps of change that can be represented with symbols such as ∆, or D.  We then may write:

a =  Dv/Dt

with the understanding that step size can be chosen small enough to meet the precision requirements of an application.

Presume that one wishes to calculate the values that falling velocity, v, and fallen distance, s, take on at successive small intervals in time, t, after a 1 kg body is dropped and that we know the values g ~= 10 metres per second and k =2.   Use a pen and paper for the calculations.

Decide to calculate at 1/10-second intervals.  Divide the paper into 5 columns headed in the first row by t, Dv, v, Ds, and s.  To begin, populate the second row with the initial values, all zeros.  Next populate the remainder of the first column with the successive values of the time t at which results are desired.

After fall has begun the body would normally arrive at the beginning of a calculation step with a non- zero velocity.  That velocity happens to be zero in the first step.  The equation for acceleration, (1),

(g - k * v) = Dv/Dt

provides that:

Dv = Dt * (g - k * v).  Then, for the first step Dv = 1.0.

From here it is easy going.  v is formed as the accumulation of the small steps Dv. Each value of v in a step moves the body an incremental distance Ds = v * Dt.  Values of Ds are accumulated, negatively, to form s.

 t Dv v Ds s 0 0 0 0 0 0.1 1.0 1.0 0.1 -0.1 0.2 0.8 1.8 0.18 -0.28

For the next row and successive rows of calculation the procedure is unchanged.

The foregoing method of solving a differential equation using pen and paper is fundamentally the same as was used by early astronomers to calculate planet orbits, planet masses and planetary distances.  Some of those calculations took a great many days.

Today, although hand calculators have been used to relieve the tedium of pen and paper calculation, the spreadsheet is better suited to the task.  The spreadsheet version, with graphs, is shown next. Click the cell addresses following for their expressions.

 . .
 C7 D7 E7 F7 G7

### Reynolds Number

There can be a question as to which model, Stokes or Lord Rayleigh's, should be used for the drag force. The essential distinguishing feature is whether the viscosity, μ, of the medium plays a role in the drag. Viscosity appears in the Reynold's numberR = p * v * r /μ , for a sphere
, (See Classical Mechanics, R. Douglas Gregory, Cambridge University Press, 2006, pp83-85.) where r is the radius of the sphere.  The reference concludes that the drag force is roughly proportional to v2 for 1,000 < R < 100,000.

Since R is proportional to velocity, and in a case where there is no initial velocity, one must conclude for a region near the very beginning of fall in air, that the Stokes model applies.  If the terminal velocity were sufficiently high there would be a transitional period followed by a region in which the Rayleigh model should be used.

The aforementioned reference provides values for the ratio μ/p for air, water, and castor oil.  The given values are: 1.5 * 10-5, 1.0 * 10-6, and 1.04 * 10-3, respectively.

### Resistance Proportional to the Square of Velocity

In this case the equation for the acceleration of the body becomes:

a = g - k * v2 = δv/δt     (2)

All that is needed to accommodate this form of the equation is to change the expression in column D from =(D\$3-F\$3 * E6) * (C7-C6) to =(D\$3-F\$3 * E62)*(C7-C6) for row 7 and subsequent rows. Calculated values will then change.

### Is there a Source for Crosschecking?

This author has only located one other numerical solution of this equation in the literature. That was in:  Euler's Method and Studies of Motion, 11-12,Source Book for Programmable Calculators, Texas Instruments Electronics Series, McGraw-Hill, 1979.  That solution was calculated at 1.0 second intervals with g = 9.8 and k = 0.005 for a 1 Kg mass.  Accordingly, Dt, g and k were changed in the spreadsheet to these values.   At t = 20 seconds, the Texas Instruments, TI, velocity given was 44.27 m/s in good agreement with the value attained in the spreadsheet, 44.2706.  The fall distance found by TI was 760.594 m. That attained in the spreadsheet was 781.511 m.  No details of the TI algorithm were provided.  Clearly the algorithms are different in some way.

Exploration of the differences ascertained that full agreement with the 20 pairs of values, (velocity, fall distance), given in the TI calculator reference could be attained by halving the Dv contribution of the first step, i.e., changing (D\$3-F\$3 * E62)*(C7-C6) to (D\$3-F\$3 * E62) * (C7-C6)/2 in row 7 only.  This then is the difference in the algorithms.

To determine a reasonable step size, trial calculations for the range 0 to 10 seconds were made with eight different step sizes Dt = 2, 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64.  Values attained on each trial for 0, 2, 4, 6, 8, and 10 seconds were then plotted on the same graph to observe their convergence. These plots for v and s are shown next. Only the plots for the four or five largest step sizes appear to be distinguishable.  The fifth of these, 0.125, was chosen for the following wider range plot of v and s. The 20 Texas Instruments pairs of values were superimposed for comparison. ### Terminal Velocity

Return from the acceleration equation to the force equation.  From the force equation:

m * a = m * g  - k * v2

The acceleration equation is: a = g - (k * v2) / m.

Terminal velocity is reached when g =  (k * v2) / m. Thus vt2 =  (g * m) / k.
For the previous example, we used m=1, g = 9.8 and k = 0.005. For these values vt = 44.271887... metres per second.  The velocity attained at 30 seconds in the previous example was 44.2717799, a tiny bit short of vt.

### A Different Formulation

Taking Lord Rayleigh's approximation for the drag force:

Fd = 1/2 * ρ * A * Cd * v2 .
Then using the force equation:

m * a = m * g  - (1/2 * ρ * A * Cd) * v2          or

a =  g - 1/2 * 1 / m * ρ * A * Cd* v2

When terminal velocity is reached:

2 * m * g = ρ * A * Cd* vt2    or:

vt = [(2 * m * g) / (ρ * A * Cd)]0.5.

a = g - [1/2 * ρ * A * Cd* v2]  / m.

Substitute 2 *m *g / vt2 for ρ * A * C to get:

a = g  *  [1 - v2/vt2],

an interesting form of the equation. Of course it is no more than a rearrangement.

Under the approximation, the k seen in equation (2)  is:

k = 1/2 * 1/m * ρ*A*Cd

The Rayleigh approximation requires ρ the density of the medium.  For air near the earth's surface this is about 1.2 kg per cubic metre. A is ~ the area that the body presents to the medium in its direction of travel.  Cd is a number describing the smoothness or streamlining of the body.

### Examples

The web-based, 1D, calculator given at the end of the next chapter and available in the upper row of tabs will be used to provide an example for a buoyant application. As that calculator does not it self account for buoyancy, a method for causing its results to include the effect of buoyancy is now given.

Buoyancy is an upward force due to the mass of the atmosphere that the object displaces. The force equation for a buoyant object is:

F = m * g - m' * g - drag  or   F= g * (m - m') - drag

Dividing the latter force equation through by m, the acceleration equation is then:

a = g * (1 - m' / m) - drag/m

Thus using g' = g * (1 - m' / m) rather than g as input to the calculator will adjust the results to include the effect of buoyancy.

A latex spherical toy balloon inflated with air has a mass of about 0.0145 kg.  Its radius is about 0.13 metres.  Guess Cd as 0.5.  Call this the Light Balloon case. Plug these values into the calculator, using a step size of 0.01.  Use g = 9.82, p = 1.2. Calculate the volume of the balloon as 4 / 3 * Pi * (0.13)^3 =  0.009203,  m' as  0.009203 * 1.2 = 0.01104 and then g' as 9.82 * (1 - 0.01104/.0145 = 2.343.  The calculator's input data is shown next: Press Calculate, and the resulting charts are: Tape a metal weight to the neck of the balloon such as a machine screw with washers added to closely match the balloon's mass thus doubling its mass. Note that when weighted, the balloon falls faster.

Plug this new value for m into the spreadsheet and employ a new value for g'. (g' is recalculated as 6.082 if the mass has been doubled) The corresponding charts are shown next. The light balloon appears to fall more slowly.  Air resistance appears to slow the heavy balloon less than it does the light balloon both for high and low velocities.  But this may not be the full story at the lower velocities.  Recall that at very low velocities we should use the Stokes model.  Would that make any substantial difference to the plotted results?

Use the 1D calculator to see how the falling rates of the two balloons compare when the value for the density p of the atmosphere is reduced.

The story of Galileo finding that an iron ball and a same size wooden ball arrived at the ground at the same when dropped together from the leaning tower is believed by historians to be fiction. Nonetheless the story has inspired a great deal of controversy over the years.

With very little additional research, and the links and material provided in this topic, the viewer should be in a position to estimate the difference in time of hitting the earth of Galileo's cannon and wooden balls.

### Next

The next topic features a web-based calculator, an emulator of an interactive spreadsheet.  It provides the viewer the opportunity to calculate and plot solutions of the Lord Rayleigh form of the falling body differential equation. The viewer may enter parameters and observe the results in a table and as plotted on charts.  When buoyancy is a consideration, the user will need to calculate and employ a g'.

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