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# Re: [Phys-L] simple Mathematica question (two typos fixed)

I’m not familiar with the syntax for Mathematica, but an easy way to avoid the fact that the derivative of the square root function does not exist at zero is to convert your equation into the system y’=v and v’=g.

On Dec 8, 2020, at 8:54 AM, Sam Sampere via Phys-l <phys-l@mail.phys-l.org> wrote:

﻿You've proven that Wile E Coyote was correct after all!

-----Original Message-----
From: Phys-l <phys-l-bounces@mail.phys-l.org> On Behalf Of Carl Mungan via Phys-l
Sent: Tuesday, December 8, 2020 11:23 AM
To: PHYS-L <phys-l@phys-l.org>
Cc: Carl Mungan <mungan@usna.edu>
Subject: [Phys-L] simple Mathematica question (two typos fixed)

Since people seem to be in a talkative mood today, maybe somebody can help me a with a simple (?) matter. It involves dropping a rock. I’m using Mathematica but you don’t have to be an expert in that to see the issue.

If we use conservation of mechanical energy, then we find the downward speed of the rock (choosing upward to be +y with the ground at y=0) is v_y = - Sqrt[2*g*(y-y0)].

Suppose we choose y0 = 100 m and g = 9.8 m/s/s and ask Mathematica to do a numerical solution from t = 0 to 4 s:

s = NDSolve[{y'[t] == -Sqrt[2 9.8 (100 - y[t])], y[0] == 100}, y, {t, 0, 4}]

Plot[Evaluate[y[t] /. s], {t, 0, 4}]

Can you guess what happens here? It’s a failure. The rock just sits at rest in the air at 100 m.

If I instead start the evaluation at 99 m height, so the rock has already fallen 1 m, then it works just fine:

s = NDSolve[{y'[t] == -Sqrt[2 9.8 (100 - y[t])], y[0] == 99}, y, {t, 0, 4}]

Plot[Evaluate[y[t] /. s], {t, 0, 4}]

But I want to start the rock at 100 m. How do I nudge the rock so it starts falling?

I must be missing something. My actual problem is like this, but more complicated. The above is just a test case for debugging purposes.

Who can tell me what I need to do? -Carl

-----
Carl E. Mungan, Professor of Physics 410-293-6680 (O) -3729 (F) Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu <mailto:mungan@usna.edu> http://usna.edu/Users/physics/mungan/ <http://usna.edu/Users/physics/mungan/>
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