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26 October 2017

Vertical motion simulator for changing g

 Screenshot from vertical motion simulatorat computercow.net
One of my favorite early-season assignments asks how much higher or lower a ball will fly when g doubles.  Most students recognize, or guess, that doubling g leads to a halved maximum height.  To their chagrin, they also discover that doubling both the launch speed and the gravitational field does NOT lead to "canceled out" effects and the same maximum height.*

* ...because height goes as launch speed squared.

Even in the first weeks of class, my students expect experimental verification of mathematical predictions, especially counter-intuitive predictions.  But, I can't easily take my PASCO projectile launcher to a planet with gravitational field 2g.

And here, then, is a perfect place to insert an animated simulation.  I stay away from simulations because they're emphatically NOT real.  Physics is in the business of predicting how the natural world works, not how programmers make it seem to work.  Nevertheless, appropriate programmed simulations can be useful for giving students a feel for experiments that can't be quickly or easily set up in lab.

Problem is: I've never found a free-fall simulation that allows me to change both g and the initial vertical speed with which an object is launched.  There are some wonderful dropped-ball simulators, and others that do a good job with projectiles.  But nothing purely vertical with a varying g and v0.  If you know of a free simulation that does what I want, please let me know in the comments.

(Barry Panas, the Official Humorist of the AP Physics Reading and master Manitoban physics teacher, is right now screaming at me the same way I scream at baseball commentators: The Interactive Physics platform will do exactly what I describe!  When I had IP installed on my old computers, I used to use it.  Barry taught me everything I know about using IP.  But, I don't have that program anymore.  I need something free and quick. Sorry, Barry.)

Good news - I have a student this year who's a programmer.  I explained what I was looking for; in a day, he had something basic but useful.  Take a look at this link. I don't exactly understand how the "pixel" button works, but it allows me to zoom in or out.

And so, after we did this problem, I projected this simulation in front of the class.  We doubled both g and v0, and wouldn't you know, the maximum height didn't remain the same.  Physics works.

Disclosure - the link is to my son Milo's site; he is the student who programmed the simulation.

1. Greg, are you familiar with thephysicsaviary.com ? If not you will be blown away! Here is the link to the exact lab you want, modeled with a firework. Everything on the site is HTML5. It is the best free physics resource on the net! Definitely take a look at the randomly generated homework problems.
http://www.thephysicsaviary.com/Physics/Programs/Labs/FireworkVerticalMotionLab/index.html

2. Yes... I saw the firework, but if I remember right their g was quantized, so I couldn’t quite get the ratios I want.

But the aviary is amazing. It is one of four sites I’d say that all physics teachers should be aware of... huh. Perhaps that’d be a good post. :-)

3. Hi Greg. I don't know if you check old comments or not, but since I'm currently reading through the old ones, I imagine at least some people do ...

There is a Phet simulation at https://phet.colorado.edu/sims/html/projectile-motion/latest/projectile-motion_en.html
that does this. It's free and HTML5. It has maybe a bit more options than you are specifically looking for, but it can be set up to do exactly what you describe. (And after the students see it with no air resistance, you can add in air resistance and check out the difference!)