Good morning Mark Rober. I want to start by saying I really enjoyed your “Egg Drop
From Space” video. I particularly enjoyed how you highlighted all of your failures;
my favorite was when there was “a single rogue negative sign in the code”. Failure is such an
important part of learning, and it is one of the reasons I structure my classes the way I do.
And he makes us always use a pencil. Because we make so many mistakes.
Right. Yeah.
Yeah. Mark, the reason I am making this video is to express my concerns about the work you showed
roughly one and half minutes into the video where you calculated the terminal velocity of a chicken
egg falling through Earth’s atmosphere. Your work looks like this. I want to talk about all the ways
I wish you would show your work better. Oh Boy.
Yeah. ♪ Flipping Physics ♪
But, maybe we should start by talking about what Mark did do well?
(Ah. Good point Billy. Could you do that, please?) Absolutely! Uh Mark, you drew the free body
diagram, summed the forces, identified the direction you were summing the forces in,
identified the net force in the y-direction equals zero, solved for the drag force,
substituted in an equation for the drag force, and solved the equation for the terminal velocity. So,
yeah, Mark, you did show a lot of work. Yes, Billy, thank you for pointing all
of that out. Okay, the first thing that caught my eye was that you did not show the conversion from
metric units to English units. Actually, let’s adjust that screenshot so we can read it better.
The equation you derived for terminal velocity is terminal velocity equals the square root
of the quantity two times the mass of the egg times the acceleration due to gravity
all divided by the drag coefficient of the egg times the cross-sectional
area of the egg times the density of air. Hey, we did an entire lesson where we derived
that equation for terminal velocity, right mr.p? (Yes, Billy, we did.)
Actually, we have done a lot of lessons about the drag force, the drag coefficient,
and even the time constant. Yeah, remember when mr.p got to
go to a water park to demonstrate drag force? Yeah! Why didn’t we get to go the water park?
Yeah, why not? Uh?
Right. Again, Mark, the first thing I noticed was that you did not show a conversion from metric
units to English units. You can see that because the acceleration due to gravity you used is 9.8
and must be in meters per second squared and your terminal velocity is in miles per hour. Then I
noticed the mass you used for your egg is 59. I am guessing you meant 59 grams. Typically, we would
use kilograms here, but, sure. Just to check, I measured the mass of an egg and it was 67 grams.
So, yeah, I bet the mass of your egg was 59 grams. I thought mr.p was vegan. Where’d he get an egg?
That’s a neighbor’s egg. Okay.
Next, you used point 5 for the drag coefficient of an egg. Very reasonable. But then there is
what you wrote for the cross-sectional area of the egg, here. I’m guessing the shape of
the cross-sectional area you used for an egg is a circle, and the equation for the area of a circle
is pi times radius squared. So, I am guessing that symbol is pi? But, what is next to it? Honestly,
I cannot read that. I am guessing it’s a decimal, two numbers, and a square. But,
I cannot read it. So, let’s move on to the density of air. You wrote one hundred twenty nine. Um. The
density of air is often given as 1.29 kilograms per meter cubed. Perhaps you meant 1.29 kilograms
per meter cubed and the decimal got absorbed into the two? That’s my best guess. But, then,
you have grams and kilograms in the same equation. That’s not good.
No. Nope.
You know what. Let’s pause for a moment and add my typed out, more detailed,
clearer solution. You can see I have listed my known values, including the mass of the egg
in kilograms. However, there is still the question of the radius of the egg. My egg
(Your neighbor’s egg.) My neighbor’s egg has a diameter
of roughly 46 millimeters. Because radius equals half of diameter, that would give a radius of 23
millimeters. When we substitute all those numbers in, with 0.023 meters for the radius of the egg,
we get 35.310 meters per second or, multiplying by 36 hundred seconds over 1 hour and 1 mile over
1609 meters, roughly 79 miles per hour. Which is close to 75 miles per hour. So,
I’m guessing the radius you used was a bit more than 0.023 meters. Mark, to reiterate,
you combined grams and kilograms in the same equation, some of your handwriting was illegible,
…and you did not show your conversion. Showing your work is so important that I have several
lessons dedicated to the importance of showing your work. At one point I remember hearing you say
you wanted to be a physics teacher. In some ways, you currently are. You have many fans who learn
from you, including my wife’s mother’s sister’s daughter’s six-year-old son Floyd, seen here
in a drawing my daughter Ryan did two years ago when Ryan was 15 years old. This kid here, Floyd,
is definitely a Mark Rober superfan. Please, Mark, for Floyd’s sake, show your work better.
As a physics teacher, I would much appreciate it. You know Mark Rober went to Mars.
No, he worked on the Curiosity rover which is on Mars. I think you’re confusing
Mark Rober with Mark Kelly. No, not Mark Kelly. It’s it’s
Mark Watney. Mark Watney is the one who got abandoned on Mars during the Ares 3 mission.
Yeah. That’s right. Really?
Thank you very much for learning with me today, I enjoy learning with you.