Photo: Rhett Allain. RC Circuit Setup

It’s pretty much a classic physics experiment. Take a resistor and a capacitor and connect it to a power supply. As the capacitor gains charge, the total current decreases. Eventually, the capacitor will get mostly charged (technically it doesn’t get to 100 percent).

But if you choose appropriate values of R (the resistance) and C (the capacitance), this thing will charge slowly enough that students can record the voltage as a function of time. With that, they can show that the following equation works.


Image: Virgin Galactic. Virgin Galactic’s First Spaceflight on December 13 2018.

Humans have decided that the boundary between Earth and “outer space” happens at an altitude of 100 kilometers (62 miles). This artificial boundary is called the Kármán line — it’s basically the height that the air is too thin for traditional fixed wing aircraft to fly. So, you need something like a rocket. If a human crosses this line, they instantly become an astronaut.

That’s exactly the plan for Virgin Galactic’s SpaceShipTwo Unity. It’s a rocket launched from a high altitude aircraft that takes humans into space. Richard Branson (billionare and founder of the Virgin Group) will be one of…


Image: Rhett Allain. My fan art version of the explosion of Death Star I

The Merriam-Webster definition of science fiction:

Fiction based on imagined future scientific or technological advances and major social or environmental changes, frequently portraying space or time travel and life on other planets.

Of course we could always argue about which films are science fiction and which ones aren’t. However, I want to instead focus on “space films”. When the plot centers around space ships, there are certain physics concepts that need to be addressed. So, what are these ideas and how do the movies deal with them?

Let me go ahead an point out something important. I’m not a hater…


Image: Rhett Allain. Created with Glowscript / VPython

Sure, you can look up the equation for escape velocity, but what does it really mean?

Let’s start with the most conceptual idea possible. Suppose you have a ball and you toss it straight up into the air. As the ball rises, it decreases in speed until it reaches its highest point. After that, it starts moving back down and increasing in speed. That’s something we have all probably experienced (throwing a ball).

The faster you throw a ball (on the surface of the Earth) the higher it will go — that is probably obvious. However, what’s not so obvious…


Photo: Rhett Allain. This is the only photo of gold that I could find.

It felt like it went on for a couple of weeks. I kept getting the same promoted tweet on twitter (I can’t even find it now). It was an ad for the show Money Heist on Netflix.

I really don’t know much about the show, but the ad shows some people scuba diving inside of a flooded vault and picking up some gold bars. Like this.


Photo: Rhett Allain. Sunrise in Louisiana.

I wish I could say this wasn’t true — but it’s true. A congressman proposed the following idea: what if we could fix climate change by moving the Earth’s orbit?

I told you it was true. Check it out.

Yes, it’s a crazy idea (for multiple reasons). However, we can’t let crazy get us down. Instead, let’s use this as a basis for an awesome physics problem. Here it is:

Suppose you move the Earth such that the intensity of light from the Sun decreases by 1 percent. How much energy would this orbital change require? …


Photo: Rhett Allain. Unit Vectors

There is a problem with many of the algebra-based physics textbooks (and courses) — they avoid unit vectors and maybe they shouldn’t.

Oh, what is a unit vector? How about I start with a physics problem so you can see why we need unit vectors. Don’t worry, I’m going to still give my full explanation of unit vectors.

One of the first topics in the second semester of physics is that of electric fields. If you have a positive point charge, the electric field would look something like this.


Photo: Rhett Allain

I don’t know who came up with this problem, but it’s actually pretty fun. Here’s the deal:

  • Start with a hoop with a radius of 0.05 meters. It rotates about an a vertical axis (in the y-direction) that passes through the plane of the hoop (see the sketch above).
  • The hoop rotates with a constant angular velocity of 10 rad/s.
  • On the hoop, there’s a bead with zero friction such that it can slide along the hoop. Let’s say it has a mass of 0.05 kg (but of course the mass doesn’t actually matter).

So, let’s find an equation of…


Photo: Rhett Allain. Area element in Cartesian and polar coordinates.

Let’s say you wanted to calculate the area of a circle. Oh, I know the answer already, but let’s just pretend. Also, for fun suppose you want to do this numerically by breaking the circle into many tiny pieces and finding the sum of the areas of the pieces. How would you do that?

I’ll be honest. I don’t want to find the area of a circle. I want to find the magnetic flux through a circle for a complicated magnetic field. This means I’m going to use a numerical calculation to find the magnetic field at a bunch of…


I remember binge-watching the show Parks and Recreation with my youngest son. It was great. But then we got to the last episode and he said he didn’t want to watch it. If we watch it, then it will be over. That’s how I feel about this episode.

Note: don’t worry — I still have a bunch more MacGyver stuff. I’m working on my paper clip videos (building stuff using paper clips). You can see those here:

Now for the science in this episode.

The Drake Equation and the Fermi Paradox

It’s not really a MacGyver-moment, but it’s an…

Rhett Allain

Physics faculty, science blogger of all things geek. Technical Consultant for CBS MacGyver and MythBusters. Former WIRED blogger.

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