# What Causes a Curveball to Curve?

If you’ve ever seen a curveball in action, you might have wondered how it works. Is it centripetal force? An optical illusion? The answer may surprise you: it’s friction.

# Air and Spin

When a non-spinning ball is thrown, the air flows around it as shown below with the blue arrows.

Now consider putting spin on the ball as depicted here.

# How Much is this Girl Slowing Down Earth’s Rotation?

xkcd’s Angular Momentum is a romantic application of the conservation of momentum. But it may lead you to wonder: How does a person spinning counterclockwise cause Earth to slow down? And by how much?

# The Bad News: It Probably Doesn’t Slow Down At All

The girl uses the friction between her feet and the ground to spin. Newton’s third law requires that the world “pushes” back, causing it to spin equally in the opposite direction but at a speed that is (roughly) proportional to their masses. This would be a very, very small change in earth’s rotation — but still a change!

The energy from the girl’s push gets transferred…

# What’s so Special about Maxwell’s Equations?

Maxwell’s equations are some of the hallmark equations of physics culture, but what makes these equations so special that people are willing to get tattoos of them?

# A Quick Summary

Maxwell’s equations are typically introduced in college-level electrodynamics courses. One thing undergraduate physics students quickly discover is that you can start almost any problem in those courses with at least one of the four equations. So, unless you’re already very familiar with them, I think it’s important to have an idea of what these four short, but potent equations do:

Gauss’ Law for Electricity: Captures the concept that charges are sources of electric…

# Why is the Sky Blue?

After light from the sun makes its lonely journey to earth, it strikes our upper atmosphere with a spectrum strongest in yellow, red, green, and blue.

As light moves through the atmosphere, it radiates the nitrogen and oxygen molecules. The electric field component of the electromagnetic radiation pulls the electrons in one direction and pushes the positively charged nuclei to the other. This process creates a tiny induced dipole.

# A Visual Guide to the Uncertainty Principle

The Heisenberg uncertainly principle captures the experimental and theoretical phenomena that you can’t ever really have full information about both a particle’s position and momentum at the same time. To increase certainty in one, you must give up certainty in the other. Quantitatively, it is described as follows,

A dive into Fourier analysis can explain this much more analytically, but for now, let’s stick to these animations.

# Where is the Wave and What’s its Momentum?

In the image below: ask yourself Where is the wave?

# Do Atoms Ever Touch?

Overall, yes. But there’s more to this question than a simple answer. So let’s break down atoms into two parts and answer this question separately.

# Do the Nuclei of Atoms Ever Touch? Yes.

The nuclei in atoms of our everyday lives usually don’t touch each other. Even the nuclei under a 20-story building aren’t in contact. Outside of our daily experiences, nuclei definitely can touch. In particle colliders, physicists throw atoms at each other to make their nuclei smash together. In space, immense forces cause atoms to squeeze together until their cores touch. …

# When Does Something Become Quantum?

The line between classical and quantum physics is a blurry line, but one general benchmark is if the wavelength of the particle is at least as large as the characteristic size of the system.

# Temperature and Wavelength

One of quantum theory’s central concepts is that everything has a wavelength associated with it. The reason why we don’t notice the wave-like properties of everyday objects because their momentum are so massive that it makes the particles’ wave unnoticeable.

Physicist Louis de Broglie proposed a relation between the momentum and wavelength of particles in 1924. He suggested that as the momentum of a particle gets…

# How to (Probably) Win at Rock Paper Scissors

The internet is riddled with articles about the best strategies for rock, paper, scissors (RPS), and they all generally suggest the same thing. But are these strategies actually effective?

# The Strategy

I should start by summarizing the applicable parts of the 2014 Chinese study where everyone else got their information. The researchers found that players will throw, on average, a distribution of 36% rocks, 33% papers, and 31% scissors (ignoring the standard deviation). This is slightly different than the Nash equilibrium for RPS, which predicts that the optimal strategy is 1/3 rock, 1/3 paper, and 1/3 scissors. …

# Why You Can’t Use Entanglement to Communicate Faster than Light (Without Math)

I often get asked to explain pop-sci concepts like how you can travel back in time. But one of the more recent ones has been to clarify how you could (or couldn’t) communicate faster than light using entangled states, so here’s a no-math explanation.

# The Scenario

Quantum entanglement is generally understood as this: Two particles have some property in which a change in one instantly causes a change in the other. So, in theory, you could change something about one entangled particle on one side of the universe and then use the change in the other particle to instantly communicate to the…

# Why is Physics so Hard?

It’s not uncommon for people to cringe when they hear the word physics. For some, it resurrects thoughts of memorizing equations and then desperately manipulating them to solve problems. For others, it’s a foreign language only understood by the geekiest lifeforms. But why does physics have such an intimidating reputation? And more importantly: is it justified?

# Your First Exposure Mattered

Let’s take a step back in time and think about your first exposure to physics. If you were lucky, you had an enthusiastic teacher who seemed way too excited about Newton’s Second Law. If you weren’t fortunate…well, you probably don’t remember much from physics…

## Colton Kawamura

Writes about science at www.groundstatecuriosity.com

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