90% of physics in 900 words

I had a physics professor who said the most important skill in science was to be able to characterize a system quickly. Be approximately right instead of precisely wrong. This has stuck with me.

Anyway, here’s physics for non-scientists, who wish to be approximately right.

Heat

Hot things want to cool off. Cold things want to warm up.

Temperature difference is what matters. When something feels cool, it’s because heat is leaving your body and warming up the thing. If something feels neither hot nor cold, it’s because you and it are the same temperature.

Nature doesn’t like differences, and will try to reduce them. Our senses are sensitive to differences.

Ice turns into water because it’s absorbing heat. Water turns into steam for the same reason.

This requires a lot of heat (energy). Your sweat, when it dries (evaporates), takes a lot of heat from your body. Your body is cooling down, and the air is warming up.

Condensation (a cold soda becoming wet on the outside) is the same thing in the opposite direction. The soda is warming up, and the air is cooling down.

Light

X-rays, Wi-fi, radio broadcasts and sunlight are all physically the same thing. We give them different names becuase we use them differently.

They differ in their frequency. They are vibrations. The frequency is how fast they vibrate. Think about ripples in a pond, or a guitar string.

Those vibrations might hit other things. The effect might be to warm up your skin, or to illuminate a room, or to cast a shadow.

Those vibrations might pass through other things, or bounce off of them.

High-frequency vibrations are more intense, but they are also easier to block. Blue has a higher frequency than red. A sunset might look red because the blue light has been blocked by the air.

Human eyes can see a small range of frequencies, represented by the rainbow, which we unsurprisingly call “visible light”. We can’t see Wi-fi or X-rays.

Most light is a combination of many frequencies. Combining lots of frequencies makes light look whiter.

Ultra-violet (UV) light has a high frequency, and is just barely outside what our eyes can see. Sunscreen mostly blocks it, as does glass.

Infra-red light, which has a low frequency, is also just barely outside what we can see.

Heat and light

Everything that is warm is also radiating light. We mostly don’t see it, it’s infra-red.

If something is warm enough, we can see the light. We call that “red hot”. Even hotter things can be “blue hot” or “white hot”.

Sound

Sound is a vibration of air. Again, think ripples in a pond.

High-frequency sound is called “treble”, low-frequency sound is called “bass”.

As with light, higher sound frequencies are easier to block. When your neighbor is playing music, you will mostly hear the bass, because the walls have blocked the treble.

Trebly things are easier for us to locate with our ears. It’s harder to figure out where bassy things are coming from.

Most sounds are a combination of many frequencies. Sounds are unique because their combination of frequencies is unique.

Speed

Your location is where you are. If your location is changing, we call that speed. If your speed is changing, we call that acceleration.

You can’t feel speed. You can feel acceleration. You get pulled back into your seat during take off, but not when you are cruising.

Gravity is a type of acceleration.

There is the thing, a change in the thing, and a change in a change in the thing.

Calculus is the math that helps us to understand changes of changes.

👉 Money is similar! Our wealth is what we have. If our wealth is changing, we call that income (or spending). If our income is changing, we call that a raise (or inflation).

Energy

If one thing is moving faster than another thing, and they collide, the fast thing will make the slow thing move faster, and vice-versa. Think about billiard balls.

Nature doesn’t like differences! The greater the difference in speed, the more violent the collision will be.

This happens with air, and with water, too. Your car is hitting millions of air molecules, causing those molecules to go faster, while those molecules cause your car to go slower.

Friction

We call these collisions air resistance, or friction. We usually want less of it. Car manufacturers design their cars so that they will hit less air.

Sometimes we want more friction. Think parachutes, or the soles of your snow boots.

Friction causes things to become hotter. Speed gets turned into heat. Usually, it’s not much heat, and we don’t notice it. Things that fall from space, like meteors and Space Shuttles, definitely do feel the heat.

When you lift something up, you are giving it energy. When you drop it, it accelerates, and releases that energy.

Burning

Burning things releases energy. It’s a chemical reaction with oxygen.

This reaction might be a flame, or it might be digestion, it might be rotting (think compost). Rotting is digestion by bacteria. Rust is metal burning slowly.

When plants grow due to sunlight, they are storing chemical energy. It is the reverse of burning. Campfires are releasing energy that used to be sunlight. A sugar rush is doing the same.