Studying physics can help you develop a scientific mindset.
Physics helps us better understand the world we live in, from atoms to the universe.

Volume 9 of the “10 Recommended Books for Teenagers” series of humanities books for teenagers.
This is an introductory physics book written for teenagers by Professor Lee Kang-young of the Department of Physics Education at Gyeongsang National University.
When we hear the word 'physics', we often think of it as a subject that only smart and brilliant people study, but physics is a discipline that explores the universal principles of the world we live in, and is closely connected to our lives.
Studying physics can teach you basic knowledge, such as why the sky is blue and why we divide the cardinal directions into north, south, east, and west, as well as how computers and smartphones, which we use every day, are made and operate.

This book examines what is studied in physics, a field that can be vague and difficult to understand, and introduces the basic knowledge that young people need to know about physics, from classical to modern physics.
This book provides an easy-to-understand understanding of major topics in physics, from the significance of Galileo's discovery of Jupiter's moons, Newton's law of universal gravitation, and Einstein's theory of relativity and quantum mechanics, which are frequently covered in various media.

Astronomy is not a discipline that developed simply to hone one's intellect.
In fact, astronomy developed for practical purposes.
They made calendars to use in farming, and to refer to them in fishing and navigation.
To farm, you need to go beyond feeling the seasons and truly understand their rules.
That way, you'll know when to sow your seeds and preserve your harvest.
Fishermen need to know when the tide comes in and when the tide goes out.
As is well known, the tides are closely related to the movement of the moon.
Also, unlike on land, it is difficult to determine direction and time when you go out to sea.
So, for sailors, stars were valuable indicators of direction.
For this reason, astronomy was one of the first fields to develop in what we now call science.
So, regardless of East or West, in ancient societies, astronomers held important positions in the country.

--- p.48

You probably have a magnet or two stuck to your refrigerator at home.
There will be decorative magnets, and there will also be advertising magnets with delivery phone numbers for Chinese restaurants or pizza places.
Now let's attach a clip to this magnet.
The clip will stay attached to the magnet without falling off.
We don't find this situation very strange.
It's only natural that clips stick to magnets.

But let's think about this phenomenon carefully.
This means that the magnetic force of the magnet is stronger than the force of gravity, causing the clip to fall to the ground.
If gravity were stronger, the clip would have fallen to the ground.
Here, gravity is the Earth's gravity, and magnetic force is the force of the refrigerator magnet.
So the magnetic force of that little fridge magnet from the pizza place is stronger than the gravitational pull of this entire huge planet.
That is, essentially, we can see that gravity is a very small force compared to magnetic force.

--- p.71

Actually, Einstein's theory of relativity is twofold.
One is the special theory of relativity, which Einstein announced in 1905 when he was a university graduate and working as a patent examiner, and the other is the general theory of relativity, which he announced in 1915 when he was a professor at the University of Berlin.
What happened in just ten years that took Einstein from being an examiner at the patent office to becoming a professor at the University of Berlin, Germany's leading university?
--- p.89

Anyone interested in physics may have heard many mysterious stories about quantum mechanics.
I'm talking about things like Schrödinger's cat or the duality of particles and waves.
But if you study quantum mechanics in college, you will understand what I mean.
So, although quantum mechanics is a special knowledge, there is no need to over-inflate it into something mysterious.
Of course, humans do not yet fully understand quantum mechanics, and we are still exploring its 'true' meaning.
We can now say that we know quantum mechanics well enough to explain atoms, nuclei, and elementary particles.

--- p.119

The most famous animal in the field of quantum mechanics is probably the cat.
You've probably heard the term 'Schrödinger's cat' at least once.
Schrödinger is one of the scientists who created quantum mechanics.
This story is a kind of thought experiment that Schrödinger came up with while talking with Einstein about the meaning of quantum mechanics, and it became incredibly famous because it very vividly illustrates many of the subtleties of quantum mechanics.
(…) There is a cat in the box.
On one hand, there are radioactive elements.
Radioactive elements are elements that emit radiation over time as they undergo radioactive decay. Here, a device is connected that injects poison gas into the box when radiation is emitted from the radioactive element.
So if the radioactive element decays, it will release poisonous gas and kill the cat, but if it doesn't decay, the cat will survive.
So whether a cat lives or dies becomes a question of whether or not the radioactive element decays.

--- p.141

The satellites' time is accurately maintained by the atomic clocks each satellite is equipped with.
However, because the satellite is moving at a very high speed, according to the special theory of relativity, time passes more slowly for it than for people on the ground.
Meanwhile, satellites are at a very high altitude and are less affected by gravity, so according to general relativity, time passes faster on satellites than on the ground.
Although these effects are small enough to be imperceptible in our everyday lives, they make a noticeable difference in a precise GPS device, so both effects must be taken into account to improve positioning accuracy.
So the GPS receiver we use takes the data it receives from the satellites, calculates the effects of both special and general relativity, calculates the exact time for each satellite, and from there, tells us our exact location.

--- pp.154~155

Once the water depth exceeds a certain point, light can no longer reach it.
So the deep sea is completely dark.
Even in very clear waters, light is almost completely absorbed beyond 100 meters.
(…)

This fact raises a very important question for our exploration of the deep sea.
Because when we go into the ocean in a submarine, it means that once we pass a certain depth, we can no longer communicate via radio.
Radar also uses electromagnetic waves, so it cannot be used at all.
In other words, once a submarine goes deep into the water, even with the most advanced modern science and technology, it is impossible to contact the submarine or find out its location.
Isn't it amazing? Even if we go to the Moon, or even Mars, we'll still be able to communicate with Earth (though it will take some time, of course).

--- p.160

The most unique experiment is one called IceCube, which takes place in Antarctica.
This experiment aims to observe neutrinos flying in from space, and what is unique about it is that it uses the vast ice of Antarctica itself as the experimental medium.
We said earlier that neutrinos rarely interact with matter.
Therefore, to observe neutrinos, a large amount of matter must be used as a medium.
IceCube drilled several holes about two kilometers deep into the Antarctic ice and placed light sensors at regular intervals inside them.
So, we observe neutrinos by detecting the tiny light that comes out when neutrinos flying in from space react with electrons or atomic nuclei in water molecules that make up ice.
Isn't it amazing how physicists can use kilometers of Antarctic ice for experiments?
--- pp.216~217

Superconductivity is a phenomenon in which the electrical resistance of a material becomes zero.
I don't know if that's such a big deal.
To put it simply, the phenomenon of electrical resistance becoming 0 is similar to the phenomenon of friction becoming 0 in our daily lives.
When friction becomes zero, it's a completely different world.
When you ride a swing, if you give it a push once, it will keep going up and down, and once you start a roller coaster, it will go up and down the rails forever.
It may sound fun, but it's actually quite difficult.
Without friction, we wouldn't be able to pick up objects, walk, or move a car forward when its wheels turn.
Anything that is slightly tilted will slide down the slope.

--- p.225