Physics mysteries

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Jaroslav Kores, Ph.D.

Text in a car’s rearview mirror (Source: © ElenaKyrylova /

Text in a car’s rearview mirror

Why is the ambulance sign on an ambulance reversed when looking in the mirror and it is clear that the sides of the displayed object do not change — what is to the right in front of the mirror is to the right behind the mirror.
A convex mirror is used in a car’s rearview mirror — this is the reason it always creates a reduced and inverted image. But the image that is created is reversed. If you want to shake your right hand with your image in the mirror, you will see for yourself. From our point of view, the hand is on the right, but in reality one would shake your hand from the left. If you take the Ambulance sign in front of the mirror, you will see that the letter A is at the end (read from left to right) and the letter E at the beginning. In order for drivers to quickly see who is following them when looking in the rear-view mirror, the inscription on the vehicles of the rescue service or fire brigade is mirror-inverted.

Jaroslav Kores, Ph.D.

Hypothetical rescue in a falling elevator (Source: © zphoto83 /

Hypothetical rescue in a falling elevator

Can I save myself in a falling elevator by jumping just before impact?
I hope that no one will have to experience this situation because it will not be possible to jump in a falling elevator — I will explain the situation where we do not take into account the air resistance. If an elevator falls, the elevator will move with the acceleration of gravity g (we won’t count air resistance). When the elevator stops, the gravity of the Earth causes our body to fall down by accelerating with gravity, but the elevator floor prevents us from doing so. We feel solid ground under our feet. When we relax the muscles in our legs, our body will go down due to gravity and our legs will still remain on the floor. The moment the elevator breaks, it starts to fall with the same acceleration that we fall with ...

Jaroslav Kores, Ph.D.

Lift forces on an aerobatic aircraft (Source: © qfiatoo /

Lift forces on an aerobatic aircraft

How is it possible for an airplane to fly horizontally during aerobatics, even when it is upside down? The lift on the wings should pull him down and not up…
It is true that the plane takes off due to the geometry of the wings. The upper part is more bulging than the lower part and therefore the air flows at a higher speed around the upper part of the wing than the lower part. And it follows from Bernoulli’s equation that the higher the velocity of the fluid, the lower the pressure at a given location. However, if that were the case, the plane could only climb. We regulate the flow speed by controlling the propeller — the faster the plane flies, the faster the flow. So it would be possible to regulate the height of the aircraft by regulating its speed. But it’s not perfect. Therefore, aircrafts have rudders (at the rear of the aircraft). Turning the elevator changes the direction of the aircraft relative to the surroundings ...

Jaroslav Kores, Ph.D.

Real plasma in flame (Source: © Aliaksandr Marko /

Real plasma in flame

Is plasma formed during the burning of a flame? And if so, can the flame react to an electromagnetic field?
Plasma refers to a gas consisting of charged particles. Due to the high temperature of the flame, a neutral gas molecule (e.g., O2) can rupture into a free electron and a positive O2 ion and a candle flame can contain plasma. Indeed, such plasma is very sparse — there are fewer charged particles in it than the recommended molecules. The process that I have described as the rupture of molecules is called ionization. Ionization can be achieved in other ways (e.g. by candle flame, high temperature, or electromagnetic radiation) and we encounter it, for example, with lightning (see the question about lightning), fluorescent lamps or arc welding. Because plasma consists of charged particles, it reacts to both magnetic and electric ...

Jaroslav Kores, Ph.D.

Creation of vacuum by explosion (Source: © kremldepall /

Creation of vacuum by explosion

An explosion (e.g. a nuclear bomb) creates a pressure wave, i.e. air spreads outwards from the epicenter. Does this mean that a vacuum is created at the site after the explosion?
Yes, that’s right. A pressure wave is typically created by an explosion (it doesn’t matter what the source is) compressing the surrounding air. Because air is a flexible environment, this compression will spread further. You can try something similar yourself — just push water in a pool (or in a bath or pond) with your palm. We will see that this compression begins to spread further through the water. Unlike our hand, the energy of an explosion is much higher and therefore occurs with stronger air compression. This compression spreads further and the energy that has been put into it can have destructive effects. At this point, we should point out that nothing like this would happen in a vacuum (e.g ...

Jaroslav Kores, Ph.D.

The unpredictable path of lightning (Source: © marcorubino /

The unpredictable path of lightning

Why does lightning sometimes hit a car on the road during thunderstorms, even though there are taller, wet and therefore conductive trees on both sides of the road?
Yes, it can happen, just as it is possible that the lightning does not hit the tree, but rather next to it, or that it does not “choose” the shortest path towards the ground. It is necessary to realize that lightning moves in smaller sections and only when the individual parts of the lightning reach the ground will what we call lightning happen (it is well seen in slow motion videos, e.g. here). We can imagine that the beginning of lightning shifts from one place to another, from there to a third, and so on, until it reaches the ground. The individual parts of the flash pass through places that have less electrical resistance than the surrounding ones ...