January 21, 2021

The Doppler effect | Science

The Doppler effect | Science



The balloon that inside a car It moves to the same side as the vehicle turns, as we saw last week, behaves so contrary to intuition – and to other objects – by the displacement of air. If the car turns to the right, the mass of air inside moves to the left, which increases the pressure on that side, and that increase in pressure prevails over the inertia of the very light globe and pushes it to the right.

We represent the teardrop-shaped raindrops (and the tears themselves, worth the redundancy) because the only drops of water that we see clearly in everyday life are those that fall from a faucet or an almost empty container, and in this yes, they have the usual tapered shape above and rounded down, because the surface tension causes the point of contact of the drop with the tap or the container to become thin until it breaks.

When a latex balloon swollen with helium rises freely in the air, it can reach, as experimentally verified, up to about 10 km (and can move up to about 3,000 km from the point of origin), if it does not explode before due to the dilation of helium inside, whose volume increases as the pressure of the surrounding air decreases.

The siren of the ambulance

You can not talk about the Venturi effect and the Magnus effect, as we did last week, without mentioning another remarkable effect also linked to the rapid and habitual displacement in our daily experience: the Doppler effect. When an ambulance approaches, its siren seems sharper than when it moves away, and the effect is even more marked in the case of a train's beep: the apparent frequency of a wave increases (or, what is the same, the length of wave decreases) if its focus approaches the observer and decreases if it moves away. In the case of sound, which travels through the air at about 340 meters per second, the effect is perceptible if the focus travels to about 50 km / h or more, as in the case of an ambulance, and it is even more so in the case of a train that travels 100 km / h or more, because we are talking about a variation of the relative speed of sound between 5 and 10%.

In the case of light, whose speed is 300,000 km / s, the source must move very fast so that the Doppler effect is perceptible; that's why we do not see color fluctuations in moving objects. But on a cosmic scale the effect is remarkable and very important in astrophysics, since the "red shift" demonstrates the estrangement of galaxies and the expansion of the universe.

And speaking of sound and light waves … Antonio shouts "Luz!" Almost at the same time that a lamp is lit. Berta thinks that Antonio has given the ignition order, while Carlos thinks he has let out an exclamation upon seeing the light. Who has the reason?

Carlo Frabetti He is a writer and mathematician, a member of the New York Academy of Sciences. He has published more than 50 scientific dissemination works for adults, children and young people, among them Damn physics, Damn mathematics or The big game. He was a screenwriter The Cristal ball

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