# Physics

Crash courses and sparks for students and explorers on the pillars of physical sciences with simplified straightforward lessons and insights.

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Most people think that inhaling helium changes the pitch or frequency of the voice. No! That is no what happens. It’s the timbre of the voice that changes.

When you inhale helium, the medium inside the vocal cavities changes from a dense medium, air, to a lighter medium, helium. And we know that in air, any vibrations travel at a speed of 343 m/s. For helium, it’s 1007 m/s. The helium medium now increases the natural frequencies of the cavities. In other words, it changes the responsiveness of the cavities to higher frequencies. This results in the amplification of a higher range of frequencies compared to that of when the air was the medium—causing the squeaky voice!

The key observation here is that the frequency at which the sound is produced by the vocal folds doesn’t change. It’s the resonant frequencies that change, forcing a change in the timbre of the voice.

What’s a timbre of a human voice? Let’s start with the voice! The human voice is created when vocal folds vibrate. It’s this vibration of air molecules when travelling through different cavities like pharynx, sinuses, nose, and mouth, that’s converted to speech. And here is the interesting part that makes them unique to a person.

Like any physical objects, these cavities through which the sound travels have their own properties as well. They have their own distinct natural frequencies because of the geometries of the muscles that are unique to a person and the composition of the air.

And the sound from the vocal folds is made of not just a uniform sine wave with a fundamental frequency, but a composite of other distinct frequencies as well. So when certain frequencies of that sound wave hit the natural frequencies of the cavities, resonance happens, and those parts alone get amplified. The end result of all this is the distinct voice of a person. In other words, the lowest resonant frequency that’s modulated by the rest of the frequencies is what gives that unique tone to your voice. And that’s what we call the ‘timbre’ of the voice.

The voice you hear when you inhale helium, that’s because of the timbre change too. Not the frequency shift!

The longest vertical straw you can drink from is 10.3 metres. Even if you use a vacuum pump it won’t suck the liquid higher than that! Here is why!

Contrary to your intuition, when you drink from a straw you are not actually sucking up the fluid here. Just the air. So, when you do that, inside the straw, the pressure drops lower than that of the atmospheric pressure (101 kPa) outside. So, it’s the outside air pressure that pushes the water into the straw.

As the liquid moves up the straw, it is fighting against the gravity that is pulling it downwards. But it still keeps rising as long as the atmospheric pressure is greater than the pressure inside the straw due to gravity (weight of the liquid column).

The more liquid enters the column, the more it weighs. And at a certain height, there’d be enough water in the straw that’d exert the same pressure as that of the atmospheric pressure. That height, at sea level on earth, for water is 10.3 m.

$$p_{atm}= 101\;kPa$$

$$p_{straw}= \dfrac{F}{A} \Rightarrow \rho g h$$

$$\rho g h = 101 \times 10^3\;N/m^2$$

$$h = \dfrac{101 \times 10^3\;N/m^2}{10^3\;kg/m^3 \times 9.81\;m/s^2}$$

$$h = 10.3\; m$$

Temperature is not the measure of the heat. Most people think that they are the same. Take a pair of small and large vessels with water and expose it to the sun. You will find the smaller vessel becoming warm quicker than the larger one. Although an equal quantity of heat is supplied to the two vessels, due to the difference in the quantity of the water, the time it takes to raise the temperature varies. This must clarify the confusion between temperature and heat.

If you have poured tea from a mug you’ll intuitively know what a Coanda effect is. To put it simply, the Coanda effect is the phenomenon where fluids like water tend to follow and stick to a contour of an object.

So what happens here? When the water flows out of the mug, the water molecules encounter the air molecules and try to drag them along due to viscosity. As the air molecules under the mug get dragged off, the pressure at that spot, which is relatively constrained compared to the other side, decreases (Bernoulli’s principle). And as the pressure is higher at the top of the water than on any other side, the water reaches equilibrium by moving towards the low-pressure region, which is what makes it to stick to the surface of the mug.

Earth’s atmosphere is leaking a few grams of helium this very moment! Yep! As helium and hydrogen are the lightest elements of all, Earth’s gravity has little effect on them in the hydrostatic equilibrium. With higher kinetic energies, hydrogen and helium reach velocities greater than that of Earth’s escape velocity in the thermosphere and they shoot into space.

If you are in space and shine a torch in an arbitrary direction, the photons (although being massless) will impart a thrust on you that will propel you in the opposite direction. This is due to the conservation of momentum as well.

In other words, when photons are ejected out of the torch, they travel outward with a momentum*. And due to this, your momentum changes to conserve the total momentum of your initial state, pushing you in the opposite direction.

* Photons have a relativistic mass and they do have momentum, given by $p = \dfrac{h}{\lambda}$.