Krackling of Knuckles

The Problem:

We all, almost on a daily basis, stretch our fingers and create a krackling sound out of the knuckles. We exclaim in hindi, "haddiyan chatak rahi hain". Meaning, "the bones make the sound". But is it really so? Not exactly. It has hardly anything to do with the bones. Read on.

The Answer:

Our bone joints of the body are lubricated using a lubricating liquid which is, of course, naturally existent in our bodies at those joints. So is the case with our finger joints too. These lubricating liquids get filled with nitrogen which seeps into them from our blood capillaries. When we suddenly stretch these joints the pressure inside the liquid falls rapidly thereby converting the dissolved gasses into bubbles which soon after explode creating shockwaves which are heard as krackles. This process is known as Cavitation. 

So next time someone says, "haddi chatkana" or "crackling BONES" you can very well correct him that it is called cavitation. 

Comment

Why do the astronauts seem to be floating inside the space stations and shuttles?

The Problem:

You must have seen, on TV shows, that astronauts seem to be floating inside the space station or the space shuttle. It seems that they are weightless. And they actually are. But the question is how and why are they in that condition?

The Solution:

This is a classic question and one of my favorites which I have asked to numerous people and I usually get one of the following answers:

1. There is no gravity in the space. (90-95% people give this answer).
2. The gravitational pull is balanced by centrifugal force. (Some more learned guys give this answer).

Unfortunately both are wrong. Lets us see why.

1. The force of gravitation is inversely proportional to the square of distance. The space stations are at a height of nearly 400 kms. Now at a height of 400 kms, if you so dome calculations, you would find that the acceleration due to gravity is 90% of that on Earth's surface. So if you weigh 100 N on Earth then on space station it would be 90 N and not zero as you might think. SO it is definitely not ZERO GRAVITY out there.

2. Take a stone attached to a string and whirl it around. Do you really think that the stone would feel weightless? Think for yourself.

SO the question is what is the real answer. 

Weight is a force with which the Earth attracts everything towards itself. The acceleration imparted by Earth is 9.8 m/s^2. Now if you are in an elevator and it accelerates upwards at an acceleration of 9.8 m/s^2 then your weight will double out.This is because the weight tends to add up only when there is a normal reaction being imparted by the surface on which you are standing. In case of falling down the surface on which you are standing also tends to be in free fall with the same acceleration due to gravity. This means there will be no normal reaction and hence the apparant weight of yours will be zero.

This is precisely what happens inside a spacecraft. It is continuously freely falling towards the center of the Earth. Which leads to a weightless condition. The only catch is that the spacecrafts have a tangential velocity of nearly 25000 km/h which tends to make them go around the Earth in an orbit.

It is like the spacecraft is falling towards the Earth and the Earth is curving away from it thereby forcing the spacecraft to never really reach the Earth's surface.

Comments

Does string telephone work? If yes, then how?

The Problem:

As a child we all have made our very own string telephone and talked to our friends with most of the time with very disappointing results. Sometimes we could hear the voice of the friend and most of the times we couldn't. The question is why sometimes we could hear a faint voice and at other times no voices at all? And how does it work at the fundamental level? 

Read on.

The Answer:

Let us start the answer from answering the last question first. How does the telephone actually work? If you remember the telephone would look like the following. 

Now there are two components to this entire system. Two pieces of cups, better if it is metallic. And one strong string, again better if it is metallic. 

They are connected as shown. 

So this was all about the construction. The second part is tricky and it is here why most of us never got to make these work.

If you look at the first figure the string attached to the cans has to be as tout as possible. The reason is given below.

When we speak in the can the can vibrates with the sound vibrations that we produce. These vibrations in turn are passed off to the string attached to the can. The vibrations then travel through the string and does the reverse process of vibrating the can and letting the other person hear what you say. 

For this entire process to be successful it is important that the strings be tout so that the tension along the string is evenly distributed and the sound/vibrations can travel properly without much resistance. 

So if you really want to make a string telephone make it using metal cans and keep the string tout. And ya you will need to practice a bit to find out what pitch of your voice will reach the other end as the cans may not properly vibrate thereby rendering the entire thing useless.

Comments

Why do chalks squeal when writing on the chalk boards??

The Problem

You all must have seen that chalks make a squealing sound when teachers use them to write on the black boards. It is an usual phenomenon which as a student and, now, a teacher I have always hated. Can this be corrected? Read on to find out. 

The Answer

As a teacher when I hold the chalk in a wrong position or orientation with the board the chalk tends to stick to the board. Then when I try to push it forward to write on the board the chalk "slips" on the board and vibrates along it. While vibrating it hits the board several times per second creating the squealing sound. As the chalk moves faster the squeal tends to die down and the chalk starts to "stick" back on to the board preparing for the next round of squealing. 

Better quality non-dust chalks can reduce the squealing considerably.

Comment

Introduction to HOWs and WHYs of Science in General and Physics in Particular.

Hi all.
I have been thinking for a long time to start a blog for everyone, but specially my students, who would love to know the answer to some intriguing problems or observations that one might come across in day to day life.
My answers are strictly based in Physics (Science) and some are borrowed from various sources which includes books as well as websites.
I hope you will find it interesting.
Do leave a comment if you like it.
Thanks.