Sink, float and wonderful things about it.

Have you ever wondered how something as huge as a ship could possibly float in water? We tend to think that lighter objects float and heavier objects sink. But, it just takes a small stone and a heavy ship to disprove that thought.

Let's start with simple questions first. When you drop an object into water, what could happen? It could either float or sink. Have you heard of the word called 'flink'? Meaning, neither float or sink. (I don't think it's an actual word though!) The object is fully immersed in water and is neither moving up nor down. Have you seen that ever?

There must be something right, that decides how much each objects gets to get wet, there must be something. And that's what we are going to read further on.

As usual, it all happened when a great guy born like long long ago, called Archimedes, said this :

When you put an object in water, the volume of water it displaces is equal to the volume of the object inside the water. 

Seems logical right. Just keep this in mind. It will come handy a little later in the post. Now, try this simple experiment at home, right now. Take an object that you know floats for sure. ( I took my son's bath duck ) Hold it in your hand and push it deep into a bucket full of water, and let go of it. What happens to it? It comes back up. Who pushes it up? The water. This force of water, or the upward push is called 'upthrust'. Now where is this force when I throw a stone or a coin into the bucket?

It is there, but it isn't enough. What did I just mean?
If the upthrust is more than the weight of the object, the object floats. If the upthrust is lesser than the weight of the object, the object sinks. Sounds elementary isn't it? If the upward force is more than the downward force, the object remains up. If the downward force is more than the upward force, the object goes down. The coin, that sinks, experiences lesser upthrust than its own weight.

How do we measure this upthrust? When everyone else in the world long long ago was puzzled with this one question, our genius, Archimedes came up with a principle, stating that

"Upthrust, that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces."

So in the coin case, the weight of the water it displaces is lesser than its own weight. Meaning, it is more denser than water. (Density = Mass/Volume.)

So, the density of the object decides the sinking or the floating behaviour. To understand why a ship floats, do this simple experiment at home. Take a vessel, and some spoons and forks. Drop them in a bucket of water. You see the vessel floats, while the spoons and forks sink. The vessel holds air inside it, making it less denser. Which is why a ship floats while a stone sinks.

Let me suggest a tweak to your float-sink experiment. Keep adding salt to your water. See if something changes. What changes? Some objects that previously sank in the water, now float. So the density of the liquid/medium matters. It's the density of the object as compared to that of the liquid it is in that decides the float sink thing.

We've learnt that there is something called upthrust which is an upward force. We also know that archimedes has taught us how to measure upthrust. We also know what makes ships float. We are not happy. We are thinkers. We think, but why does this upthrust exist at all?

The explanation goes on something like this.... In a column of water, the lower layers are under greater pressure than the upper layers. (because the lower layers experience the weight of the upper layers) So when an object is placed in the water, the bottom part of the object experiences more pressure than the top part of it. Which means to say, the object experiences a net pressure from the medium/water in the upward direction. And that is this upthrust. "Aahaa..."

So.. does upthrust increase with water depth then? Technically, once the body is fully immersed, the amount of water it displaces is constant. So the upthrust should also be constant. But we just said the lower layers exert a lot of pressure. So shouldn't there be a lot of upthrust as the body goes deeper?? What is wrong in the statement I just made?

Yes, the lower layers exert a lot of pressure. But upthrust is the difference in pressure experienced by the top and bottom layers of the object. That difference remains a constant! And so does the upthrust.

Now.. have you noticed the behaviour of tall objects, like when I place an empty glass tumbler in water, the tumbler tilts and maybe even topples off. Add some water to the glass, and you see that the glass stabilises. What is this tilt and stability and who do they depend on?

Apparently, there's something called centre of buoyancy and centre of gravity.

Centre of gravity or centre of mass is the one point where the weight of the object seems like it is acting from. Let's say you have a medium size plate in your hand. The centre of gravity will be the spot where you could balance it on your one finger. (perpendicular-ly)
Centre of buoyancy, similarly is the point where the buoyancy force seems like it acts from. That is to say, just consider the part of the object immersed in the water and find its centre point.

If the Centre of gravity is located below the centre of buoyancy, the object wouldn't topple. Do watch this wonderful video from this wonderful website for a lovely animation regarding the very same point.

For the interested, do go through these wonderful TedEd videos about archimedes and the history behind his principle here and here. Nice interesting story! I found them a delight. And for the ever curious minds, read the next post on submarines after having said "Hail Archimedes"!

Save the turtles.

Today, we are to learn about the endangered turtle species, how to help conserve them and also learn something new and fascinating about them.

A normal morning. After dropping kids in the school, my friends and I decided to let the steam off by getting together for a cup of coffee. "One of my friends (who we fondly call captain, coz she is one, she flies planes!!)   said, "I'm planning to take my kid to the turtle conservatory today. Anyone cares to join?" I hadn't even heard of the conservatory, despite the 4 well spent years in the city. I sent some rapid fire questions her way, in the quest to gather some information. Where? Why? What happens there and stuff. She patiently answered most of them, and she along with the other friend at the table jointly said, " It's a lovely experience, you should come. Do bring your kid along." And boy, am I glad that conversation happened. It was a wonderful experience.

A group of nice minded and like minded people got together and formed an organisation called Students Sea Turtle Conservatory. This is their webpage. Their page has details on how they work. There are seven species of turtles it seems. And they seem to be in the endangered status. Chennai sea coast attracts olive riddley turtles, it says. One of the seven species. These turtle moms, when ready to lay eggs, walk to the shore, dig a hole in the coast, lay the eggs, cover them up and disappear into the sea. When the eggs hatch, the hatchlings come out and find their way back to the sea. The problem in this whole scenario being, there is no control over when the eggs would hatch, as in, the time of the day. As the hatchlings don't yet have a hardened back, and are so vulnerable to attack, they easily fall prey to the crows, eagles, seagulls. Also, itseems sometimes stray dogs dig up the eggs and eat them thereby sending the species towards extinction. Sad isn't it?

It says, the egg laying happens in the december to april period. And during this time, these nice people from the conservatory walk the entire sea coast as a team, and look out for these nests. Carefully dig them out and bury them back in fenced enclosures, thereby protecting them from dogs. After a particular amount of time, they carefully dig the mud around the hole, take the hatchlings out carefully and let them reach the shore after sunset, a time where the birds already found their way back home.

Here's a pic of a volunteer carefully digging up the hatchlings.

This one is a basketful of these tiny little hatchlings ready to go to the sea.

Their final walk.

At the time we went to the hatchery, there were two volunteers. A guy who supervised the whole thing and made sure things happened according to the plan. A girl who so patiently answered all our questions and kept giving us lot of information about these turtles.

I asked the volunteer, "when you guys put it so much effort to take care of the hatchlings, why not leave them in the sea, why are u making them walk the beach? " she said," The hatchlings imprint the beach they were born in while walking. Which means to say, when they have to lay eggs, they come back to the very same stretch of beach." Amazing isn't it?

She told us that we shouldn't touch the hatchlings and cause them discomfort. Because they wouldn't eat for the next two days and need every bit of their energy for swimming. And you know, the gender of the hatchlings depends on the heat the eggs are exposed to. It seems, one can say with a little certainty that the eggs that are towards the centre of the heap have a little more temp than the ones that form the periphery. I asked her why they didn't use incubators to regulate the temperature to make sure the distribution of female turtles were the same as the male ones. She said they didn't want to play God and wanted to help conserve the turtles with minimum intervention possible.

These turtles she said live for quite long like other turtles do. Adult grows upto 24 inches. It seems 100,000 turtles were reported dead over a period of ten years ( 93-03) along the sea coast of Orissa. Imagine how many could've perished all over the world. No wonder their status is endangered or threatened.

I asked her what could've killed so many of them over the years. She said many reasons. Fishing expeditions, discarded nets, sometimes humans dig the eggs out to eat it ( though it's illegal ), some nests get plundered by raccoons, stray dogs, sometimes the nests get infested with bacteria. The very next day I saw an article in the paper about a turtle who had lost his flipper to a discarded net. Sad isn't it?

That day after the visit, my son and I had quite a conversation about how cute and vulnerable the hatchlings were and what we could do to save them. He wanted to adopt one. I'm not sure if that's allowed or if it's causing the turtle more trouble than helping him. Instead, we vowed to spread the awareness amongst our friends and to bring them to this conservatory to let them see for themselves. You know itseems even the balloon that we discard ( let fly in the air) if it happens to fly over the sea before it bursts, the debris the balloon leaves also kills many turtles. I din't know that. Let's spread the word, cause the awareness and save the cute turtles! 

Loss of weight or weight-less

You must be thinking, "physics posts, biology post, now tips to lose weight? Wonderful!!" Well, no tips to lose weight in this post, ( sorry for that, maybe later ) but I'm going to help you understand a new concept called weightlessness. That is, how to feel 'like' you've lost all your weight, when you very much haven't. And where could we see examples of this concept?

To start with the basics,  what is weight? Colloquially we use the terms 'mass' and 'weight' interchangeably. The amount of matter in an object. But, actually mass is the amount of matter in an object and weight is the amount of force with which an object is attracted towards the centre of the Earth ( could be any planet/moon, Earth in our discussion) , and is measured using a spring balance. That said, when we walk, or run or jog for that matter, do you feel any force pulling you down? No. Gravity force cannot be felt. Isn't it? If that's the case, why do you experience soreness when you sit for a long time? What is that pressure that is felt?

The force that acts in the upward direction when you sit is the contact force offered by the chair. Contact force is the force that occurs when two objects are in contact. Gravity on the other hand is not a contact force. Two objects needn't be in contact for Gravity to act. When we sit in our chair, the net force acting on us is zero, so the gravity force must equal the upward contact force. Gravity is not felt, so the force we feel is the contact force equal to our weight. Similar is the case when we stand on the weighing machine. The weighing machine measures the contact force, which is equal to our weight.

Think about this, what if the contact force turns out to be zero. The weighing machine would actually read zero!!?! And we might not feel our weight. Right? Maybe we get the floating-in-the-air feeling? That's good. Now how to make the contact force read zero? Maybe if the chair was falling? Free falling? Then it won't be stable enough to give the contact force. Contact force zero. The person sitting on it will feel weightless!! Ooh la la.

To understand this further, take a paper cup. Punch two holes towards the bottom. One on either side. Fill it with water. Notice water leaking out from either sides. Now drop the glass. How does the water behave? Watch the video from here to learn. Water was momentarily weightless.

Same is the case with a free falling elevator. Elevator whose cable has just snapped and while it's free falling, the unfortunate person or object inside would experience floating and weightlessness.


One another instance is skydiving. When the skydiver jumps from the plane, nothing to give him his contact force. Hence, he feels like he/she is floating. The familiar stomach in your mouth feeling. After a while, the drag force keeps increasing and the skydiver reaches the terminal velocity. In which case, the net force on him being zero, he feels his weight. Just like he were lying down on his bed or sitting in his chair. Then the parachute, sudden increase in drag force, the skydiver feels heavy, momentarily. Finally he reaches a lower terminal velocity and glides down safe.

Have you seen astronauts floating in their spaceship? In movies? That's because of weightlessness video. Explains it wonderfully too. Not because there is no gravity there. There definitely is some amount of gravity there, coz if not for gravity, the spaceship wouldn't orbit the Earth, but fly off into space.( the same as why moon orbits the Earth ) The astronauts feel weightless inside the ship because the ship is falling towards the earth due to gravity and so are the astronauts inside. No contact force. See this

Been to a roller coaster ride? The butterflies in your stomach feeling, which occurs when you've just begun a big descent is also the same weightlessness concept. When the roller coaster takes the descent with you, if fails to offer you your contact force, and you feel like you are lifting off from your seat.

Wonderful! To add on to your excitement, imagine getting into a lift with a weighing machine. And as the lift goes up and down, see if the weighing machine shows you different weights !! Sounds impossible isn't it. Just try it! The weighing machine will actually show weights corresponding to you feeling light or heavy when the lift moves down or up.

Wow! we've learnt a lot of things today. A new force called contact force. The concept of weightlessness. Why we feel light on a roller coaster just at the start of a big drop. Why astronauts float in space, and that it's not because there is no gravity there. And we know how a skydiver feels when he jumps off a plane. Have I inspired you to become a skydiver today? Have I inspired you to become an astronaut today? Do you feel like taking the lift till the highest floor of a tall building just to correlate your experience with what we just learnt? Too much to ask for!! I agree. Have I inspired you to ask thought provoking questions about the simple things that we see in our everyday life? Did you say 'yes'? I say 'mission accomplished'. I can see myself next to  Tom Cruise, with a gun in my hand, wearing all black and mission impossible music in the background! Oh gosh, that is mission impossible isn't it! And this is mission accomplished. I think I went too far!! (Sheepish grin) Never mind. Forget that! But remember the lift, the roller coaster, the astronauts and the skydiver and more importantly, the zero reading on the weighing scale!