Answers to Last Issue's Do You Know?
1. Our bones are so heavy. Yet, how do we float?
Ans: This is because of Archimedes' principle. There are different forces acting on an object that is either fully or partially submerged. The water exerts an upward (buoyant) force on the object, while the weight of the object acts downwards. Depending on which is greater, the object either floats or sinks. Archimedes was the first to describe the buoyant force. He said that this upward force is equal to the weight of the fluid that the body displaces.
What does this mean? If you take a metal plate, and try to float it on water, it displaces very little water, so it sinks. If you take the same mass of metal and shape it into a cup, it will float. How does this happen? The cup encloses a larger volume, so it sinks until the volume of water it displaces has a weight equal to itself. At this point, its weight is balanced by the equal and opposite buoyancy force and it remains at that depth, floating.

The mass of the fluid that is displaced is equal to its density D times the displaced volume V (because the density is described as mass per unit volume).  So the weight of the displaced fluid is D V g, where g is the acceleration due to gravity. If M is the mass of the object, its weight is M g. So the forces are balanced when
M g = D V g
or M = D V.
Two things can be seen. One, the balance of forces is independent of acceleration due to gravity, g. Second, it is tempting to say that the mass of the object can be written in terms of its density, so if its density is less than that of water, then it will float. But is the second statement correct? The density of steel is much more than water, but a steel cup floats! So we have to take into account the shape of the object, because that is what determines the volume of the displaced fluid. For example, that is why a ship floats. It is shaped so that the volume of water it displaces is large enough to provide the required buoyancy force for it to float. The formula above, in terms of mass of the object is always correct. But for objects like plates, cylinders, and even human beings, it is a good approximation to say that it will float if its density is less than that of water.

Now we come to human beings. Humans have a density very close to that of water. We are mostly bone, muscle and fat. Bone (3 gm/cc) and muscle (1.055 gm/cc) are dense compared to water (1 gm/cc) while fat is less dense (0.8-0.9 gm/cc) than water. So depending on the proportion of these three in your body, you will either barely float, or easily float. If you fill your lungs with air, you will become more buoyant, and if you float with head down and arms and legs spread out, you will displace more fluid and float more easily (just like cup versus plate).


2. An apple falls down but the Moon doesn't. Is gravity different on Earth and the Moon?
Ans: This question was not properly posed. The question should have been, "An apple falls down, but the Moon doesn't. Why? Doesn't gravity act on the Moon as well?" The answer is that gravity acts the same way on an apple and the moon.  Since the same physics applies, the interesting part is that the Moon is also falling, but doesn't fall on Earth! How do we understand this?

The simplest way to understand what is really quite a complicated array of forces is to look at it the way Newton did.  Newton believed that the force of gravity was universal, and it was the key force for planetary motion.  Newton's cannonball was a thought experiment. A thought experiment is one where the experiment is not actually done, but the scientist imagines what would happen if he did the experiment.

The experiment appears in his book, "De mundi systemate", published after his death in 1728, nearly 300 years ago! (It was also published in English as A Treatise of the System of the World.) In this experiment from his book, Newton imagined a stone being projected from the top of a high mountain, and that "that there is no air about the Earth", so he ignored air resistance (think parachutes).

As a gravitational force acts on the projectile, it will follow a different path depending on its initial velocity. If the speed is low, it will simply fall back on Earth. As you throw it with more and more horizontal initial velocity, the stone travels farther and farther before it falls back to Earth. If the speed reaches what is known as the orbital speed at that altitude, it will go on circling around the Earth along a fixed circular orbit "and return to the mountain from which it was projected".  

If the speed is higher than the orbital velocity, but not high enough to leave Earth altogether (lower than the escape velocity), it will continue revolving around Earth along an elliptical orbit. If the speed is very high, it will leave Earth in a parabolic (at exactly escape velocity) or hyperbolic trajectory. This is how spacecraft are sent to the Moon and Mars, overcoming Earth's gravity.


3. Do dolphins and whales sleep?
Ans: Sleep is crucial for all living beings. Like humans, dolphins have two hemispheres in their brain. But while we rest the whole of our brain (and body) through sleep at the same time, for dolphins and whales it is not so easy as all that.

Humans are involuntary breathers because our breathing is automatically controlled by our brainstem. This means that we breathe without thinking about it and breathe automatically even when we fall asleep!  

Whales are voluntary breathers meaning they must think about taking each breath. If they were to fall completely asleep, their brain would rest but they would drown without their brain reminding them to breathe! Instead, they have a fascinating adaptation known as ‘unihemispheric sleep’. To be able to sleep, whales shut down half of their brain at a time. Being partially awake allows them to continue breathing and be aware of their surroundings.  

Whales will usually stay in place as they sleep near the surface. As they rest, they are switching which hemisphere of the brain is awake.  They usually float vertically just below the water surface. This resting behavior is often called ‘logging’ because they resemble a log floating at the water’s surface.

Different species have different sleep methods and requirements and the amount of sleep needed can vary greatly between species. For example, researchers believe that sperm whales spend only ~7% of their day sleeping, usually in short naps of 10-15 minutes at a time! This finding makes sperm whales the species that needs the least amount of sleep in the planet! Giraffes are a close second who spend around 8% of their day in slumber.

Dolphins also have to come to the surface when they rest in order to breathe, which they do consciously (by choice). If they didn’t surface, they could drown.  They also have to be alert to changes in local conditions and to the presence of predators such as sharks, or even humans.

Dolphins close the left eye when the right half of the brain sleeps, and vice versa, alternating which half of the brain is sleeping so that they can rest without losing consciousness.  Dolphins often lie motionless at the surface, breathing regularly, or swim slowly close to the surface.  In shallow water, they might sleep on the seabed, rising regularly to breathe. So, they do sleep, but not like we do.

Marine mammals such as whales and dolphins spend their entire lives at sea. Observations of bottlenose dolphins in aquariums and zoos, and of whales and dolphins in the wild, show two basic methods of sleeping: they either rest quietly in the water, vertically or horizontally, or sleep while swimming slowly next to another animal. Individual dolphins also enter a deeper form of sleep, mostly at night. It is called logging because in this state, a dolphin resembles a log floating at the water's surface.

When marine mammals sleep and swim at once, they are in a state similar to napping. Young whales and dolphins actually rest, eat and sleep while their mother swims, towing them along in her slipstream--a placement called echelon swimming. At these times, the mother will also sleep on the move. In fact, she cannot stop swimming for the first several weeks of a newborn's life. If she does for any length of time, the calf will begin to sink; it is not born with enough body fat or blubber to float easily.

Other methods help marine mammals to hold their breath longer than other types of mammals can. Marine mammals can take in more air with each breath, as their lungs are proportionately larger than those in humans. In addition, they exchange more air with each inhalation and exhalation. Their red blood cells also carry more oxygen. And when diving, marine mammals' blood travels only to the parts of the body that need oxygen--the heart, the brain and the swimming muscles. Digestion and any other processes have to wait.

Finally, these animals have a higher tolerance for carbon dioxide (CO2). Their brains do not trigger a breathing response until the levels of CO2 are much higher than what humans can tolerate. These mechanisms, part of the marine mammal diving response, are adaptations to living in an aquatic environment and help during the process of sleeping. Cetaceans reduce the number of breaths they take during rest periods; a dolphin might average 8 to 12 breaths a minute when fairly active only to have their breathing rate drop to 3 to 7 per minute while resting.


4. Which are the smallest and largest birds in the world?
Ans: Would you like to see the world’s smallest bird? Then you’ll need to travel to Cuba. Once on the island, your best bet for tracking down the tiny wonder is to visit a forest edge hung heavily with vines and bromeliads. There, hovering at the flowers — if you squint hard enough — you’ll find the Bee Hummingbird.

The Bee Hummingbird, which is found only in Cuba, is an absolute miniature, even among hummingbirds. It measures a mere two and a quarter inches long. Bee Hummingbirds are often mistaken for bees. They weigh less than two grams — less than a dime. That’s half the weight of our backyard hummers, like the Ruby-throated or Rufous. The female builds a nest barely an inch across. Her eggs are about the size of a coffee bean.

In flight, the Bee Hummingbird’s tiny wings beat 80 times a second. And during a courtship flight, they beat up to 200 times per second! The male’s entire head and throat shine in fiery pinkish-red, and blazing red feathers point like spikes down the sides of the breast. A sight to behold!

What about the biggest bird? That award goes to the ostrich. Ostriches are large flightless birds. Two living species are recognised, the common ostrich, native to large areas of sub-Saharan Africa, and the Somali ostrich, native to the Horn of Africa.

They are the heaviest and largest living birds, with adult common ostriches weighing anywhere between 63.5 and 145 kilograms and laying the largest eggs of any living land animal. With the ability to run at 70 km/h, they are the fastest birds on land. They are farmed worldwide. Ostrich leather is a lucrative commodity, and the large feathers are used as plumes for the decoration of ceremonial headgear. Ostrich eggs have been used by humans for millennia. 

The ostrich is the biggest of all the birds on Earth, both in size and weight. These behemoth birds grow up to 2.7 m tall and can weigh up to 130 kg. Despite having a wingspan of up to 7 feet (2 m), ostriches are unable to fly. Instead they use their wings in a similar way to how a ship uses its sails. During a fierce 70 km per hour run, these birds open out their wings and use them as air-rudders for rapid braking and steering. This agility enables them to escape some of the many threats they face on the African savanna, including predatory species such as lions and jackals. In some situations, ostriches will go on the offensive and use their powerful clawed feet to deliver a blow strong enough to kill a lion.

The marabou stork from Sahara in Africa, the tall Emu of Australia, Happy Eagles and the Greater Rhea of South America, the Albatross ad Cassowary of Antarctica are among the world's biggest. The Dalmatian Pelican of Europe is among the biggest flying birds in the world.

Sources:
https://www.usms.org/fitness-and-training/articles-and-videos/articles/why-do-you-float-in-water,
https://wikipedia.org,
https://www.scientificamerican.com/article/how-do-whales-and-dolphin/,
https://www.discoverwildlife.com/animal-facts/marine-animals/how-do-dolphins-sleep,
https://www.audubon.org/news/get-know-bee-hummingbird-worlds-smallest-bird
https://www.livescience.com/biggest-birds-on-earth