Wednesday, 27 November 2019

Can gravity be infinite? if not why?

Niagra waterfall
The kinetic energy that water gains when it falls (and can therefore be converted into electricity by a hydroelectric plant) comes ultimately from sunlight and not from gravity. As a force, no energy can be extracted from gravity itself. Public Domain Image, source: Christopher S. Baird.

No, gravity can not be used as an infinite energy source. In fact, strictly speaking, gravity itself can not be used as an energy source at all. You are confusing forces with energy, which are very different things. Energy is a property of objects, such as balls, atoms, light beams, or batteries. In contrast, forces describe the interaction between objects. Forces are the way that energy is transferred from one object to another when they interact, but forces are not the energy itself. Gravity is a force, so it just provides one way for objects to exchange and transform energy to different states.
If I lift a bowling ball to the top of a hill and let it go, the ball falls, speeds up, and seems to gain energy. Isn't this a case of gravity giving energy to the bowling ball? No. Again, gravity is just a force, so it just describes how objects interact. The energy that the ball displays as a falling motion came from my muscles when I hefted the bowling ball to the top of the hill, and not from gravity. Gravity just provides a way to temporarily store energy in an object. We call the energy that an object gains when you lift it against a force "potential energy". The energy comes from the lifting agent and not from the force. The force just provides a way to transfer energy from one object (my muscles) to another object (potential energy in the lifted ball). When I let go of the ball, gravity converts the potential energy of the ball to the kinetic energy (motional energy) of the ball. But the ball can never gain more kinetic energy than the total potential energy that I put into it by lifting it.
This concept is true of all forces, and not just gravity. Two magnets attract each other and fly together, speeding up and seeming to gain energy. You may think that the energy has come from the magnetic force. In truth, the energy comes from your hand pulling the two magnets apart against the magnetic force. The magnetic force just provides a way for potential energy to be stored in the magnet (by virtue of you pulling them apart, not just by virtue of them being magnets), and then converted from potential energy to kinetic energy. Any time you push an object to a new location against a force, you are giving it potential energy.
It is true that gravity is "unlimited" in the sense that it never turns off. Earth's gravity will never go away as long as it has mass. But since this is just a force and not an energy, the never-ending nature of gravity cannot be used to extract infinite energy, or any energy at all, for that matter. Think of gravity loosely like a rubber band. Stretch the rubber band and let go and it snaps back into place. You can therefore store potential energy in a rubber band by stretching it, and this potential energy becomes kinetic energy when you let go. But an unstretched rubber band just sitting there won't move at all, and can't create any energy. The energy you see in the rubber band snapping comes from you stretching it and not from the rubber band itself. Neglecting heat losses, the kinetic energy that comes out of the rubber band (how much it snaps) is exactly equal to the potential energy that you put into it using your muscles (how much you stretch it). Lifting an object against gravity is just like stretching the rubber band.
Confusing energy and forces leads to non-sensical ideas such as free energy (perpetual motion) machines. Such machines always fail precisely because forces are not energy, and you can't extract one single bit of energy from a force itself. For instance, a "free energy" machine could consist of a ball that rolls down a hill and hits a paddle, which turns a wheel. The problem with this machine is that the ball has to be returned to the top of the hill for the process to continue, and the amount of energy you have to put into your machine to put the ball back at the top of the hill equals the energy you get out of your machine from the spinning wheel. Actually, the amount of energy you get out of your machines is always less than the energy you put into it because some of the inputted energy is wasted to heat energy through friction. Free energy proponents devise ever cleverer ways to get the ball back to the top of the hill (or the magnets separated again, or the rubber band stretched again, etc.), hoping that just one more extra gear or wheel will somehow magically create energy out of nothing. But they can never get around the fact that forces are not energy and you can never get more energy out of a system than you put in.
What about hydroelectric plants that extract energy from the falling water in rivers? Don't they extract energy from gravity for free? No. The water in the river is no different from the ball that you have to haul up the hill. The water got its energy not from gravity but from some external agent that placed it high up in the mountains against gravity, so it could fall down the river bed. The external agent in this case is sunlight. Sunlight warms the ocean, causing the water to evaporate and float into the sky. The energy contained in the photons of sunlight is converted to the potential energy of the water molecules that are lifted high in the sky. These water molecules then rain down to the ground, form rivers, and flow back down to the ocean, converting their potential energy to kinetic energy, heat, and (in a hydroeletric plant) electricity. Ultimately, therefore, hydroelectric plants extract solar energy from water.


Tuesday, 19 November 2019

When stuck in water, bees create a wave and hydrofoil atop it, study finds

Walking on Caltech's campus, research engineer Chris Roh (MS '13, Ph.D. '17) happened to see a bee stuck in the water of Millikan Pond. Although it was a common-enough sight, it led Roh and his advisor, Mory Gharib (Ph.D. '83), to a discovery about the potentially unique way that bees navigate the interface between water and air.
Roh spied the bee during California's years-long drought, when the pond's fountain was turned off and the  was still. The incident occurred around noon, so the overhead sun cast the shadows of the bee—and, more importantly, the waves churned by the flailing bee's efforts—directly onto the bottom of the pool.
As the bee struggled to make its way to the edge of the pond, Roh noticed that the shadows on the pool's bottom showed the amplitude of the waves generated by the bee's wings, as well as the interference pattern created as the waves from each individual wing crashed into each other.
"I was very excited to see this behavior and so I brought the honeybee back to the lab to take a look at it more closely," Roh says.
Working with Gharib, Caltech's Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering, Roh recreated the conditions of Millikan Pond. They placed water in a pan, allowed it to become perfectly still, and then put bees, one at a time, in the water. As each bee flapped about in the water, filtered light was aimed directly down onto it, to create shadows on the bottom of the pan. Roh and Gharib studied 33 bees individually for a few minutes at a time, carefully scooping them out after a few minutes to let them recover from their swimming efforts.
A paper describing what they found was published in the Proceedings of the National Academy of Sciences on November 18.
"The motion of the bee's wings creates a wave that its body is able to ride forward," Gharib says. "It hydrofoils, or surfs, toward safety."
Slow-motion video revealed the source of the potentially life-saving asymmetry: rather than just flapping up and down in the water, the bee's wings pronate, or curve downward, when pushing down the water and supinate (curve upward) when pulling back up, out of the water. The pulling motion provides thrust, while the pushing motion is a recovery stroke.
In addition, the wingbeats in water are slower, with a stroke amplitude—the measure of how far their wings travel when they flap—of less than 10 degrees, as opposed to 90-120 degrees when they are flying through the air. Throughout the entire process, the dorsal (or top) side of the wing remains dry while the underside clings to the water. The water that remains attached to the underside of the wing gives the bees the extra force they use to propel themselves forward.
"Water is three orders of magnitude heavier than air, which is why it traps bees. But that weight is what also makes it useful for propulsion," Roh says.
The bees do not seem to be able to generate enough force to free themselves directly from the water, but their  motion can propel them to the edge of a pool or pond, where they can pull themselves onto dry land and fly off. Hydrofoiling is a lot more taxing for the bees than is flying, says Roh, who estimates that the bees could keep up the activity for about 10 minutes, giving them a fixed window to find the edge of the water and escape.
The motion has never been documented in other insects, and may represent a unique adaptation by bees, Roh says.
"On hot days, bee hives require water to cool off," Roh says. "So when the temperature rises, workers are sent out to gather water instead of pollen." The bees will find a water source, swallow some into a special chamber in their bodies, and then fly off. Sometimes, however, they fall in. And if they cannot free themselves, they die.
Roh and Gharib, who work in Caltech's Center for Autonomous Systems and Technologies (CAST), have already started applying their findings to their robotics research, developing a small robot that uses a similar motion to navigate the surface of water. Though labor-intensive, the motion could one day be used to generate robots capable of both flying and swimming.
The study is titled "Honeybees use their wings for water surface locomotion."