What is Gravity?
Gravity is a downward vertical force that keeps an object on the earth, or pulls it towards the surface of the earth. It is responsible for the weight that objects have, for the suspension of earth in space, its rotation round the sun, and also for the rotation of the moon round the earth.
It is a very powerful force that helps us get a lot of things done: like walking, eating, building, among others. An instance where there is no gravity is the moon, and if you have seen footages of the Apollo Lunar Landing, you would observe how the astronauts floated about in lunar space, and how it took them longer to perform the simple task of hoisting a flag.
But it seems there are exceptions you ask. How is it that birds, airplanes, kites and hot air balloons fly, and even remain suspended in air for hours? Does gravity become cancelled in these instances? No. Gravity is a constant, natural force. But there is another set of forces that make for these possibilities, it is called aerodynamics.
What is Aerodynamics?
Aerodynamics is the study of forces that allow objects move through air. An object – say a bird, or airplane – can only overcome gravity with the aid of aerodynamic forces, not cancel it.
What Are These Forces?
Lift is the push that lets something move up. It is the force that is the opposite of weight. Everything that flies must have lift. For an aircraft to move upward, it must have more lift than weight. A hot air balloon has lift because the hot air inside is lighter than the air around it. Hot air rises and carries the balloon with it. A helicopter’s lift comes from the rotor blades at the top of the helicopter. Their motion through the air moves the helicopter upward. Lift for an airplane comes from its wings.
Taking airplanes as an example, have you noticed that their wings are curved and rounder at the top, while they are flatter, and sharper at the bottom. That shape makes air flow over the top faster than at the bottom, because air cannot turn around a sharp edge. So, the oncoming air divides, then rejoins beyond the wings. The fast-moving air over the wings has less pressure than the slow-moving air at the bottom. This difference in air pressure lifts the airplane off the ground.
Because an airplane moves through air, it experiences drag, which is a force that tries to slow something down, making it hard for the airplane to move. Also, the faster an object moves through air, the greater the drag. This is where winglets at the tips of planes come in, they have a curved, tapering shape – from narrow at one edge, to wide at the other edge – and this reduces the mass of rotating air produced beyond the wings, thus reducing drag.
Thrust is the force that is the opposite of drag. Thrust is the push that moves something forward. For an aircraft to keep moving forward, it must have more thrust than drag. A small airplane might get its thrust from a propeller (the rotating fan blades seen in the picture above), but a larger airplane gets its thrust from its engines as seen in the picture below.
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