# Materi Pelajaran

A. Aerodinamika

Why the aeroplane can fly ?

If you know about Bernoulli Laws, we can explain about that. Please read about Bernoulli Laws.

An airplane flies because its wings create lift, the upward force on the plane, as they interact with the flow of air around them. The wings alter the direction of the flow of air as it passes. The exact shape of the surface of a wing is critical to its ability to generate lift. The speed of the airflow and the angle at which the wing meets the oncoming airstream also contribute to the amount of lift generated.An airplane’s wings push down on the air flowing past them, and in reaction, the air pushes up on the wings. When an airplane is level or rising, the front edges of its wings ride higher than the rear edges. The angle the wings make with the horizontal is called the angle of attack. As the wings move through the air, this angle causes them to push air flowing under them downward. Air flowing over the top of the wing is also deflected downward as it follows the specially designed shape of the wing. A steeper angle of attack will cause the wings to push more air downward. The third law of motion formulated by English physicist Isaac Newton states that every action produces an equal and opposite reaction (see Mechanics: The Third Law). In this case, the wings pushing air downward is the action, and the air pushing the wings upward is the reaction. This causes lift, the upward force on the plane.

Lift is also often explained using Bernoulli’s principle, which states that, under certain circumstances, a faster moving fluid (such as air) will have a lower pressure than a slower moving fluid. The air on the top of an airplane wing moves faster and is at a lower pressure than the air underneath the wing, and the lift generated by the wing can be modeled using equations derived from Bernoulli’s principle.

Lift is one of the four primary forces acting upon an airplane. The others are weight, thrust, and drag. Weight is the force that offsets lift, because it acts in the opposite direction. The weight of the airplane must be overcome by the lift produced by the wings. If an airplane weighs 4.5 metric tons, then the lift produced by its wings must be greater than 4.5 metric tons in order for the airplane to leave the ground. Designing a wing that is powerful enough to lift an airplane off the ground, and yet efficient enough to fly at high speeds over extremely long distances, is one of the marvels of modern aircraft technology.

B. Medan dan Potensial Listrik

Seorang anak pernah bertanya, berapa medan dan potensial listrik pada konduktor bola berongga yang berjari-jari 4 cm dan bermuatan listrik 4 mikro coulomb pada a) jarak 2 cm dari pusat bola,b) di kulit bola dan c) pada jarak 10 cm dari pusat bola.

Jawab :

Oleh karena kecenderungan muatan listrik tersebar merata di permukaan konduktor bola berongga, maka tidak ada muatan di dalam bola konduktor, sehingga kuat medan listrik di dalam bola adalah nol, dan potensial di dalam bola sama dengan di kulit bola, sehingga :

a) E = 0 , V = 9.109 . 4.10-6 / 0,04 = 900.000 V

b) E = 9.109 . 4.10-6 / 0,042 = 57,6 N/C dan V = 9.109 . 4.10-6 / 0,04 = 900.000 V

c) E =  9.109 . 4.10-6 / 0,12 = 3.600.000 N/C dan V = 9.109 . 4.10-6 / 0,1 = 360.000 V