an airplane fly?
Four forces keep an airplane in the sky: lift,
gravity, thrust, and drag. As an airplane speeds down the runway, the
airplane’s wings disrupt the flow of air. The disruption causes the air
above the wings to move faster than below the wings. As a result, the
air pressure above the wings is less than below the wing. This gives the
airplane lift, and lift pushes the airplane up.
Gravity is the force that acts against lift.
Gravity pulls the airplane toward the Earth. Airplanes are built so that
their weight is spread out from front to back keeping the airplane
balanced. If the downward gravitational force is less that the upward
lift, the airplane stays in the air.
Thrust is the force that moves the airplane
forward. Engines provide thrust. Sometimes the engine turns a propeller,
or sometimes it is a jet engine. Thrust keeps air moving over the wings
so lift can be created.
As an airplane moves forward, there is a backward
force, called drag, caused by the resistance of the air. Drag slows the
airplane down. You can feel drag when you walk against a strong wind.
Airplanes are designed to let air pass around them with less drag.
All four forces work together. In order to remain
in flight, the thrust must be greater than the drag, and lift must be
greater than the force of gravity.
How does a
microwave cook food?
A microwave oven uses microwaves, a type of
electromagnetic radiation, to heat food. Waves in the frequency range of
microwaves are absorbed by water, fats, and sugars. When they are
absorbed, microwaves are changed into atomic motion. This means that the
molecules that make up the food vibrate, and vibrating molecules produce
heat. They are not absorbed by most plastics, glass, or ceramics. Metal
reflects microwaves, and this is why metal pans cannot be used.
Microwave ovens cook food from the inside out. To
better explain this: let’s say you want to bake a cake in the oven.
Normally, you would bake a cake at 350 degrees, but what if you
accidentally set it to 600 degrees. The outside of the cake would burn
before the inside ever got warm. In an oven, the heat has to move from
the outside to the inside. The inside of an oven has hot dry air that
evaporates moisture. This allows the outside to be crispy while the
inside is moist.
In microwave cooking, the microwaves enter the food
and cause water and fat molecules to vibrate evenly throughout the food.
The vibrations cause heat everywhere all at once because all the
molecules are excited together, so the food heats from the inside out.