The Physics of Everyday Things by James Kakalios

Author:James Kakalios
Language: eng
Format: epub, pdf
Publisher: Crown/Archetype
Published: 2017-05-15T16:00:00+00:00

You board the aircraft and make your way down the aisle to your assigned seat. It’s not hard to spot the only unoccupied one in the full plane, way in the back, a window seat. You place your small bag underneath the seat in front of you and fasten your seatbelt. The main flight attendant announces that the airplane door has been closed and that all personal electronic devices should be turned off or switched to airplane mode (you do the latter for your smartphone and check that your tablet is off). After a few minutes the plane begins taxiing toward the runway. The pilot comes on the public address system, welcomes you onboard, and indicates that your flight is number two for takeoff. The flight attendant begins the well-practiced safety presentation, and almost as soon as she says the last word, the pilot tells the crew to prepare for takeoff. The plane idles for a moment at the start of the runway, and then the throttle is opened and the craft begins its acceleration. Gaining speed, the airplane begins, gently at first, to angle up toward the sky. Soon you are airborne.

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If you want to leave the surface of the Earth, whether in a hot air balloon or a supersonic jet, you need more air molecules to strike the bottom of your craft than the top, leading to a net unbalanced upward force, and if this force is greater than your weight, it will lift you up, up, and away.

Hot air balloons and blimps rise for the same reason ice cubes float at the top of your drink rather than sit on the bottom of the glass—a difference in density leads to an upward force, called “buoyancy.” The magnitude of the buoyant force depends on the size of the object, that is, how much of the fluid or air is displaced by the volume of the object. If the weight of the object is greater than the weight of the medium it displaces, it will sink; if it is lighter, it will float.*2 To get a balloon to rise, one fills it with a gas (typically either hot air or helium) that has less mass per volume (density) than the surrounding atmosphere. For transportation in a device that has a density greater than air, one has to work a little harder in order to arrange for the atmosphere to provide a net upward force.

An airplane’s wings are shaped and curved, with a larger profile on top of the wing, so that the air moving beneath the wing is deflected downward and the air moving above the wing turns upward. The air beneath the wing is jammed into a smaller volume, and when this happens the air pushes back on the wing in the opposite direction. Here we make use of Newton’s third law of motion: For every action there is an equal and opposite reaction—or, forces come in pairs. You cannot push on something without that something pushing back on you.

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