7: Weight, Perceived Gravity, and Weightlessnes

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Topics covered: This lecture explores weight, perceived gravity, weightlessness, free fall, zero perceived gravity in orbit.

Instructor/speaker: Prof. Walter Lewin

Date recorded: September 22, 1999

Video Index

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  • What is Weight?
    Weight is the force exerted on you by a bathroom scale. Your weight increases when you are in an elevator which is accelerated upward; you weigh less than your normal weight when the elevator is accelerating downward, and your weight is zero when the elevator is in free fall. When you hang from a rope, your weight is indicated by the tension in the rope.

  • Tension in Massless String
    Consider a string of negligible mass suspending two objects with different mass on either side of a frictionless pin. The tension in this string is everywhere the same because the string is "massless" and the pin frictionless. The masses of the two objects are NOT equal but the weight of the two objects IS THE SAME!

  • Weight, when Swinging around on a String
    An object is swirled around on a string in a vertical plane. The tension in the string is evaluated when the object is at the top and when it is at the bottom of its circular trajectory. The tension at the bottom is always higher than the normal weight of the object, but the tension can be zero when the object is at the top in which case the object is weightless.

  • Objects in Free Fall are Weightless
    Exploring the weight of a tennis ball being tossed in the air, and of a bottle of water in Professor Lewin's hands when he jumps off a table. The bottle and Lewin are in free fall, thus both are weightless.

  • Weight Measurements of a Free Falling Object
    Bathroom scales have too slow a response time to indicate your zero weight when you weigh yourself while jumping off a table. Professor Dave Truemper has built a scale with a response time of 10 ms (it uses pressure gauges instead of springs). Professor Lewin places a 10 lb object on the scale, and we see that the scale indicates this weight. He then tapes the object firmly to the scale and drops the scale with the object from about 2 meters. The weight of the object is zero during free fall!

  • Professor Young's "Zero Gravity" Experiments
    NASA has sponsored "zero gravity" experiments to study motion sickness. These are zero weight environments (not zero gravity) created in air planes which free fall for 30 seconds. The air plane's (a KC135) trajectory is discussed in detail, and one of Professor Young's video clips is shown.

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