8.01x - Lect 7 - Weight, Weightlessness in Free Fall, Weight in Orbit

Lectures by Walter Lewin. They will make you ♥ Physics.・2 minutes read

The lecture focuses on weight, defined as the force exerted on an object by gravity, with examples of weight changes in different scenarios, such as free fall and acceleration in elevators, using Newton's Second Law and tension in strings. Zero gravity experiments on airplanes like the KC-135 study motion sickness under weightless conditions, with participants alternating between weightlessness and increased weight every 90 seconds.

Insights

  • Weight is directly linked to mass and gravity, with the force exerted on an object by gravity defining its weight, as seen in scenarios like elevator accelerations and free fall.
  • Tension in a string can indicate weight, remaining constant in a frictionless system, with acceleration affecting weight readings, leading to situations like objects of different masses having the same weight when suspended.

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Recent questions

  • What is weight?

    Weight is the force exerted on an object by gravity, often measured on a bathroom scale. It is directly related to an object's mass and the force of gravity acting upon it.

  • How does acceleration affect weight?

    Acceleration can impact an object's weight. When accelerating upwards in an elevator, the force from the scale must exceed gravity for acceleration to occur, increasing weight. Conversely, accelerating downwards results in decreased weight as the scale force is less than gravity.

  • What is free fall?

    Free fall occurs when acceleration due to gravity cancels out, resulting in weightlessness and a reading of zero on the scale. During free fall, all objects are weightless, like in a spacecraft in orbit or an elevator with a cut cable.

  • How is tension in a string related to weight?

    Tension in a string can indicate weight, with tension increasing when accelerating upwards and decreasing when accelerating downwards. The tension remains constant due to the absence of friction and the masslessness of the string.

  • What happens during zero gravity experiments?

    Zero gravity experiments are conducted on airplanes like the KC-135 at high altitudes, creating a cycle of free fall, increased weight, normal weight, and weightlessness every 90 seconds. Participants experience weightlessness and increased weight alternately to study motion sickness under such conditions.

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Summary

00:00

Understanding Weight and Gravity in Physics

  • Weight is the focus of the lecture, defined as the force exerted on an object by gravity, often measured on a bathroom scale.
  • Weight is directly related to mass and gravity, with the force from the bathroom scale defining an object's weight.
  • On Earth, weight is typically measured in pounds, but can also be calibrated in newtons.
  • When accelerating upwards in an elevator, the force from the scale must exceed the force of gravity for acceleration to occur.
  • Newton's Second Law is applied in this scenario, with the bathroom scale indicating an increase in weight due to acceleration.
  • Accelerating downwards in an elevator results in a decrease in weight, as the force from the scale is less than gravity.
  • Free fall occurs when acceleration due to gravity cancels out, resulting in weightlessness and a reading of zero on the scale.
  • Weight can also be determined by the tension in a string, with tension increasing when accelerating upwards and decreasing when accelerating downwards.
  • The tension in a string remains constant throughout due to the absence of friction and the masslessness of the string.
  • Objects of different masses can have the same weight when suspended by strings, with tension indicating weight and acceleration affecting weight readings.

17:19

Weightlessness and Tension in Free Fall

  • When m1 is much larger than m2 and m2 approaches zero, a goes to g and T goes to zero, causing the object to go into free fall.
  • In the case where m2 is larger than m1, m1 gains weight while m2 loses weight due to acceleration, leading to changes in tension in the string.
  • Using easy numbers like m1 as 1.1 kg and m2 as 1.25 kg in a frictionless system with a negligible mass string, the acceleration of the system is approximately 0.064 g.
  • Substituting the numbers for m1 and m2, the tension in the system is calculated to be 1.17 g, with both sides having the same tension.
  • When an object is at the bottom or top of a circular motion, there is a centripetal acceleration that must be present for the motion to occur.
  • At points P and S in the circular motion, the tension in the string is determined by the centripetal acceleration and gravity acting on the object.
  • If the centripetal acceleration is exactly 10 m/s^2, the object will be weightless, but if it's larger, there will be tension in the string.
  • During free fall, all objects are weightless, like in a spacecraft in orbit or an elevator with a cut cable.
  • When jumping off a table, there is a brief moment of weightlessness, followed by a significant increase in weight upon impact due to acceleration.
  • A specially designed bathroom scale with a fast response time can show weight dropping to zero during free fall, as demonstrated by Professor Dave Trumper.

34:00

"Weightlessness and Gravity in Zero-G Experiments"

  • The bathroom scale has a cover on it, and the actual mass on top of the spring is 4½ pounds.
  • A barbell weight of 10 pounds is placed on top of the scale, resulting in a reading of roughly 14½ pounds.
  • The weight is taped down and dropped from about 1½ to two meters, causing the scale to show zero weight upon impact.
  • The weight jiggles due to slight accelerations upwards and downwards, affecting its measured weight.
  • Zero weight is achieved during free fall, with the weight increasing upon impact and then returning to weightlessness.
  • Zero gravity experiments are conducted on an airplane, such as the KC-135, at an altitude of about 30,000 feet.
  • The airplane goes into free fall, creating a parabola trajectory, with passengers experiencing weightlessness.
  • The cycle of free fall, increased weight, normal weight, and free fall repeats every 90 seconds during the experiments.
  • The experiments aim to study motion sickness under weightless conditions, with participants experiencing weightlessness and increased weight alternately.
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