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APPARENT WEIGHT

AP TOPIC 2.6 (APPARENT WEIGHT)

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AP TOPIC COVERED

Topic 2.6: Gravitational Force

Learning Objectives:

  • Describe the conditions under which the magnitude of a system’s apparent weight is different from the magnitude of the gravitational force exerted on that system.

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THINK ABOUT IT

Can you think of a circumstance where your motion causes you to feel heavier or lighter than your actual weight?

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APPARENT WEIGHT

  • A change in the force acting on an object due to acceleration in the direction of gravity is called “apparent weight”.
  • The magnitude of the apparent weight of a system is the magnitude of the normal force exerted on the system.

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MEASURING NORMAL FORCE

  • Imagine standing on a scale.
  • What force is the scale measuring?

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MEASURING NORMAL FORCE

  • Imagine standing on a scale on a stationary elevator. What will the scale measure?
  • On an elevator starting to move upward?
  • On an elevator starting to move downward?

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MEASURING NORMAL FORCE

Lab Time!

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PRACTICE #1

A person of mass m is standing on a scale in an elevator that is accelerating downward with an acceleration of magnitude a. Which of the following is a correct expression for the normal force exerted on the person by the scale?

  1. mg C. m(g-a)
  2. ma D. m(g+a)

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PRACTICE #2

Elevator A is moving upward at a constant speed of 4m/s and Elevator B is moving downward at a constant speed of 2m/s. In each elevator, a 10kg object is on a scale. The scale in Elevator A reads FA and the scale in Elevator B reads FB. Which of the following correctly compares FA and FB, and provides a valid justification?

  1. FA > FB because elevator A is moving up while elevator B is moving down
  2. FA > FB because elevator A has a greater speed than elevator B
  3. FA = FB because two objects with the same mass always have the same apparent weight
  4. FA = FB because both elevators are traveling with a constant speed

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PRACTICE #3

A 100 kg astronaut will be sitting his seat during takeoff of a spaceship. If the spaceship has a maximum acceleration of 25 m/s2, what apparent weight of the astronaut should the mechanical engineer designing the seat take into consideration?

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