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First Year Physics Assignment - Theory

  1. Sep 20, 2015 #1
    1. The problem statement, all variables and given/known data
    Answer true, false, or Cannot tell to teach of the five statements below.
    A) A 2 kg ball and a 3 kg ball are dropped from a tower. The net forces on each other are the same.
    B) An astronaut stands on a bathroom scale on the surface of the moon; the scale reads the astronaut's true weight.
    C) A box with a 100 kg passenger is catapulted straight into the air. The passenger's true weight is zero once the box is airborne.
    D) You are standing on a jet-powered platform that can move straight up or down. Your apparent weight is greater than your true weight if the platform is ascending and speeding up.
    E) The mass of an object depends on its location.

    2. Relevant equations
    No equations, just theory.

    3. The attempt at a solution
    A) False, net force in free fall = weight of an object, and a 2 kg ball and a 3 kg ball have different weights.
    B) False, gravity is different on the moon and the bathroom scale would read differently.
    C) False, the passenger's apparent weight would be zero once box was airborne, but their true weight would remain the same.
    D) Unsure about this one, but I believe it would be "Cannot tell", because I'm unsure if "speeding up" = acceleration.
    E) False. Mass is constant.

    I have already tried entering these answers (False, false, false, cannot tell, false) and it is not correct. Does anyone have any advice or comments about how my reasoning is incorrect?
     
  2. jcsd
  3. Sep 20, 2015 #2

    Doc Al

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    Staff: Mentor

    What is the meaning of "true weight"? As compared to what?

    What's the definition of acceleration?
     
  4. Sep 20, 2015 #3
    "True weight" would be the "real weight" on earth of the person (their mass x 9.8 m/s^2), as opposed to apparent weight. And oh! Okay, D is true.
     
  5. Sep 20, 2015 #4

    Doc Al

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    I would say that "true weight" reflects the actual force of gravity, as opposed to the "apparent weight". Does "true weight" change on the moon?
     
  6. Sep 20, 2015 #5
    No, it does not! Therefore, B is true! Thank you very much!
     
  7. Sep 20, 2015 #6

    Doc Al

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    Careful now. I would not agree with that. Reread my response above. Is the actual force of gravity different on the moon's surface compared to earth's?
     
  8. Sep 20, 2015 #7
    Yes, it is. The force of gravity on the moon is 1.622 m/s^2, while gravity on earth is 9.8 m/s^2.
     
  9. Sep 20, 2015 #8

    Doc Al

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    Right. So would an object's true weight be different on the moon versus on the earth?
     
  10. Sep 20, 2015 #9
    The weights would be different, but I think they would still both be true weights. I think true weight is just mass x gravity wherever a person/object is.
     
  11. Sep 20, 2015 #10

    Doc Al

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    I agree with that. So what's your answer for B?
     
  12. Sep 20, 2015 #11
    True!
     
  13. Sep 20, 2015 #12

    Doc Al

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    Staff: Mentor

    Good!
     
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