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Homework Help: What If We Had Negative Mass?

  1. Jan 4, 2008 #1
    This isn't really a homework question... but it is homework... and I have a question... so I thought I might as well post it here.

    This is part of my grade 12 Physics curriculum. For our project, we're supposed to ask ourselves "What if <this happened>?"

    I chose to do mine on negative mass.

    I have attached my progress so far (.doc format), and I wanted to hear if someone knowledgable could verify some of my theories...

    I think everything is right... but I'm not so sure about my conclusion about friction.

    If [TEX]F_{f} = \mu mg[/TEX] (F_{f} = \mu mg) and m is negative, then friction might actually be in the opposite direction, cancelling out the negative acceleration. But I'm still not so sure. Give er' a read if you'd like to help me out, and let me know.


    Edit: I also realized my Free Body Diagram is incorrect for the freefall question. Gravity should be up because Fg = mg. But acceleration is still down because accel is opposite of Fnet.

    Attached Files:

    Last edited: Jan 4, 2008
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  3. Jan 4, 2008 #2

    Andrew Mason

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    Consider the gravity of a negative mass by applying Newton's Law of Universal Gravitation. What would the direction of acceleration be between two negative masses? If there was matter with negative mass, would its gravity attract or repel ordinary mass?

  4. Jan 4, 2008 #3
    A negative mass and a positive mass would both exert repelling forces against eachother, correct?

    But the effect of a repulsive force on a negative mass still results in acceleration towards the positive body, because Fnet = ma, and m is negative, so acceleration is in the opposite direction of the unbalanced force. The positive body would accelerate away due to the repulsive force.

    A negative and a negative would exert attractive forces. Therefore, accelerration would be away from eachother.

    I talk about this in the .doc file, and have another section that will talk more about this. But currently the attachment is still pending approval...

    I'd like to know about my friction question though. And the validity of the statements I've made once you can see the .doc file.
    Last edited: Jan 4, 2008
  5. Jan 4, 2008 #4
    Now think about what that would do to inertia.
  6. Jan 5, 2008 #5

    Shooting Star

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    Consider this situation -- positive and negative masses of magnitude m are placed at rest at a certain distance from each other. What happens?
  7. Jan 5, 2008 #6

    Shooting Star

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    This just occurred to me. Would you be able to measure the negative mass of a body using a common balance, which is used to measure mass? What weight will it show on a spring balance?
  8. Jan 5, 2008 #7
    Well this is what I have for my current chart of how bodies of different masses affect eachother.

    For each negative object, the acceleration opposes the direction of the force of gravity, since Fnet = ma and m is negative.

    For each positive object, the acceleration is in the same direction as the force of gravity.

    http://img409.imageshack.us/img409/5420/fgsz7.png [Broken]

    Using a common balance, the object would weight the same, because conventionally the scale measures the upward Normal Force. In this case, it would measure the upward Gravitational Force, because it still accelerates downward into the scale with that upward force.
    Last edited by a moderator: May 3, 2017
  9. Jan 5, 2008 #8

    Shooting Star

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    So, a common balance does not measure mass, as we had been taught. It only measures the magnitude of the mass?

    What about the spring balance?
  10. Jan 5, 2008 #9
    Well what I said is that it measures the magnitude of the normal force. This is then internally proportioned to "weight" by the balance...

    It should be the same.

    I'm wondering why I'm being quizzed. Could someone begin to correct me? I'm sure I've made mistakes all over the place. ;)
  11. Jan 5, 2008 #10

    Shooting Star

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    What gave you the idea that you're being quizzed. I was pointing out what I thought were interesting phenomena, so that you'd think of more "what if" scenarios.
  12. Jan 5, 2008 #11
    Ah, I gotchya. I've taken note, and have lots more ideas for additions. :)

    Unfortunately I'm going to have to restrict the report now to kinematics and planetary motion. It's already getting really bulky. But I'll try to fit in more "what if" scenarios for the scale and such.

    Too bad I can't edit my attachment. I'll have to upload the revision here.

    It's fixed lots of mistakes, incorrect terminology, etc, very much improved... but it's still incomplete.

    Attached Files:

    Last edited: Jan 5, 2008
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