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Math Help - mass, and accelerating elevator

  1. #1
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    mass, and accelerating elevator

    A 96.0 kg person stands on a scale in an elevator. What is the apparent weight in each of the following situations?
    when the elevator is accelerating upward with an acceleration of 1.90 m/s^2
    N

    when the elevator is moving upward at a constant speed
    N

    when the elevator is accelerating downward with an acceleration of 1.50 m/s^2
    N


    i thought the accelerating upward problem was as simple as 1.90 m/s^2 * 90 which would give the newtons, but i don't think i'm right here.

    need some help with each of these please.
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  2. #2
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    Quote Originally Posted by rcmango View Post
    A 96.0 kg person stands on a scale in an elevator. What is the apparent weight in each of the following situations?
    when the elevator is accelerating upward with an acceleration of 1.90 m/s^2
    N

    when the elevator is moving upward at a constant speed
    N

    when the elevator is accelerating downward with an acceleration of 1.50 m/s^2
    N


    i thought the accelerating upward problem was as simple as 1.90 m/s^2 * 90 which would give the newtons, but i don't think i'm right here.

    need some help with each of these please.
    As always draw a FBD, set positive directions, and apply Newton's 2nd. Make sure that you set the acceleration to 0 for the constant velocity part.

    (And acceleration is not measured in N. Check your units!!)

    -Dan
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    ya, for the first one, the net force is equal to the mass * g and then added to the mass * the acceleration.

    so g is that gravity?

    thanks alot.
    Last edited by rcmango; October 20th 2007 at 01:53 AM.
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  4. #4
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by rcmango View Post
    ya, for the first one, the net force is equal to the mass * g and then added to the mass * the acceleration.

    so g is that gravity?

    thanks alot.
    You've got the idea. However I'm a stickler for proper language here: g is not "gravity," it is the "acceleration due to gravity." And if you want to be really precise it is the "acceleration due to the Earth's gravity at the surface of the Earth."

    Many people just shorten the phrase "acceleration due to gravity" to just "gravity." (I've caught a number of professor's doing this too!) But I warn people against it since they are calling g, an acceleration, by the name "gravity," which is a force. I have seen a number of students giving the units of g as Newtons because of this, which is (of course) incorrect.

    -Dan
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  5. #5
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    ya, thanks for the explanation too.
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