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Finding weights on other planets

  1. Sep 26, 2005 #1
    Finding weights on other planets URGENT

    yeah, here comes a hw problem my class had that both me and my friends couldnt get. please help us :bugeye:

    A bag of sugar weighs 5.00 lb on earth. what should it weigh in newtons on the moon, where the free-fall acceleration is 1/6 that of earth. repeat for jupiter, where g is 2.64 times that on earth. find the mass of the bag of sugar in kilograms at each of the three locations.

    The book answer is 3.71N, 58.7M, 2.27Kg

    What just happened. can anyone walk me through this. My e-mail is madperseid@yahoo.com, and my aim sn is themadperseid. Thanks
  2. jcsd
  3. Sep 26, 2005 #2

    James R

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    Weight is mg, where m is the mass and g is the gravitational acceleration.

    For example, if an object has mass 5 kilograms, and is on a planet with g=10 m/s^2, then it weighs 50 Newton.

    The problem is getting you to think about the difference between mass (the amount of stuff in an object) and weight (the force of gravity on an object).

    You need to know that the freefall acceleration on Earth is 9.8 m/s^2. You'll also need to convert pounds to Newtons. (I don't know the conversion factor for that, since I live in a country which uses sensible units. I think pounds are a unit of weight, not mass, though.)
  4. Sep 26, 2005 #3


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    Depends on the system. In one version, pounds are force and mass is in poundals. In a more common system, there is the pound-force and the pound-mass: one pound-mass weighs one pound-force under one standard g.

    And people say those aren't sensible units! Hmph.
  5. Sep 27, 2005 #4
    riru, i hope u read this this morning. IVE GOT IT. ok, so here goes

    thus 5lbs=m*(9.8m/s^2)
    since 5lbs is not ci units, convert, using the ratio that 1n=.2248lb
    the conversion turns 5 lbs into 22.24n
    due to the nature of n (kg*m/s^2), you can divide out g [m/s^2], leaving only the kg mass of the bag on earth (22.7 kg)
    then, since w=mg
    you merely substitute the mass in, and multiply the gravity by the given ratio
    example for moon (2.27kg*((9.8)*(1/6)))=3.7N
    For the Jupiter one, you do the same, and multiply the gravity by the given ratio
    example for jupiter (2.27kg*((9.8)*(2.64)))=58.7n
  6. Sep 27, 2005 #5


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    I didn't bother to check the arithmetic, but you've got the concept down. Bravo!
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