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Thread: Question about weight

  1. #1 Question about weight 
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    The radius of the earth is r. A satellite of mass 100kg is at a point 3r above the earths surface. What is the satellites' weight?

    6.3 N
    110 N
    61N
    38 N
    240 N
    Thanks! Please show what calculations you are using to solve

    PS: What i dont understnad is why they are asking for the weight while the state teh mass is 100kg? Grr, i dont understand!


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  3. #2  
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    I could be wrong but it seems as if this problem also lacks sufficent information for an answer. Why are you asking for weights and then listing day's as answer choices? I know how to do the other problem but I don't even have a clue as to what is being asked in this one.

    Also I have found that I learn physics best when I do it myself, not get other to do it for me. Invest in a good text book with sample problems in the chapters and you should be able to learn alot of it yourself. Also is this a precalculus physics class or Calculus based physics class? Most problems have a calculus solution and a more direct approach.


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  4. #3  
    Forum Radioactive Isotope MagiMaster's Avatar
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    Weight is (mass) * (acceleration due to gravity).
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  5. #4  
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    Quote Originally Posted by GenerationE
    I could be wrong but it seems as if this problem also lacks sufficent information for an answer. Why are you asking for weights and then listing day's as answer choices? I know how to do the other problem but I don't even have a clue as to what is being asked in this one.

    Also I have found that I learn physics best when I do it myself, not get other to do it for me. Invest in a good text book with sample problems in the chapters and you should be able to learn alot of it yourself. Also is this a precalculus physics class or Calculus based physics class? Most problems have a calculus solution and a more direct approach.
    Omg, i put the answers from a diff question hahahaha
    sorry sorry sorry, i am very stressed, i dont understand a thing!
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  6. #5  
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    Thats not to bad then, all you need is the gravitational equation. Gravity is a constat 9.8m/s*s near earths surface but since your satellite is far above the surface of the earth you have to use a different equation. I Think its is this

    F= G (mass of satellite* mass of earth)/r^2

    G is the gravitational constant and R is the distance between the two objects.

    Edit: remember G is 6.67*10^-11 in this problem not 9.8
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  7. #6  
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    omg is that the equation??
    I thought that was just when you were trying to find the distance or something between 2 objects/satellites....
    omg im so scared for my test tomorrow
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  8. #7  
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    If you learned something else use it. I think its the right equation to use but I could be mistaken. It's been over a year since I have had physics. Remember I doubt few people on this forum have a degree in physics. I am working towards a degree in Electrical Engineering but that by no means makes me a expert on physics.
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  9. #8  
    Forum Radioactive Isotope MagiMaster's Avatar
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    F=ma or a=F/m, so you should be able to use those equations together for your answer.
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  10. #9 Re: Question about weight 
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    Quote Originally Posted by Jprojectrunway
    PS: What i dont understnad is why they are asking for the weight while the state teh mass is 100kg? Grr, i dont understand!
    Oh, by the way grams is measured in mass. Mass and weight are different. Weight is the samething as saying Newtons or Force. Weight = M*g; Force = M*gravity in certain situations.

    so saying that something has a mass of 100kg's doesn't tell you the Weight. In this problem you have to figure out what gravitation to use to multiply by the 100kg mass.
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  11. #10  
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    Quote Originally Posted by MagiMaster
    F=ma or a=F/m, so you should be able to use those equations together for your answer.
    I'm pretty sure that equation wouldn't be enough to answer the question as it is posed. you posted one equation with two unknowns. Your right in assuming that those equations go with a situation like this. But your missing acceleration(gravity) what is it's value that high above the earths surface?
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  12. #11  
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    I meant F=ma together with the equation for gravity and that for the speed of a circular orbit.
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    The weight of a 100 Kg mass at the earth's surface is mg or 100*9.8=980N.

    Gravity follows the inverse square law so at 3r the weight would be 980*[(r/3r)^2]. That's about 110.
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  14. #13  
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    Quote Originally Posted by Harold14370
    The weight of a 100 Kg mass at the earth's surface is mg or 100*9.8=980N.

    Gravity follows the inverse square law so at 3r the weight would be 980*[(r/3r)^2]. That's about 110.
    Yeah, that does equal 110, why can you use 9.8 m/s*s when the satellite is so far away from earths surface harold? I thought you would of had to use

    The Force to to gravitation = GMm/r^2 ?
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  15. #14  
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    Quote Originally Posted by GenerationE
    Quote Originally Posted by Harold14370
    The weight of a 100 Kg mass at the earth's surface is mg or 100*9.8=980N.

    Gravity follows the inverse square law so at 3r the weight would be 980*[(r/3r)^2]. That's about 110.
    Yeah, that does equal 110, why can you use 9.8 m/s*s when the satellite is so far away from earths surface harold? I thought you would of had to use

    The Force to to gravitation = GMm/r^2 ?
    The simple answer is that I have the value of g memorized, but I would have had to look up the values of G and M. I didn't need to know those, only the ratio based on the relative distances and the inverse square law.
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