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Newtons law of gravity?

  1. Jun 3, 2008 #1
    1. The problem statement, all variables and given/known data

    Imagine taking your dog for a walk. You feel a real bond for man's best friend. Could this be due to gravitational attraction? Estimate the force of gravity between you and your dog.

    2. Relevant equations

    force~mass1 x mass2 divided by distance^2

    3. The attempt at a solution

    So am I just supposed to estimate the weight of the man/dog, and the distance?

    so, 100kg x 30kg divided by 5m^2? giving me an answer of.....

  2. jcsd
  3. Jun 3, 2008 #2
    Right down the formula of gravitational attraction from your handbook. You will see you forgot a very important part of the formula.

    Now you say 120N. This means that something of 120 kg would accelerate at 1 m/s² at your dog. Now, you dont see big garbage containers flying at your dog now do you ;).

    Try and relate to the reality. Look at the important part of the formula you forgot, and look at how big it is.
  4. Jun 3, 2008 #3
    ok, I forgot the G, sorry, so the awnser would be.... 0.000000008?
  5. Jun 3, 2008 #4


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    Building on Swatje's post, you've shown the proportionality (the ~ symbol) between force and mass/distance.

    But you have not shown the formula equating (the = symbol) force with mass/distance.

    And the difference between the two you will see, as Swayje ponits out, there is a very significant bit of the formula missing.

    [Edit: never mind. You got it.]

    1] what are the units of that number, and
    2] does this value seem right to you?
  6. Jun 3, 2008 #5
    ok wait, so i got it right?
  7. Jun 3, 2008 #6
    1.) N? cause its a force?
    2.) its looking for the force of attraction so i think it would be small?
  8. Jun 3, 2008 #7


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    Hi vandorin! :smile:

    Suppose you're carrying the dog …

    what's the gravitational attraction then? :smile:
  9. Jun 3, 2008 #8
    I have no idea....can someone please give me a hint :'( ?
  10. Jun 3, 2008 #9


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    While I haven't done the calculation myself it does appear plausible. Certainly it should be tiny.

    And yes, I wanted you to list the units so you understood the meaning of the number. It is a force.

    I think he's just kidding. Don't let it confuse you.
  11. Jun 3, 2008 #10


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    I'm not kidding!

    The gravitational force will be greatest when the dog is closest (and most loved :!!) ).
    Well, how far is your centre of mass from the dog's centre of mass? :smile:
  12. Jun 3, 2008 #11
    Ok, back to the problem, G = 6.67 x 10^-11 so i Did that times 120 and got the .000000008
  13. Jun 3, 2008 #12
    Yes you could do that and calculate it. But that's not the purpose of the exercise.

    The purpose is to look at G. G = 6.67 * 10^-11. This means 0.0000.... A very small number. And just by looking at this small number you should be able to guess that it's probably not the case, just as we see in real life. It only counts if m1*m2/r² is very very very large. But like really large, in maginuted of billions. And this is why gravitational force is only noticeable between planets, because they are large enough.

    If you would compare it to, for example electrostatics, like, your hair that goes all wierdishly when statically charged, you see that very little can cause huge forces, in comparison to gravitational force. So actually gravity is quite weak.
    Last edited: Jun 3, 2008
  14. Jun 3, 2008 #13
    but was my answer right?
  15. Jun 3, 2008 #14
    I didnt check. But you didnt have any concrete data to start on, but it seemed plausible.
  16. Jun 3, 2008 #15


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    Regardless, this is a distraction to a serious poster who has not gotten a satisfactory answer yet.
  17. Jun 3, 2008 #16

    D H

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    G = 6.67 x 10^-11 what? .000000008 what? Many of us sincerely want you to express units when you work out an answer.

    G is not a number. It is a physical constant; its value depends on the system of units. For example, G might be (and is) 39.478 in some other system of units.
  18. Jun 3, 2008 #17
    Ok, so how would I figure out G for this equation?
  19. Jun 3, 2008 #18

    D H

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    In the international system of units (SI, for short), G is 6.6730*10-11 meters3/kilogram/second^2 (or 6.6730 newtons-meters2/kilogram2). Always carry the units around. If nothing else, doing so will tell you when you cannot possibly have the correct answer. For example, in your first post you calculated the answer to be 120. If you had explicitly written down the units along with the numbers, you would have seen that that 120 was actually 120 kilogram2/meter2, which is not a unit of force.
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