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Homework Help: Two quick questions about gravity

  1. May 1, 2005 #1
    We feel a gravitational force of attraction wards the earth but no electrical attraction because gravity is in general a much strong force right?
    Last edited: May 1, 2005
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  3. May 1, 2005 #2


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    Actually, the gravitational force is relatively weak!
    If you calculate the ratio of the electrical repulsion to gravitational attraction of two electrons, you'll find that this ratio is very small.

    It turns out that a lot of everyday matter is effectively electrically-neutral (since there are negative charges that can balance positive charges). By contrast, there is nothing to balance out mass, the gravitational charge.
  4. May 1, 2005 #3


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    Depends on what you mean by "stronger force"! Given two particles of the same mass and charge, the electrical force between them will be much, much larger than the gravitational force. In that sense, gravity is a very weak force. However, in bulk, because positive and negative electrical charges cancel (while there is only one kind of gravity) gravity is much "stronger" than electrical force.
  5. May 1, 2005 #4
    thanks, so I gather that we feel a gravitational force but not an electrical attraction since everything is basically neutral.
    Last edited: May 1, 2005
  6. May 2, 2005 #5
    Coulombs law gives the attraction between two charges:
    \vec F_{12} = \frac{k q_1 q_2}{r^2}\hat{r}

    Where [tex]q_1[/tex] and [tex]q_2[/tex] are point charges, and [tex]k[/tex] is a proprtionallity constant.

    Then we have the gravitational force between two masses:
    F = \frac{G m_1 m_2}{r^2}
    Where [tex]m_1[/tex] and [tex]m_2[/tex] are "point" masses, and [tex]G[/tex] is a the gravitational constant.

    Now if we are looking at the attraction between two objects, we need to use the equations a LOT of times. We need to add up the forces exerted from each point. So we will integrate with the equations with respect to the object in question. Now [tex] q [/tex] can be either positive or negative. So when we start adding some positives with negatives we are going to get some cancellations.

    On the other hand, can we have a negative mass? nope. So [tex] m_1 [/tex] and [tex] m_2 [/tex] will always be positive. So each of those "points" keep adding and adding, and because they are positive...they will never cancel. So the more mass, the more gravitational force it will have. But with the electro-force if we are examining a planet, we might start adding up a huge amount of charges in the planet, but some will be positive and some will be negative, so the electrical force balances more or less.
    Last edited: May 2, 2005
  7. May 2, 2005 #6
    I always thought of it as comparing the two constants. If you hold two 1kg masses a meter apart, they will have a gravitational attraction of 6.67 x 10^-11N, whereas if you held two charges of opposite polaritya nd charge magnitude of 1C a meter apart, their attraction will be 9x10^9N.
  8. May 2, 2005 #7
    Coloumb force is 10000000000000000000000000000000000000 times stronger than gravitational force. :smile:
  9. May 2, 2005 #8
    "We feel a gravitational force of attraction wards the earth but no electrical attraction because gravity is in general a much strong force right?"

    okay, so... I'm standing here. Do I feel a force of attraction? no.. also, I have a practically neutral charge, so the statement is false right?
  10. May 2, 2005 #9
    Well, feel is a difficult quantifiable statement (in the sense of how to quantify it). I guess you do not feel the force of gravity in the sense of jumping into a cold pool. You most definitely notice the effects of gravity everyday (for example jumping doesn't cause you to continue on out into space. Now do you feel the electrical force on a daily basis? ... no. Do you notice the electical force? well kind of no... not in the big picture sense of gravity. It definitely isn't pulling you anywhere like gravity. It depends on the domain you are dealing with.

    Also in general is also a difficult statement to express. Coulombs force IS stronger, would I say "in general" it is stronger? no probably not. Just because talking about gravity usually doesn't involve talking about gravity with such small masses.

    But in general is a tricky statement to say. Would you say that in general a Honda Civic is faster then a Porsche? Well a Porsche is most definitely faster, but usually those cars are restricted to the street, so you never top out the Porsche. Also, everyday experience will show you that the Honda-Civic- driver drives their car around much faster then Porsche owners. Now the Porsche is obviously faster (just like coulombs force being stronger) but in general, when driving around in a big city (noticing how people drive) the honda civic is faster. So if you want to get all liberal with the words, the statement could be true or false. If you are looking for a straight up answer, I would say it is false because Coulombs force IS stronger.

    Who gave you this question by the way?
  11. May 2, 2005 #10
    I was given this question through a vauge hw assignment. It was a followup question for a reading on gravity, kepler's laws, and Einestein's General Theory or Relativity. It's only a true or false question, I still dont know how to answer it :confused:
  12. May 2, 2005 #11
    "We feel a gravitational force of attraction wards the earth but no electrical attraction because gravity is in general a much strong force?"

    Go with false.

    We feel a gravitational force of attraction towards earth, but no electrical attraction because the graviational force exerted by the earth is greater then the electrical force exerted by the earth. Not because gravity is a stronger force. You can prove that gravity is a weaker force by examining the proportionallity constants as whozam pointed out.

    If you get it wrong, then express your concern to the teacher and show them why you are right.
  13. May 2, 2005 #12
    so if the word stronger was switched with greater... it would be true?
  14. May 2, 2005 #13
    also, another one of these true and false questions is here

    I'm guessing form the response that it's false, but I heard that these paths could intersect because of the curvature.
  15. May 2, 2005 #14
    I like to think of it this way:

    It takes the whole mass of the earth to pull me back to the ground via the gravitational interaction when I jump, and it takes it a pretty long time too (like, a few whole seconds!). On the other hand, if I were to jump off of the top of a ten-story building, and gravity accelerated me through all that distance, the few particles in the section of sidewalk that I hit would still bring me to a dead stop in a millionth of a second via the electromagnetic interaction.

    You definitely "feel" the electromagnetic interaction in your everyday life - it's what stops you from walking through walls!
    Last edited: May 2, 2005
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