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Gravity, center of planets, stars

  1. Nov 25, 2013 #1
    Hello , it is said that at the center of planets and stars there is zero gravity because , assuming spherical geometry, the gravity from all sides comes together and cancels out in the middle and everything that is in close approximation to it.
    Now that said I also read that the pressure at sun and possibly other stars is the greatest at the center , now gravity is the force responsible for this pressure yet it cancels out at the center due to similar forces from all sides how come that add to the greates pressure at center ? Thank you.
     
  2. jcsd
  3. Nov 25, 2013 #2

    russ_watters

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    Staff: Mentor

    Welcome to PF!

    Gravity and pressure are two very different things. The gravitational force ON YOU is what pulls you toward the center of the earth. But the pressure on you is the weight (gravitational force) on what is above you. So the pressure is due to the fact that most of the mass of the earth is NOT located at its center.
     
  4. Nov 25, 2013 #3
    When you are in the center, both halves of the planet(star) attracting each other and can crush you, but can not move you in any direction. In the center pressure is the same from any direction.
    If you are not exactly in the center, in some directions mass will be greater than this in the opposite direction. Forces and pressure will not be in equilibrium and you will be pushed to the center.
     
  5. Nov 25, 2013 #4
    Okay so every mass attracts every other mass , now in a star the force of gravity in the center cancels out as it is coming in similar strength from all sides , now as I understand the pressure arises from the fact that two masses are attracting so they exert pressure on themselves, the higher the mass the bigger the pressure , just like adding weights on a measuring device.

    But as I said in the center of a star the gravity cancels out because it is coming from all sides equally , but since the gravity is coming equally from all sides the pressure too must come equally from all sides , so what are you saying that gravity cancels out but the pressure which is a result of gravity attracting mass doesn't cancel out yet squeezes with double force as one mass is pushing against other mass in the center and so from all possible directions?
     
  6. Nov 25, 2013 #5

    Nugatory

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    Yes. It might be easier to visualize if you think in terms of just two large masses, one on each side of you. The left hand mass is pulling on right-hand mass, and the right-hand mass is pulling on the left-hand mass, so they want to move towards one another and squash whatever has the misfortune to be caught between them. That's pressure.

    However, if we could somehow hold them apart so that they didn't come together to crush you... right in the middle of the open space between them the gravitational field will be zero as their pulls cancel out. A small object at that point will be attracted towards neither.

    [edit: except that I didn't notice the word "double" - that part is wrong, as russ_waters points out below]
     
    Last edited: Nov 25, 2013
  7. Nov 25, 2013 #6

    russ_watters

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    No. Pressure acts in all directions, equally. If it pushes from the left at X and pushes from the right at -X, the pressure is just X, not 2X.
     
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