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When you drop a ball from above, why doesn't it drill through earth?

  1. Jul 28, 2013 #1
    First of all, I want to say that I am totally ignorant about physics. And I do not understand the general concepts of physics at all.

    Now coming to the question.

    Suppose, you drop a green tennis ball from above.

    Now It usually lands on earth and stop there ( ignore the bounces for now. ) Suppose, the point at which the green ball lands is a red ball. Since the earth is made of ''a mass of balls'' ( to get the concept, I am using such metaphors ), below the red ball should be another ball, and below that another ball, and so on, till you reach the opposite point on the earth, which is, suppose, a pink ball.

    Now since all the balls from the red ball upto the pink ball is drawing the green ball toward them, how can this one little green ball stop just over the red ball ignoring the pull of all the balls between the red and the pink one?

    Shouldn't it just ''break the red ball'' and keep reaching toward the next ball and break that too? I mean, drilling through earth?

    I know the earth is more or less a sphere. So are the pulls from balls around the red ball preventing the green ball from drilling through earth?
  2. jcsd
  3. Jul 28, 2013 #2
    I have a strong urge to ask you if you are high on pot :P

    Anyways, what exactly mean by 'pulls from the balls' and 'break the balls'?
  4. Jul 28, 2013 #3


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    There's a couple of factors at work here, but I think the main one is that objects cannot pass through other objects because there is a repulsive force between them when they touch. They cannot break through objects unless they can break the bonds holding the objects atoms and molecules together. The force of gravity simply cannot accelerate a tennis ball to a high enough velocity for it to break through another tennis ball.

    Does that make sense? It is very difficult to explain if you know nothing about physics. An understanding of at least the very very basic fundamentals is required if you truly want to know why.
  5. Jul 28, 2013 #4


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    I assume he's asking why gravity does not cause objects to drill through the Earth. The first tennis ball breaking the second would be like it breaking through the surface of the Earth. That's what I'm getting from it at least.
  6. Jul 28, 2013 #5
    Oh that way.

    Well if you know a bit of physics, if you apply a force on an object and the object can apply an equal and opposite reaction force, it wont budge. If your force is higher that what it can provide, it'll move/break.

    The the force we apply can easily be countered by the Earth.

    But consider the construction of buildings. A strong foundation is built first. If that is not done or if the bearing capacity of the ground is not high enough, the building begins to sink in. Why? Because an equal opposite force cannot be provided by the ground and there is a resultant force in the downward direction.
  7. Jul 28, 2013 #6
    What kind of force is this ''repulsive force''? Is it related to gravitational force? Between which things is this repulsive force acting? And is this single repulsive force between the green and the red ball enough to counteract all the attractive gravitational forces exerted by all the balls between the red and the pink balls?
  8. Jul 28, 2013 #7
    'Elasticity' is a physical property of materials which return to their original shape after they are deformed. Like a tennis ball or a rubber band for example. They are not very stiff under load. So some stuff 'bounces'.

    Dirt is sometimes rather elastic, more so when wet, but tends not to restore while rock tends to shatter rather than deform. All these behaviors result from the chemical bonds of the individual materials.

    For one everyday material to penetrate another you need rather a lot of energy, like a bullet, and certain material composition...a 'bubble gum' bullet won't do much, and rubber bullets used by authorities to disperse crowds are fired at lower energy [less gunpowder is used] and deform on impact, so are far less lethal than metal bullets.

    A tennis ball does not have sufficient gravitational attraction, not enough force, not enough energy to do much of anything..... but bounce. Likewise you sitting on your chair at a computer luckily don't 'drill though the earth because the gravitational effects between you and earth are far to weak.

    You'd need a lot of gravitational attraction, a lot of force/energy, to overcome the repulsive force Drakkith mentions. The forces holding materials together are in general a lot stronger than gravitational forces at the surface of the earth. And those forces are a lot stronger than you...so push as you might,for example, you can't get your hand to go into a concrete wall.
  9. Jul 28, 2013 #8
    Yep, that's what I'm trying to say. Thanks for helping.
  10. Jul 28, 2013 #9
    As far as I understand, according to the law of gravitation, all the things in the universe are attracting each other. In other word, pulling each other. That's what I meant by ''pulls from the balls.''

    And by ''break the balls'', I meant due to ''pull from the balls between the red ball and the pink ball'', I thought, the green ball would keep falling down through the red ball, thus breaking the red ball. And then, break the next ball, and then the next, and so on.
  11. Jul 28, 2013 #10
    Thank you very much for your reply. I didn't get this repulsive force though, and I made some questions to Drakkith.
  12. Jul 28, 2013 #11
    Repulsive force is the Force of reaction acting on the ball when it falls to the ground.

    It's like this. Consider yourself wearing a metallic armour. In front of you is a wall. Behind that wall is Magneto (from X-men).

    He tries to pull you towards himself. But the wall stops you at a certain distance. He continues to pull you, like the Earth pulls the ball, but the wall prevents it. The wall will be broken down only when the magnetic force wll exceed a certain limit.
  13. Jul 28, 2013 #12


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    There is electrostatic repulsion at the microscopic level between the interacting bodies. This repulsion is orders upon orders upon orders upon orders of magnitude stronger than the gravitational interaction.
  14. Jul 28, 2013 #13


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    See what WannabeNewton said.
  15. Jul 28, 2013 #14
    So is the repulsive electromagnetic force ( as WannabeNewton said ) acting between the outer electrons of the green ball and the outer electrons of the red ball? And if so, is this repelling force between these two balls greater than the pull from all the balls between the red and the pink ball toward the green ball combined?
    Last edited: Jul 28, 2013
  16. Jul 28, 2013 #15
    So is the repulsive electromagnetic force acting between the outer electrons of the green ball and the outer electrons of the red ball? And if so, And if so, is this repelling force between these two balls greater than the pull from all the balls between the red and the pink ball toward the green ball combined?
    Last edited: Jul 28, 2013
  17. Jul 28, 2013 #16


    Staff: Mentor


    Yes, otherwise it would break the red ball, as you said.

    Consider two protons placed one meter apart. Their gravitational attraction is 1.9E-64 N but their electrical repulsion is 2.3E-28 N, which is 36 orders of magnitude larger. It is very easy for the electrical force to overwhelm the gravitational force.
  18. Jul 28, 2013 #17
    Thank you very much. I wonder why electromagnetic force is not as ''famous'' as or more so than the gravitational force.
  19. Jul 28, 2013 #18
    That's a subjective comparison but the gravitational 'force' is 'famous' [unique] it that it stands distinct and apart from the other three fundamental forces [strong, weak, electromagnetic]. Gravity affects everything whereas the other forces are specific: for example the electromagnetic force is limited to electrically charged particles, like the proton and electron. And gravity is described via a geometrical theory rather than a field theory which has so far made it impossible to mathematically combine gravity with the other three forces in the Standard Model of particle physics.
  20. Jul 28, 2013 #19
    Umm for one, it's the gravitational force that keeps us on the Earth.
  21. Jul 28, 2013 #20


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    Some of it is historical, some if it is practical.

    The practical reason is that Newtonian gravity is easier to teach and understand without advanced math, so is taught first. Even high-school physics includes a competent introduction to the basic concepts of Newtonian gravity; electricity and magnetism is usually a college-level course. So there's just a lot more people out there who have a nodding acquaintance with gravity; for many people their only experience with how math can be used as a tool for understanding the world is from studying gravity.

    The historical reason is that the mathematical formulation of Newtonian gravity was discovered and put to use solving practical problems (ballistic trajectories, falling objects, planetary motion) two centuries before we had an equivalent understanding of electromagnetism.
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