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Rock Tumbling

  1. Jun 13, 2012 #1
    In a rock tumbler, the stones collide with each other and break apart. Eventually they become smooth. I was thinking about exchange of energy like when billiard balls hit. When two billiard balls collide, they exchange their energy. It's a little different if they have different amounts of momentum. This makes me question how a rock tumbler works. Can someone explain what is it about rock tumbling that causes the rocks to break up when they collide?
  2. jcsd
  3. Jun 14, 2012 #2
    Yes its true that the energy of an isolated system remains constant. But here , in this case , the rock tumbler is to be shaken by some external force, hence the system is not isolated.
    Further, the answer to your question is that the force is the force is the cause of breaking up of stones.
  4. Jun 14, 2012 #3


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    Staff: Mentor

    When two objects collide and exchange energy and momentum, there must be a force involved - thats' how the energy and momentum is transferred.

    For example, suppose I drive an automobile into a brick wall. Before the collision, the automobile is moving with some speed v. A very short time later, the automobile is at rest. Therefore, the wall has exerted a force on the automobile to decelerate it during that time, and (if I assume for simplicity that the force is constant) the force and the time are related as: [itex]F=P/\Delta T[/itex] where F is the force, P is the momentum the automobile started with and ΔT is the time it takes for the collision. The force is very large, and sheet metal deforms plastically, so when the wall exerts that force on the automobile it crushes the sheet metal and smashes the car.

    When two billiard balls collide, the same thing happens, except that the momentum and hence the forces are much lower, and a billiard ball is tough enough that the forces aren't sufficient to physically damage it.

    What happens in your rock tumbler is somewhere in between. The force at the moment of impact is large enough, and the stone is brittle enough, that impacts between large pieces may break them up; but as the pieces become smaller and the momentum and force is reduced, the stones don't break but instead any projecting irregularities are knocked off.
  5. Jun 14, 2012 #4


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    It is the grit in the tumbler that causes the wear and smooth finish. A more coarse girt for the beginning tumble to remove projections and sharp corners, and moving down to finer grit for the smoother finish. Some collisions between the stones will cause pieces to break off but that is a minor feature and an inconvienience when going thru the pollishing phase. Rock tumbling is a similar to sanding a piece of wood of sand blasting a metal surface. In all cases a grit is used as the medium to achieve the desired affect. You will note that most of the removed material ends up as dust.
  6. Jun 14, 2012 #5
    Thank you.
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