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Homework Help: Does an object make tons of micro movements while moving?

  1. Jan 25, 2014 #1
    1. The problem statement, all variables and given/known data

    This isn't homework, just pursuing some physics on my own and curious.

    2. Relevant equations
    So I've got 2 questions which have been sorta bothering me.

    1-When I throw something into the air, does it stop for a tiny time at each molecule it meets (Newtons third law) during which the molecule moves away and the object keeps moving?

    2- In regards to gravity, If I just spin a wire with a ball connected to the edge and spin it there's a force acting perpendicular to the speed vector, isn't that the same on earth (I realize it's more complicated due to many things, like not having a wire connected to us..) but in general isn't it the same thing (asking this since heard it's unknown where gravity comes from).

    3. The attempt at a solution
    Those are my "solutions" look to verify and get better!
  2. jcsd
  3. Jan 25, 2014 #2
    1- I'm not exactly sure about the explanation you are seeking. The object you toss will have some kinetic energy, and it will collide with air molucules thus giving some energy to them, so that they are pushed away from the object (and into more air molecules). For instance if you wave your hand hard enough, you can even feel a "wind", which is molecules moving faster due to the kinetic energy, you gave them. I don't think you can say, that anything "stops" for a short while. Their direction is changed, but they were already moving pretty randomly.

    I am not really sure, how the 3rd law of motion works here, and if you can even apply it to gas molecules (as you would usually use statistics here). Personally I understand it better, by looking at energies, as that are typically, what you do regarding gasses. I guess the 3rd law would be okay, but it also applies for the molecules, as they also exert a force on the object as they are also in motion.

    Not a short or accurate answer I guess, maybe someone else can be clearer regarding the 3rd law of motion.
  4. Jan 26, 2014 #3


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    No. Newtons law doesn't say an object will stop, just that it will experience a reaction force as it pushes the air out of the way.
  5. Jan 26, 2014 #4


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    Centripetal force is not the origin of gravity. Gravity doesn't require the objects to be spinning. Two stationary objects in space are still attracted to each other. On earth Gravity provides the centripetal force to stop you flying off into space.
  6. Jan 26, 2014 #5


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    Good questions, and: cherish that curiosity!
    On 1: An object you can toss into the air is generally a solid and generally contains of the order of 1020 to 26 atoms or molecules. In a solid the molecules are fairly close huddled up, bound by some force, most of the time electromagnetic. This maintains the shape of the object so much we can threat it as if it were, well, an object. We deal with it in a macroscopic way: weight, position, orientation, velocity, temperature.

    Air is a gas, meaning the forces between the molecules are smaller and the average distances between them a lot greater. They move about at high speed and collide zillions of times per second (it is very instructive to calculate average speeds, speed distributions, number of collisions per time, per volume etcetera). Sometimes we think to know that the speed is dependent on temperature, but in fact it's the other way around. Pressure has to do with the number of molecular collisions, be it among each other or against the walls of a container, or an object tossed into the air: molecules, all of them.

    For the subject at hand you can treat these collisions as hard-ball collisions, which on a molecular scale they are to a great extent, but on a submolecular scale they are not. They are spring-like interactions that change the trajectories of the particles involved. Far too many to worry about. Especially when the size differences between the colliding participants are of the order of magnitude of > 106, let alone of the order of magnitude of the number above.

    In a (imaginary) hard-ball, head on collision of two equal billiard balls with exactly equal speed, there is an indivisible moment both have speed 0. The kinetic energy is zero, but since we believe in conservation of energy, it must be somewhere else. Going from colliding blocks of butter, via rubber balls to hard balls, we can convince ourselves that it's been converted to energy in compression, a kind of energy as stored in a compressed spring. Spring energy is re-converted into kinetic energy by expansion force and re-appears as kinetic energy, ideally without losses. The billiard balls have swapped all momentum. Ideally.
    You could say that the balls have stopped, but under an imaginary submolecular microscope the shuddering and shaking of the molecules in the solids has not stopped at all: there is no dropping of the temperature to absolute zero, therefore motion must have continued.

    There is no concept of stopping when dealing with molecules. Changing speed, direction of motion, slowing them down, ok.

    Now think of the collision between an ideally hard billiard ball and an ideally hard wrecking ball, again head on. Billiard ball at high speed and wrecking ball hangs still. Do you think one could notice the effect on the big guy? No way. Momentum differences after and before are imperceptible. Not zero though!
    But that imperceptible effect increases if we start firing thousands of billiard balls per second at the wreching ball. At least it wouldn't hang completely vertical from its chain any more.

    Wikipedia has nice gif animations of brownian motion http://en.wikipedia.org/wiki/Brownian_motion
    that are a beautiful intermediate step between molecules among themselves and an object tossed up in the air. (Note that the pictures do NOT have gravity).
    5 molecules, dust speck, grain of salt, coin, billiard ball form a sequence of objects that 'feel' relatively less and less of individual collisions with molecules. Not only because of the bigger and bigger weight, also because of the area: they get hit more and more frequently from all sides.

    Someone like you was curious enough to wonder about the chaotic motions of pollen in water under a microscope and had the effect named after him. Someone else did some quantitative work on this and it now counts as confirmation of the existence of molecules. He became the most famous scientist of all times!

    Back to our wrecking ball in a gas of billiard balls.
    Hanging still: collisions from all sides, average net result zero. But wait, gravity makes air pressure height dependent! So a bit more from below than from above. Eureka! In the passing we've come by Archimedes' discovery as well: in a vacuum the chain tension would be a little higher than in air.
    Throwing it up means the 10 to the umpteenth molecules bound together have an average speed upwards. More collisions from above than from below. There's what we call the air resistance in supra-molecular circumstances. Dust takes longer to settle than a cloud of wrecking balls...;-) Do I have the sensation of Galilei enthousiastically nodding his head ?

    Bottom line: there is a lot of exchange of momentum on the fly. Newton's third law in action! 'Object' behaviour is determined by averages over 10^many collisions that have almost no effect on object (but the individual air molecules experience a ruddy hard collison with a pretty hard rubber wall). No stopping.

    On 2. You are absolutely right. Same thing. I have a hard time imagining what you mean with "a wire with a ball connected to the edge and spin it ". But yes, as the world turns, we don't fly off into space in a straight line because earth and we attract each other. (Newton 3 again!). That's a force vector, perpendicular to the velocity. It doesn't change the speed, only the direction of the velocity.

    As an feeble attempt at humour: If the population on one half of the world jumps up at the same time, the earth moves a little in the other direction. Not for long, but still. However, the center of gravity of world + population) follows the same path as in the case nothing happened.
  7. Jan 26, 2014 #6
    Thanks for the great answers, just a few more questions:
    Shouldn't gravity cause the pressure on top to be greater? I mean particles being pulled into the object.

    Secondly, I believe I'm misunderstanding something and would love clarification:
    Say I hit a billiard ball with a stick, it starts going in some direction, it hits another ball and causes that ball to move forward while it itself moves backward- but following me hitting the ball it has 0 force acting on it (ignoring friction) so why would it bounce back? What third law of Newton is present here? My assumption is that the energy it has becomes elastic and that's the "force" acting on the 2 balls.

    That bothers me with a ball bouncing as well, to begin with, doesn't hitting the floor cause it to bend? I mean if not is there stored an infinite amount of energy in those chemical bonds? (Note i'm not talking about hitting it hard, just repeated small hits so it doesn't break, but by newtons third law return the same force each time).
    Also, I understand bouncing with energy, using forces however-
    The ball comes with some speed and gravity force acting upon it, once it hits the floor, gravity is acting in the upward direction, the ball stays in the place (Speed)/10 seconds (like every second 10 M/S is removed), then it is shot upwards with gravity, wouldn't this mean all balls (with the same mass from different heights) bounce the same height? Moreover wouldn't mass be irrelevant? (since once it starts rising the same gravity is acting upon it), and lastly, wouldn't EVERY ball hit the ground and rise for exactly a second?

    Thanks in advance, sorry for spamming questions :P
  8. Jan 30, 2014 #7


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    Sounds reasonable, but not so. There's two interpretations possible for the second sentence:
    pulled towards the center of gravity of the object by gravitation force
    pulled towards the center of gravity of the earth by gravitation force

    The first is negligible (squared: object has a lot smaller mass than earth and air particles are all around, so forces cancel if the distribution in space is uniform).
    I take it you mean being pulled down by the earth (its mass is a bit bigger).
    But that is also the case in the surroundings of the object. The effect of the earth gravitational filed is a pressure gradiënt: the higher you go, the lower the atmospheric pressure.
    (by the way, it's nice to calculate a few things there. The entire air column above 1 cm^2 of earth weighs 1 kg). I refer to the glimpse we caught of Archimedes....
  9. Jan 30, 2014 #8


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    Yuch, I lost yet another long story about bouncing billiard balls. Rain check.
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