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Good old fashion beer drinkin' physics help settle the bet

  1. Jul 11, 2010 #1
    Two questions, answer either or both;

    1) Will an airplane in flight sustain more damage (due to more total kinetic energy) from hitting a turkey vulture size bird (that's big for those non-ornithologist types)? Or will the bird simply be "vaporized" on impact and the airplane sustain less damage the faster the airplane is flying?

    2) If a balloon full of water also containing a ping-pong ball were to be placed in space, what would the ping-pong ball do? Ignoring the material science of what the balloon and ping-pong ball are made of. Would the ball float loosely around (the middle of the balloon) or seek the edge of the balloon and float around the parameter?

    Thanks for the help, all are welcome to comment but please include your credentials if possible.

    Last edited: Jul 11, 2010
  2. jcsd
  3. Jul 11, 2010 #2
    1) The plane will suffer more damage. The faster the plane goes the more damage it will sustain.

    2) Here I must make idealized assumptions. That the balloon is indeed fully in freefall, it's not rotating, and there are no electrostatic or other forces to consider. The ping pong ball will float around in the balloon much like it would without water. However, if you accelerate the balloon one way, the ping pong ball will accelerate the other way.

    There's an experiment you can do that'll demonstrate the backward acceleration in your car. Tie a helium balloon to the center console, so that it floats just below the roof. Now hit the gas pedal and, your pushed to the back of the car, the balloon will dart to the front of the car. Hit the break and the opposite will happen.
  4. Jul 11, 2010 #3


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    First thing to consider: if a ping pong ball is exposed to the vacuum of space, what would happen to it?

    In the manufacturing process ping pong balls are not pressurized beyond atmospheric pressure; there is no pressure difference between the inside and the outside. Ping pong balls are airtight; if the ball is dented then you can outdent it by carefully heating the ball with steam. The heat will soften the plastic and it increases the internal pressure.

    Ping pong balls are made of very tough material, so in all probability the ping pong ball is strong enough to withstand the force from the inside pressure when it is exposed to the vacuum of space.

    The water and the balloon make no difference.
    Assume that you prevent temperature effects. (That means working inside the cargo bay of some spacecraft. Every objects radiates thermal energy in the form if infrared light, so if you want the temperature of the balloon to remain the same you need to irradiate to make up for the loss.)

    The balloon is exerting pressure on the water inside it, but only a very slight pressure. The vapor pressure of water at room pressure is very low too. So it's a close call, maybe the balloon will rupture, maybe it won't, it will depend on the quality grade of the balloon. (I'm assuming now that the gases that are dissolved in the water won't go out of solution.)

    As I said, the water and the balloon will make no difference, so for the ping pong ball it won't matter whether the balloon ruptures or not.
  5. Jul 11, 2010 #4
    1. Airplane will always sustain more damage the faster it goes.

    2. Having never tried this I cannot say for sure but I do have lots of expirience with high vacuum as the company where I work uses vacuum in many processes. Based on its effects on anything else I am 99% sure that both the balloon and and the pingpong ball would instantly rupture and release their respective gasses into the vaccum of space (the water would also be gas at this pressure).

    We have tried placing a number of objects in vaccum chambers at work with amusing result... Marshsmellows are especially funny... Maybe I need to try ping pong balls and balloons although they would have to be full of air as I am guessing a water balloon would rupture at a point where MOST of the water was still liquid and then there was be a bunch of water getting in the vacuum pump.
  6. Jul 11, 2010 #5
    Both of you completely missed the point of the question... or purposely went off on a tangent because you didn't know the answer.

    To the OP:

    The reason objects float or rise to the surface is due to gravity and their density. If an object displaces more water than it weighs, then it will float.

    Think of it like this: Gravity is always pulling down on the water, so it is "squished" to a flat surface. When you displace water, you are raising the water level. This causes the water to push back on whatever object is displacing it. If that force is greater than the weight of the object, the object will rise.

    So you can see, without gravity, the ping pong ball has no preference for any particular area in the balloon and will float around freely.
  7. Jul 11, 2010 #6


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    Error fixed in bold. You have it right in the example: you accelerate forward, the balloon also moves forward.
  8. Jul 11, 2010 #7


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    Oops - I only read half the question. :-(
    I never even got to the part where the ping pong ball floats...

    I agree, in free motion there is no buoyancy, so the ping pong ball will tend to remain in the same position inside the water filled balloon.
  9. Jul 11, 2010 #8
    Actually no, I fixed it back. The helium balloon always accelerates the opposite direction the car accelerates, in an enclosed car. This I have bought many helium balloons to demonstrate. As another mentor pointed out to me, it can be explained by buoyancy. I was describing the effect in terms of the velocity distribution of the air molecules, which is just as right.

    Oops, I need sleep. You were talking ping pong ball. But the ping pong ball acts much like the helium ballooon because in is in water, which is heavier than the ball. Just like the air is heavier than the balloon. But it only accelerates the other way relative to the water balloon.
    Last edited: Jul 11, 2010
  10. Jul 11, 2010 #9
    The bird test is a standard one when qualifying airplanes. They fire grocery-store chickens (thawed) out of pneumatic cannons at the plane. The chickens "fly" around 350mph as they impact. They do not vaporize, rather they splatter into a fine mist on the windows and wings as if they were (ironically enough for this thread) water balloons. The chicken going through the jet engine is a different story. The blades of the compressor fan slice through the bird with such elegance and grace that I dare say it should be the model for a hybrid kitchen appliance / art exhibit.

    Without gravity the ping pong ball should float but would probably tend to the edge and roll around there- not due to gravity but due to tiny differences in the surface energies between the ball and balloon.
  11. Jul 11, 2010 #10
    I appreciate your input thus far. As far as the example of Boeing shooting the chickens at their planes, I think it might be a little misleading. I have flown jets in the Navy for the last 9 years and have seen small birds destroy engines and go through wings/ canopies....I have a roommate who is touting his "astro-engineering" academy degree as his basis as why I am wrong. Not the most solid of arguments. As one who majored in Econ I cannot claim credentials, however it does seem pretty basic....feel free to correct me if I'm wrong, but the faster an object is going, the greater the total energy it will impart during the collision.

    His argument was we can get going so fast that the bird just splatters instead of causing serious damage....I say go ahead and speed up and let me know how the bird tastes when it crashes through the canopy and hits you in the face at 500kts.

    Additional note about likening the bird to a water balloon hitting the airplane. I have dropped water balloons out of Cessna's, one of which nailed my buddies metal folding chair....it was destroyed.

    Thanks again.
  12. Jul 12, 2010 #11
    And just like my wan said , Another way to do it is take a jug and tape a string to the bottom and then tape that string to a ping-pong ball and then submerge it in water
    till it is over the top of the ball so it is floating in the middle of the jug , and then put it in your car and drive around and when you brake the ball goes towards the back and when you hit the gas it goes forward . But when you take a turn it goes towards the inside of the turn which is the direction of acceleration .
  13. Jul 12, 2010 #12
    The reason a submerged body will go up or down is not because of gravity but because of the density gradient of the surrounding fluid. Gravity just happens to be the mechanism we're most familiar with that causes density gradients. Think of a submarine that's not moving through the water, thus has no dynamic control. If it wants to go up or down it must make itself less or more dense by ejecting or taking in water. What's more it will only go up or down until its density matches that of the surrounding water.

    In the absence of a local gravitational field or acceleration, and ignoring all other forces, the ping-pong ball will remain in the middle of the water balloon. There is no density gradient in the water to push the ping-pong ball one way or another. This situation is analogous with releasing a golf ball in a space capsule. It will remain where it is release even though it is much denser than the surrounding air and it should fall, but in what direction should it fall?

    Strictly speaking even in a small water balloon the water will, under it's own gravity, have a spherically symmetrical density gradient with the centre being more dense, so the ping-pong ball would tend to move towards the outside of the balloon; however, in a small water balloon this effect would be so tiny as to be negligible.
  14. Jul 12, 2010 #13


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    I wonder if, even ignoring this density gradient due to gravity, a ping pong ball would still stick to the interior of the balloon (if it happened to touch the edge first), not due to any density effects, but cohesion between the water molecules (assuming your plastic ping pong ball is hydrophobic).
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