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Old July 11th, 2007, 20:21   #41
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Originally Posted by Flatlander View Post
(I'm a mechanical engineering student).
The moment of inertia of a sphere is 2/5*m*R^2.
1) My bad, I usually deal with circular cross setions of beams (Civil Eng Student).

Originally Posted by Barf View Post
Momentum = mass/velocity = kg/(m/s)
A small car (assume 1 metric tonne) moving at 1 m/s has the same momentum as 1 kg block at 1000 m/s, but the block has 1000 times the kinetic energy.
2) ...Well, actually it's mass multiplied by velocity, but that's ok.

Exactly. And personally, I think that 1kg block moving at 1000m/s will hurt ALOT more than the 1 tonne car moving at 1m/s. Would you not agree (since that's nearly 3 times the speed of sound...with general conditions). This kind of proves that impact energy is the main basis for hurt. Remember this, I'll be bringing it up later.

Originally Posted by Flatlander View Post
Kinetic energy (.20 @ 90m/s) = 810 joules -> From this I can calculate the theoretical velocity of .25's (neglecting rotational energy)

Velocity of .25's with same KE = 80.5 m/s
3) I realise you're that you're just doing a basic comparison and that the extra powers of ten would just disappear anyways, but the BB has .81 joules (.2g is .0002kg, which is nessecary for J).

Originally Posted by Flatlander View Post
Topic of what to use for calculating how much “hurt”:

-So again, I have no idea what to use for calculating “hurt” but I’m leaning towards impulse/momentum.
4) Summary: 1kg Block moving at 1000m/s verses 1000kg car moving at 1m/s. Momentum is equal, kinetic energy has a 10^3 difference in magnitude. 1kg block with 1000m/s velocity would porbably hurt more. For full details, see quote reply number two.

Originally Posted by Flatlander View Post
I'm not completely sold that impact energy (kinetic energy) is the definitive way of saying what will hurt more. I could be wrong and will look into it as I'm quite curious....
5) I would say so. What else could cause hurt? When something has momentum, it also has energy. The transfer of energy to the tissue of the body would cause pain. Why can't momentum do this alone? Think electricity. Electircity doesn't have momentum (don't give me any quantum electron movement theories, because the momentum of an electron is rediculously small, since it has such a tiny tiny tiny tiny mass and they don't even move that fast either...not as fast as most people might think anyways). For all intents and purposes electricity is just energy. Pure energy. Yet when you're connected to a live wire or two, it can really hurt. Now, I'm not entirely sure on what I just said, since the closest thing to an electricity course I've taken is a first year intro to electricity and magnetism and a really simple electricity and thermophysics course, but it seems to make sense to me.

Originally Posted by Flatlander View Post
Just look at the Mythbuster's "frozen vs thaw" chicken test. They thought that the penetration should be equal as they will have the same kinetic energy when they impact the windshield. This was obviously not the case as they saw the frozen had MUCH greater penetration. True, they do have the same "impact energy", but they transfer this energy completely different! Not quite a good analogy for our BB case but it gets your mind around energy being the "be all, end all"....
6)But then collision thoery has to be accounted for. The thawed chicken is a softer medium so it will absorb some of the impact energy (which would probably cause it to explode into little bits). The frozen chicken is a harder medium so it will not absorb nearly as much of the impact energy, which will force it all onto the object it is impacting. This is definately a very bad analogy for our BB case, since the BBs are of nearly the same hardness, and would produce the same type of impact.

Originally Posted by Flatlander View Post
This is a light-hearted debate (on my mind) and enjoy getting the brain going even though it is summer time! This is a huge portion of what I study at school so it quite interests me.
7)Ditto. I may not be in Mech and may not need to know a whole lot about the way things move (aside from structures), but I certainly do enjoy knowing it. It's good to get out and strech the old brain out during the off season.

Out of Sport. Have Fun!

Last edited by Nik12; July 11th, 2007 at 21:58..
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