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This is to post your overused words. Interestingly, I definitely have to say interesting and definitely.

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I didn't guess it, that one's just obvious... I'm still trying to not say that one, it's getting harder with each sentence, however I'm still rambling without say that word, very common, most common used in english, so I've heard anyway. Maybe it's time I stop before this gets so complicated I can't find a way around that word.

Sorry to spell that magical word... ...but I say 'therefore' too often in explanations. A few science tests ago I had a problem that asked me "What is the difference in momentum between two balls of equal SIZE when said balls are moving at a velocity of (ball1) 0m/s and (ball2) 100m/s?". I stated that difference in momentum between a stationary object and a moving object is impossible to calculate. I also stated that, according to relativity, you could say that those two balls had the same momentum in opposite directions. If ball one and two are equal in mass, you can say that ball1's momentum is equal to ball2's momentum, and each has a momentum of (whatever ball1's mass is multiplied by 50m/s). If you are moving towards ball1 (which is 'stationary') at 50m/s along the same plane that ball2 is moving towards ball1, it seems that ball1 is moving towards you at 50m/s, and ball2 is headed straight towards ball1 at an equal velocity.

I received a 90% grade, rather than 100%, for writing an answer that excelled to the point that even the teacher didn't understand it.

It's a pity that you were wrong. The question as posed implicitly specifies the reference frame to be used (namely, yours). In that reference frame, one ball is moving at 0 m/s, the other at 100 m/s. Since 100 m/s << 30000000 m/s, relativistic corrections are pointless and obfuscatory, and serve only to make you sound clever.

The difference in momentum is (100 - 0) m/s * mass. Since the mass of the two balls wasn't specified (or at least, you didn't quote it), the correct answer must be left symbolic: delta p = 100*m kg-m/s.

I would have given you 0 for that answer.

It's always best to keep the problem fully symbolic (algebraic) until the very end, then substitute known values. Otherwise, you could (not this problem, but others) end up in a situation where something looks like 0/0 or some other pathological case, but isn't really.

The problem statement, according to "chriskarr" is that the two balls are "the same size." I'm assuming that means the same mass (otherwise there is insufficient data to solve the problem at all). Then we have

p1 = m v1 (where v1 = 0 m/s)
p2 = m v2 (where v2 = 100 m/s)

dp = p2 - p1 = mv2 - mv1 = m(v2-v1)

Now substitute to obtain dp = (100-0 m/s) m = 100×m kg-m/s

Oh, by the way, the problem as written by "chriskarr" is also incorrect in using "velocity." Velocity is a vector quantity -- it has both a magnitude and direction. The correct term should have been "speed."

For this special case, you're right. Zero is just zero :-) However, consider a very similar problem, with ball #1's speed 10 m/s, and ball #2's speed 110 m/s. The difference in speeds is the same, but the momentum difference will clearly not be the same as before if the masses are unequal.

Absolutely! The problem is trivial (a grossly overused word in my field :-). My posting above is the whole solution, no matter what the mass and velocities are. Chemistry? Urgh...I can probably remember some of my undergrad p-chem, but I can't do o-chem at all. I know stoichiometry and the "theory" behind activation energy and rate-limiting intermediates, and I'm a firm believer in conservation of energy (so no, I won't help you design a water-fueled car :-).

That's ok then, I was wondering where the unsolvable came in...

It's a bit like squaring the circle, until I tried it, however some notions about using pi formula seem plausible, though it's impossible it's interesting because it seems so impossible...

I'll PM you, the physics might be need too, it's not exactly conservative but it's not perpetual motion...

I left out some bits with that comment. This particular problem, the one "chriskarr" wrote, is too simple to really need to be run through symbolically.

However, there are more complex situations, where you want to leave things in symbolic form, work through the algebra and calculus, factoring or combining terms, and only at the end plug in actual numbers. The main reason is that you could have a problem posed where the particular numbers coincidentally "cancel," but if you leave things symbolic, the cancellation doesn't cause problems.

Here's a specific example. Suppose, in the course of solving a physics problem, you find an expression that involves

m1 - m2
--_--_--_--_
m12 - m22

If the problem stated that m1=m2=50 kg, and you plug that in at the beginning, then you end up with 0/0 and you don't have a solution. But leave it symbolic, and you know m12 - m22 = (m1+m2)(m1-m2), so you factor the denominator and the expression above becomes 1/(m1+m2), and there's no division by zero at all.

Feel free to PM or use my Orangeboard, or even start a topic in Physics, if you want :-)

I'm probably not in your school. I read Gilgamesh in high school, 20 years ago, and then again in college 10 years ago, and then again last year. I'll probably have my kids read it next year (they are home-schooled). I like "flood stories" from the various cultures. The similarities and differences are fascinating!