Physics teaser #2
Elder User
Joined: Mar 2005
Posts: 735
Likes: 1
From: SW Iowa
OK, simple
answer:
The train doesn't stop to 0 velocity ever! The bee's forward velocity stops at the sudden impact point in time, and matches the trains velocity resulting in death for the bee, but the train's velocity is never changed.
In a traffic accident, when a speeding heavy truck hits a light car head on, the truck is slowed by operator braking and by the energy force expended by the truck upon the car at impact, but the heavy truck never stops. The car does suddenly match the remaining truck's velocity and direction, but the impact and sudden change in velocity and energy dissapation from the truck hitting the car results in certain distruction.
The train doesn't stop to 0 velocity ever! The bee's forward velocity stops at the sudden impact point in time, and matches the trains velocity resulting in death for the bee, but the train's velocity is never changed.
In a traffic accident, when a speeding heavy truck hits a light car head on, the truck is slowed by operator braking and by the energy force expended by the truck upon the car at impact, but the heavy truck never stops. The car does suddenly match the remaining truck's velocity and direction, but the impact and sudden change in velocity and energy dissapation from the truck hitting the car results in certain distruction.
Elder User
Joined: Nov 2005
Posts: 597
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From: San Antonio, TX
The frame of reference isn't stated in the question. In reference to the ground, which would normally be assumed, the train never stops. In reference to the bee, it's dead so it doesn't have a frame of reference 
Elder User
Joined: Mar 2003
Posts: 588
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From: AZ.
As I see it, we're compairing apples to oranges. In the first post two golf ***** tied in contact with each other travel at the same velocity. True, but the poor bee is going the wrong way. He definitely has to stop to change directions to stay in contact with the train but the train tends to stay in motion. ... JMHO..
Post Fiend
Joined: Apr 2004
Posts: 6,473
Likes: 4
From: Lafayette, IN
For an infinitesimal amount of time, the bee's velocity is zero while it changes driections, but never the train's. Ask any physics prof (not a HS fizzics teacher).
Trains only slow down visibly when hit by an object IN THE MOVIES.
Jason
Trains only slow down visibly when hit by an object IN THE MOVIES.
Jason
Elder User
Joined: Jul 2005
Posts: 969
Likes: 1
From: North Texas
I think your problem can be solved with one realization. Basic physics, whether highschool or college undergrad, uses simplified formulas. These formulas are simplified based on certain assumptions. Perfect materials is a key one in this instance.
Were the train and bee both perfect materials there would be no deformation. In that case, for an instant, the train's energy would be used to accelerate the bee. The train would instantly drop from 60mph to 59.9999999mph. Acceleration would go negative, but speed would still not hit 0. The trains enegy acts on the bee, not on slowing the train! On opposing trains the speed would equal 0 because the opposing forces were equal. Anything of less energy will only slow the train to a new speed.
In the real word the bee and train are not perfect materials. The bee deforms(massively), the train deforms(imperceptibly), and the passengers cannot tell a difference.
Were the train and bee both perfect materials there would be no deformation. In that case, for an instant, the train's energy would be used to accelerate the bee. The train would instantly drop from 60mph to 59.9999999mph. Acceleration would go negative, but speed would still not hit 0. The trains enegy acts on the bee, not on slowing the train! On opposing trains the speed would equal 0 because the opposing forces were equal. Anything of less energy will only slow the train to a new speed.
In the real word the bee and train are not perfect materials. The bee deforms(massively), the train deforms(imperceptibly), and the passengers cannot tell a difference.
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Post Fiend
Joined: Jun 2001
Posts: 5,228
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From: Running Springs CA
Right, the bee deforms, otherwise it would bounce off, like a baseball off a bat. The other factor that violates textbook theory is the fact that the train is not a free body in space. It is connected to the rails and has constant propulsive force mantaining its velocity relative to the earth. No, you are not going to get slippage at the instant of impact.
Postmaster
Joined: Jan 2005
Posts: 4,743
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From: Peoria, IL
After sobering up this morning and reading the question againI have realized that I was wrong.
The first part of the story is put there to do exactly what it did to me last night, confuse you.
Just because the bees velocity reaches zero at a certain point in time it does not mean that the trains velocity is zero also. The Bees velocity reaches zero at some point in time while the train is accelerating the bee in the opposite direction, and losing a minute amount of it's own velocity. When all is said and done then both objects are traveling together at an even rate slightly less than that of what the train was originally traveling at.
Dang it I hate admitting I was worng.
The first part of the story is put there to do exactly what it did to me last night, confuse you.
Just because the bees velocity reaches zero at a certain point in time it does not mean that the trains velocity is zero also. The Bees velocity reaches zero at some point in time while the train is accelerating the bee in the opposite direction, and losing a minute amount of it's own velocity. When all is said and done then both objects are traveling together at an even rate slightly less than that of what the train was originally traveling at.
Dang it I hate admitting I was worng.
Laughing Gas
Joined: Dec 2003
Posts: 796
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From: KCMO
Originally Posted by Beerstalker
The bee stops the train because for a very small amount of time all of the train's energy is used to accelerate the bee in the opposite direction. Once the bee has began accelerating in the opposite direction the train starts moving again.
The bee theoretically experiences zero velocity, only because the direction reverses. In reality, parts of the bee would begin to slow and deform while other parts of the bee would still be travelling at the original speed. By the time the entire bee becomes deformed, all of its energy is imparted to the train (and mostly converted to heat) - the bee becomes "one with the train".
Postmaster
Joined: Aug 2003
Posts: 2,873
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From: St. Peter
i have the answer. This is how the bee stops the train... The Train engineers forgot to fill the windshield wiper fluid, and when the bee hit the train it deformed and made a mess, thus making it very difficult for the train engineers to see. So they had to stop the train, wipe off the windshield and then start going again.
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Postmaster
Joined: Aug 2001
Posts: 4,532
Likes: 5
From: Trussville, Alabama
Bingo, the initial assertion is 100% true in certain situations. The bee clearly can not stop a train, but the initial assertion can sure get folks thinking down those lines.
It is all a matter of kinetic energy, momentum and vector addition.
you know, all that cool PHYSICS stuff!
Thanks all - it was a fun thread.
It is all a matter of kinetic energy, momentum and vector addition.
you know, all that cool PHYSICS stuff!
Thanks all - it was a fun thread.
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