Sigma's post got a lot of folks talkin', so here's another......

Two objects moving while in physical contact with each other, will be moving at the same velocity.

EXAMPLE - You tape two golf ***** together. The are in contact with each other. You throw them across the yard. They stay in contact. They travel at the same velocity.


So here's the puzzler:

A train is riding down the tracks at 50 mph. (moving say, L to R)
A honey bee is flying down the track in the opposite direction from the train. (R to L)
The bee flys head on into the windshield of the train.
The bee's velocity goes from moving R to L, down to zero, then reverses direction to move (along with the train) from L to R.


So, for an instant, the bee's velocity is zero.
At that same instant the bee is smacked flat against the train's windshield (obviously in full contact with the train).
So, how does the bee stop the train for that instant in time?


(I know the answer but am asking just offering it up for folks to discuss)

 

It doesn't stop the train.

The bee will reduce the trains velocity by a small amount, as it accelerates the bee in the direction L to R, but it will not stop the train, as the bee does not supply sufficient force via momentum (velocity X mass) to stop the train.
The train on the other hand has a sufficient momentum to accelerate the bee.

Assuming the same velocity (for simplicities sake)

(Large Mass X Velocity) + (Small Mass X -Velocity) = Joint Mass X Velocity

 

I know what the last thing to go through the bee's mind is!

 

Yeah, the bee doesn't stop the train. The overall force from the collision is still in the train's initial direction with an extremely negligible slowdown resulting from the force of the bee on the train.

Let's see if we can get some more brain teasers in here. This is fun.

 

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.

 

Think about it, Beerstalker.

If that were true, and the train stopped for a very small amount of time, everything in the train that isn't tied down would fly forward. Objects in motion will stay in motion.

If the train actually did come to a stop for a very small amount of time, it would require that the train's engine start the train moving and accelerating again. You know how long it takes for one of those to accelerate? You know how many bees hit trains?

 

That's just it, all of that actually does happen, it's just that it is for such and incredibly short period of time it is unnoticable.

Think about it in a larger scale like an automobile accident.

A 5000lb truck moving 50mph and a 3000lb car moving 40mph hit head on, both vehicles will slow down until all of the cars kinetic energy is removed from the trucks kinetic energy. For an instant everything stops, then the remaining kinetic energy of the truck is used to accelerate both vehicles in the direction the truck was originally going, at a slower rate (31mph assuming no friction or other lossses).

 

Doesn't it have something to do with the transfer of energy? Kinda like hitting a fastball... the ball comes in at 100mph goes out even faster.

 

Within the reaction, everything stops relative to each other (as in the car reaches the same speed as the truck.This point here proves it.
For the truck to still have kinetic energy it MUST still be moving (relative to the ground).

 

Actually the fastball thing is quite a bit more complicated because you have to get into the fact that the ball deforms and then returns to it's original shape.

Since I'm assuming the bee isn't returning to it's original shape I didn't take that into consideration.

 

Bzzzzt. Try again.

No train stoppage. At most, there is going to be some flex in the metal or glass where the bee hits, but inertia is inertia--Newton's first law of motion: bodies in motion tend to stay in motion, bodies at rest tend to stay at rest. The bee's impact will reduce the speed of the train by a miniscule amount, but that loss of speed is quickly recovered by the engines. The larger the train or the smaller the bee, the smaller the reduction in speed. The larger the bee or the smaller the train, the larger the reduction in speed.

Now, if the bee were moving fast enough, it could stop the train, but only if the bee's mass times its velocity is the same as the train's mass times its velocity.

 

Beerstalker, I think you have drunk too much Kool-aid. A bee that weighs about 1/100 of an ounce running into a 50,000 ton train is very negligible in slowing it, let alone stopping it for a nanosecond. Imagine 100 billion bees running into the train in a 5 second period. This you wouldnt even feel if you were riding in the train.

 

That's just it you guys physics is physics. Size doesn't matter as to what happens, just the scale of how it happens. Just because you cant feel the train stopping and then speeding back up doesn't mean it didn't happen.

If the bee and train did not stop it would mean that the bee instantaneously went from traveling in one direction and started travelling in another. This is impossible. Time is infinite not finite.

 

Yes it does.
You would feel HUGE inertia forces.
It didn't stop.
The bee got accelerated in the right direction by the train. (which brings the bee to almost the same speed as the train was initially travelling)
As a result of this, the train gets slowed by a miniscule amount, as the bee accelerates the train in the left direction.

 

There is no way the bee stops the train, at least the train relative to the ground. Because the law of conservation of momentum says "if no external forces act on a closed system, the total momentum of the masses in the system will not change." This means that the momentum of the train and the bee before the collision have to equal the momentum of the train and the bee after the colision. Physics would not allow the bee to stop the train relative to the ground. If you have a 50,000 kg train going 10 m/s, it has a momentum of 500,000 kgxmxs. If the bee had a mass of 1 kg and was traveling 10 m/s, it has a momentum of 10 kgxmxs. since they are traveling in opposite directions, you have to subtract the momentum of the bee from the momentum of the train, so 500,000 kgxmxs - 10 kgxmxs =499,990 kgxmxs. If the bee stopped the train relative to the ground, the momentum of the train would be 50,000kg x 0 m/s which give the train a momentum of 0 kgxmxs, and the bee is also stopped on the trains windshield, which would give it a momentum of 10kg x 0 m/s = 0 m/s. ITS NOT POSSIBLE.

BUT!!!!!, For the bee relative to the train, like the first post stated, two objects moving in physical contact with each other will be moving at the same velocity. So at that instant when the bee's velocity was zero, and in that same instant it was in FULL contact with the windshield, the trains velocity was zero. Right? If 2 objects are in contact with each other, then they have the same velocity. So if the bee is in contact with the train, and the bee's velocity is zero, then the trains velocity must be zero.