Riddle me this...
Discussion
...a question no doubt asked and answered by people far cleverer than me but here goes: A train traveling north at 100mph; a fly travelling south at 10 mph splats against the windscreen of the train. Now it would seem to me that during the transition of going 10mph in one direction to 100mph in the other, at some point it becomes stationary(?) but at this point it is attached to the train, so does this mean the train is also stationary?
If your explanation doesn't involve cats in boxes, that would be good; this is also my first time in the science section so be gentle!
If your explanation doesn't involve cats in boxes, that would be good; this is also my first time in the science section so be gentle!montecristo said:
Using a fly may confuse things because it's squishy, it could be better to think of a hard projectile. Maybe the part of the train that collides with the fly does stop, hence it dents. The dent would be more visible with a hard projectile.
What about a bullet fired at an oncoming train, one that doesn't puncture the metal.motco said:
An infinitesimally small part of the glass for immeasurably small instant of time will be stationary together with molecular scale parts of the fly sequentially - probably...
The last thing that goes through the fly's mind? Its a
hole!
Surely everything is stationary for an immeasurably small instant of time? However fast it's moving.The last thing that goes through the fly's mind? Its a
hole!Given that the train is made up of trillions of atoms, not a single 'thing', is it not viable that atoms where the fly strikes on the train screen stop but the train continues as it was but with a tiny bit of speed rubbed off i.e. 0.0000000000001mph
As an example I guess if you are running and someone throws a ball at you from the other direction your hand will cushion the impact and travel at a different speed relative to the ground vs your body which is still maintaining its speed before you pull your hand forwards again to match the speed of your body?
As an example I guess if you are running and someone throws a ball at you from the other direction your hand will cushion the impact and travel at a different speed relative to the ground vs your body which is still maintaining its speed before you pull your hand forwards again to match the speed of your body?
coldel said:
Given that the train is made up of trillions of atoms, not a single 'thing', is it not viable that atoms where the fly strikes on the train screen stop but the train continues as it was but with a tiny bit of speed rubbed off i.e. 0.0000000000001mph
That's what I was struggling to convey.The train will be fractionally slowed by the impact, but obviously it's almost incalculably small. 
The other thing to consider is the flex of the glass window on the train as the fly hits it, the squashing of the fly etc, but yes in the answer to the OP's question, at some point all the bits of fly have to stop at some point during the collision, even if it only takes a tiny fraction of a second to accelerate from -10 to +100mph.
The other thing to consider is the flex of the glass window on the train as the fly hits it, the squashing of the fly etc, but yes in the answer to the OP's question, at some point all the bits of fly have to stop at some point during the collision, even if it only takes a tiny fraction of a second to accelerate from -10 to +100mph.
Dr Jekyl. Not a maths or physics Ph.D I assume.
You are wrong. Think about adding together an immeasurably large number of consecutive immeasurably small periods of time and see if your train has moved.
It might move an immeasurably small distance in an immeasurably small time but it still moves. 100 km/hour is about 0.028mm per microsecond
You are wrong. Think about adding together an immeasurably large number of consecutive immeasurably small periods of time and see if your train has moved.
It might move an immeasurably small distance in an immeasurably small time but it still moves. 100 km/hour is about 0.028mm per microsecond
wag2 said:
Dr Jekyl. Not a maths or physics Ph.D I assume.
You are wrong. Think about adding together an immeasurably large number of consecutive immeasurably small periods of time and see if your train has moved.
It might move an immeasurably small distance in an immeasurably small time but it still moves. 100 km/hour is about 0.028mm per microsecond
The point I'm making is that it makes no sense to talk about speed being zero for an immeasurably small instant because speed is distance/time. In the train collision scenario you end up saying 'well it wasn't quite stationary, it did move a bit, but that's because the time interval was longer than the time it was stationary', which can apply to anything.You are wrong. Think about adding together an immeasurably large number of consecutive immeasurably small periods of time and see if your train has moved.
It might move an immeasurably small distance in an immeasurably small time but it still moves. 100 km/hour is about 0.028mm per microsecond
I had lectures from a maths professor, his name sadly escapes me, who dreamed of being called as an expert witness in a speeding case. Any measurement of speed, he said, is a measurement of an average speed whereas the law sets limits on instantaneous speed. He had a two blackboard proof to demonstrate that if average speed was over a particular value, so must be instantaneous speed.
The key feature regarding the instant the fly is stationary is that it changes direction. There is a point on a graph where it has stopped moving one way and not started moving the other.
The key feature regarding the instant the fly is stationary is that it changes direction. There is a point on a graph where it has stopped moving one way and not started moving the other.
Replace the train and the fly with a big magnet and a small magnet. As they approach each other their magnetic fields start repelling each other. That repelling force initially slows the "fly" magnet down, then it stops for an instant, then reverses the flies direction of travel and pushed it away in front of the train. There is no actual impact. The interaction between the two objects takes place over time as the forces between the two objects acts a bit like a spring.
In reality at a microscopic level that's what's happening with the atoms in the fly and those in the front of the train. There are no actual impacts. Particles are just acting on each other at a distance.
Once you stop thinking of it as being two solid objects literally colliding there's no need for the front of the fly to undergo an infinite acceleration in order to immediately match the speed of the train, which is really what you're trying to work around by thinking that maybe the train somehow stops too, etc
In reality at a microscopic level that's what's happening with the atoms in the fly and those in the front of the train. There are no actual impacts. Particles are just acting on each other at a distance.
Once you stop thinking of it as being two solid objects literally colliding there's no need for the front of the fly to undergo an infinite acceleration in order to immediately match the speed of the train, which is really what you're trying to work around by thinking that maybe the train somehow stops too, etc
Exactly as above. Imagine even simpler, you are walking and someone from the opposite direction throws a ball at you, your hand can catch the ball and cushion it so that it changes the direction of the ball to eventually being moving at the same speed as you walking, but your hand movement changed the speed and direction of the ball without you failing to stop walking. The problem with the problem is that it tries to convince you the train and fly are two single 'things' when they are in fact made up of trillions and trillions of individual things.
Edited by coldel on Thursday 21st February 22:17
Dr Jekyll said:
Surely everything is stationary for an immeasurably small instant of time? However fast it's moving.
If time is immeasurably small then your saying it cannot be measured, the same must apply to an object being immeasurably stationary It cannot exist as it cannot be measured therefore it’s a state of transition from one state to another.......or is it a case of for one trillionth of a nano second both objects are stationary.....either something can be measured (this is science) or it cannot. Edited by Toaster on Friday 22 February 07:53
Toaster said:
If time is immeasurably small then your saying it cannot be measured, the same must apply to an object being immeasurably stationary It cannot exist as it cannot be measured therefore it’s a state of transition from one state to another.......or is it a case of for one trillionth of a nano second both objects are stationary.....either something can be measured (this is science) or it cannot.
Surely everything can be measured in some way, it's just that we currently may not have the tech/methods to measure it or indeed the knowledge to know what to measure.Edited by Toaster on Friday 22 February 07:53
wolfracesonic said:
Now it would seem to me that during the transition of going 10mph in one direction to 100mph in the other, at some point it becomes stationary(?) but at this point it is attached to the train, so does this mean the train is also stationary?
An example of Zeno's arrow paradox?Gassing Station | Science! | Top of Page | What's New | My Stuff