Theoretical question
Discussion
Dr Jekyll said:
What about a fly inside the ISS? Presumably the flapping of it's wings to press itself against one end of the ISS would push the air against the other end and cancel out.
What if the fly was on a conveyor belt? 
Talking of flys, this happened a few days ago and it's been making my brain itch :-
Sat on the train on my may to work. Train pulls into station and doors open. A wasp that had been buzzing about the platform flew into the train and remained flying around inside the train after the doors closed and we pulled away. At no point did the wasp land on any surface in the train. For the wasp to be buzzing around inside the carriage, it must have been accelerated at the same rate as the carriage, yet at no point has it touched anything to have an acceleration imparted, as it's always been in the air. What force accelerates it inside the train so it's moving at the same speed as the carriage?
Sure it's an obvious answer (open to closed system?), but it's been driving me mad thinking about it!
marshalla said:
hornet said:
as it's always been in the air.
There's a hint of the answer to your question in the question itself - how much air movement do you feel inside the train ? Why ? Probably sounds like a stupid question, but it just wouldn't dislodge itself from my brain!
Small things like wasps live in a world that is dominated by friction, not gravity. Think of the ratio of their surface area to their volume. It is much higher than ours. As a result, a breeze that ruffles our hair will send them flying. Similarly they can walk up the side of a glass or can stick to the ceiling with their feet.
V8LM said:
One second the fly was going south. The next north. The velocity of the fly has changed sign. The velocity of the fly at one instant, therefore, must have been zero. This was because it had contacted the car. If the fly had a velocity of zero, and was in contact with the car, then the car must also have a velocity of zero.
This totally fails the 'bulls
t' check though. This goes something like:- I have constructed a scenario in my head
- Does it match what I see in the real world?
t and we can stop any kind of 'OMG I've found a hole in physics' and take a look at your statement.The thing which leaps out at me is '..was in contact with the car, then the car must also have a velocity of zero' which is, with a second's thought, shown to the prime source of bulls
t here. We can all I'm sure think of many, many things that are in contact but not moving at the same velocity (consider a glancing blow of two cars, for a start). Therefore, the fundamental construct of the argument is bullst and physics can breathe a sigh of relief.But then I'm sure you knew that anyway
As a, possibly interesting, aside to the original question, since everything is, probably, quantised then presumably there is a minimum force (call it the Planck force, to fit in with other quantum measures) which will cause acceleration. Of course since everything is quantised I would expect that to be some, rather obvious, term such as the momentum imposed by a single photon.
Yes I've been drinking, but this still feels right to me, if you know better please shoot it down, that would be science in action.
Yes I've been drinking, but this still feels right to me, if you know better please shoot it down, that would be science in action.
Einion Yrth said:
As a, possibly interesting, aside to the original question, since everything is, probably, quantised then presumably there is a minimum force (call it the Planck force, to fit in with other quantum measures) which will cause acceleration. Of course since everything is quantised I would expect that to be some, rather obvious, term such as the momentum imposed by a single photon.
Yes I've been drinking, but this still feels right to me, if you know better please shoot it down, that would be science in action.
p = E / c = h v / c = h / lambdaYes I've been drinking, but this still feels right to me, if you know better please shoot it down, that would be science in action.
Aren't photons massless? Newtonian physics would suggest they impart no force (F=ma). If Einsteinian physics disagrees, then do satellites/rockets have to adjust for sunlight force when they come out of the Earth's shadow?
However, quantised force is an interesting idea. Does the concept of "force" play a major role at a quantum level? When an electron is excited into a higher energy state, has a force acted? I think the standard model suggests a photon is absorbed - does this constitute a force acting? Is there any acceleration?
However, quantised force is an interesting idea. Does the concept of "force" play a major role at a quantum level? When an electron is excited into a higher energy state, has a force acted? I think the standard model suggests a photon is absorbed - does this constitute a force acting? Is there any acceleration?
Edited by ewenm on Monday 29th June 21:52
TheEnd said:
Yeah I've seen that before, can't go wrong with a bit of xkcdV8LM said:
One second the fly was going south. The next north. The velocity of the fly has changed sign. The velocity of the fly at one instant, therefore, must have been zero.
On average, at any rate. There was probably a point when its head was moving north but its arse was still moving south. There was possibly a point where its head went through its arse.Not that this adds anything to the discussion.
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