Theoretical question
Discussion
Was thinking about ion propulsion and it kind of snowballed.
So given no other forces at work (no gravity or photons), let's imagine an ISS floating in space. and then have a fly that can survive and even fly in space is trying to push the ISS in one direction, given infinite time, will it push the ISS off the spot?
Yes?
what about a stead stream of snowflakes hitting the same spot?
what about one molecule hitting the same spot?
What I am trying to ask I guess, is there such a thing as a force too small or mass too big to make a difference?
So given no other forces at work (no gravity or photons), let's imagine an ISS floating in space. and then have a fly that can survive and even fly in space is trying to push the ISS in one direction, given infinite time, will it push the ISS off the spot?
Yes?
what about a stead stream of snowflakes hitting the same spot?
what about one molecule hitting the same spot?
What I am trying to ask I guess, is there such a thing as a force too small or mass too big to make a difference?
isee said:
So given no other forces at work (no gravity or photons), let's imagine an ISS floating in space. and then have a fly that can survive and even fly in space is trying to push the ISS in one direction, given infinite time, will it push the ISS off the spot?
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.Dr Jekyll said:
isee said:
So given no other forces at work (no gravity or photons), let's imagine an ISS floating in space. and then have a fly that can survive and even fly in space is trying to push the ISS in one direction, given infinite time, will it push the ISS off the spot?
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.Eric Mc said:
Yes.
Even light can exert a pressure on an object.
Comets and asteroids are always having their trajectories altered by small impacts or even by light pressure.
Eric I understand that, I was asking if there is a force too small to do anything (yet never the less still a measurable force). Let's say, ISS is stationary and gets hit by a single photon. Will that one photon hitting the station translate into it being shifted say 1 metre a 1000 years later?Even light can exert a pressure on an object.
Comets and asteroids are always having their trajectories altered by small impacts or even by light pressure.
Galileo said:
I've just been watching about the NASA Dawn mission. It has a Xenon Ion propulsion system that pushes the craft with the same pressure as a piece of A4 paper held in your hand. Given several years that pressure pushed the craft upto 75000 km/h.
i always wondered what that ion thrust is actually like in comparable terms. Never realised it was that small.It's been experimentally verified down to 10^-14 N.
http://physicsworld.com/cws/article/news/2007/apr/...
That's roughly (depending on who you believe) the weight of a bacterium. Though you can measure the mass of less: http://news.bbc.co.uk/1/hi/sci/tech/3607993.stm - but that IS measuring mass, not force.
http://physicsworld.com/cws/article/news/2007/apr/...
That's roughly (depending on who you believe) the weight of a bacterium. Though you can measure the mass of less: http://news.bbc.co.uk/1/hi/sci/tech/3607993.stm - but that IS measuring mass, not force.
isee said:
Eric I understand that, I was asking if there is a force too small to do anything (yet never the less still a measurable force). Let's say, ISS is stationary and gets hit by a single photon. Will that one photon hitting the station translate into it being shifted say 1 metre a 1000 years later?
I came to this concept years ago by wondering how much the temperature would go up on Pluto if I struck a match on Earth. I decided that quantum theory would mean it was 0.Regarding the fly, you may also need to consider whether the medium it's in is Newtonian or non-Newtonian.
Not sure where you're going to find somewhere free of gravity either unless you're in deep inter-stellar space 
Being in constant free-fall (i.e. orbit) is not being in a gravity-free environment although it may appear to be to those falling/orbiting.
The space-sail idea requires the force of the solar wind (IIRC) which is v. small but present and potentially effective.
Being in constant free-fall (i.e. orbit) is not being in a gravity-free environment although it may appear to be to those falling/orbiting.The space-sail idea requires the force of the solar wind (IIRC) which is v. small but present and potentially effective.
In classical physics you'd use Newton's 3rd Law that states that a body's acceleration is proportional to the net force applied to it; F = m × a.
You'll see there aren't any conditions in that expression. It doesn't say something like "only if the ratio of the mass to the force is less than some limit".
So according to Newton it doesn't matter how big the mass or how small the force; any net force on a mass will cause it to accelerate.
The ISS is a pretty good example of a classical system.
You'll see there aren't any conditions in that expression. It doesn't say something like "only if the ratio of the mass to the force is less than some limit".
So according to Newton it doesn't matter how big the mass or how small the force; any net force on a mass will cause it to accelerate.
The ISS is a pretty good example of a classical system.
Any force no matter how small will exert a force on that object. The motion of that object is determined by the net force it feels.
If we use the example of a car on earth and a fly trying to move it( trying to keep it as relatable to the original question as possible) The the car feels the force of the fly, and the effect of weight(this being the combination of the mass of the object x the force of gravity) when stationary. This weight component is massive on earth due to the strength of gravity. Something the fly would also have to cope with is air resistance if it ever did begin to move. All of these components would massively outweigh the force exerted by the fly.
If we now look at the International space station, the weight component is tiny due to the massive reduction in the gravitational force. Space is almost a perfect vacuum (more perfect that anything currently achievable on earth) so there is also no air resistance when the fly starts to move the space ship. Therefore in this model the fly/ion would have the greatest force and would be capable of moving the station.
I was once told by my Physics teacher to carry a bag of tennis balls in space, so if you float off you can throw them in the opposit direction and propel yourself to where you want to go...
If we use the example of a car on earth and a fly trying to move it( trying to keep it as relatable to the original question as possible) The the car feels the force of the fly, and the effect of weight(this being the combination of the mass of the object x the force of gravity) when stationary. This weight component is massive on earth due to the strength of gravity. Something the fly would also have to cope with is air resistance if it ever did begin to move. All of these components would massively outweigh the force exerted by the fly.
If we now look at the International space station, the weight component is tiny due to the massive reduction in the gravitational force. Space is almost a perfect vacuum (more perfect that anything currently achievable on earth) so there is also no air resistance when the fly starts to move the space ship. Therefore in this model the fly/ion would have the greatest force and would be capable of moving the station.
I was once told by my Physics teacher to carry a bag of tennis balls in space, so if you float off you can throw them in the opposit direction and propel yourself to where you want to go...
maxjeff said:
If we use the example of a car on earth and a fly trying to move it( trying to keep it as relatable to the original question as possible) The the car feels the force of the fly, and the effect of weight(this being the combination of the mass of the object x the force of gravity) when stationary.
But this is where things go wrong. If your car is travelling north at 70 mph and meets the fly going south at 2 mph, the fly will end up stuck to the car travelling north at ever-so-slightly less than 70 mph.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.
V8LM said:
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.
Speed is distance/time. So saying the velocity must have been zero for an instant doesn't really mean anything and is nothing to do with the argument. Even if the fly was an entity with no force at all it would still change direction, the measure of force is how fast the car is going after the collision compared with before. maxjeff said:
I was once told by my Physics teacher to carry a bag of tennis balls in space, so if you float off you can throw them in the opposit direction and propel yourself to where you want to go...
Not quite tennis balls, but close-https://what-if.xkcd.com/85/
maxjeff said:
If we now look at the International space station, the weight component is tiny due to the massive reduction in the gravitational force.
Actually whilst the space station experiences a smaller gravitational force than a comparable object on the ground - the difference isn't that great.The ISS at 400km altitude experiences a gravitational force of around 8.7m/s/s (compared to 9.8 for an object at sea level) - that's what keeps it in orbit.
Objects inside the ISS appear 'weightless' because they are in the same free fall trajectory as the ISS. They have a zero net acceleration relative to the ISS. A mass only appears to have weight if it is subject to a net acceleration due to gravity relative to the measuring device (a balance in this case). For any given mass - the weight changes depending on the acceleration due to gravity (i.e. an object with 1kg of mass will have different weight depending on whether you measure that weight on the surface of earth or the surface of the moon).
Edited by Moonhawk on Friday 26th June 09:28
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