Drag in a Vacuum

  • #1
hi ....this is may be stupid question citing that there is so much information about the subject..

i understand that friction is possible in space...ie rub two things together and a resisting force will create drag on both surfaces and heat will be produced....

But can space vacuum create drag on an object that moves through it....can space vacuum be touched or mechanically interacted with like a gas?

Or better put can a mechanical force be applied to a space vacuum..?
 

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  • #2
Drakkith
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But can space vacuum create drag on an object that moves through it....can space vacuum be touched or mechanically interacted with like a gas?
It cannot. The only interaction you have with space itself is that which is governed by General Relativity, and it doesn't include drag.
 
  • #3
How is a mechanical force (thrust) applied to a space vacuum...recoil from mass flow( mass number 1) out the rocket nozzle would be canceled by the equal and opposite reaction of the rocket mass (mass number 2)
ie if i was at rest in a space vacuum and took off my shoe and thew it ....it would not leave my hand as my body would cancel the force with an equal and opposite reaction to that force....
 
  • #4
Drakkith
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ie if i was at rest in a space vacuum and took off my shoe and thew it ....it would not leave my hand as my body would cancel the force with an equal and opposite reaction to that force....
Not true. Your shoe would go flying off one way and you would be pushed backwards. Note that the equal and opposite forces don't cancel each other out, as they act on different bodies. The force from the shoe on your hand acts on you. The force of your hand on the shoe acts on the shoe. Since the forces act on different bodies they do not cancel each other out.
 
  • #5
a rocket relies on massflow a mechanical force...and the recoil of the mass of the exhaust gasses are what pushes the rocket forward... how can the mass flow effect the rocket if there is zero resistance to both sides of the equasion...how can the rocket impart resistance to the mass flow and vice versa?
 
  • #6
force needs leverage or resistance to do work .......my body throwing the shoe has no resistance to push another mass the shoe and the shoe has no resistance to push back on my body....to me that sounds like they cancel each other.. to say my shoe would go flying needs there to be a resistance to either me or the shoe..and in a space vacuum there are no such things as resistance or drag.
 
  • #7
Drakkith
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The rocket applies a force on the exhaust to accelerate it out of the engine and nozzle, which is matched by an equal and opposite force from the exhaust on the rocket, as required by netwon's laws. The force of the exhaust on the rocket accelerates the rocket forwards.

And that's pretty much it. The basic idea is that simple. Resistance has nothing to do with it.
 
  • #8
A.T.
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  • #9
Drakkith
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Note: I've consolidated all the posts above this one into a single thread, so the order and content may not match up exactly.
 
  • #10
Drakkith
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force needs leverage or resistance to do work
As A.T. said, this is not correct. As an example, an electron moving close to another electron would feel a repulsive force from the 2nd electron on itself and would likewise exert a repulsive force on the 2nd electron. Both electrons would be accelerated away from each other by an equal and opposite force.

A rocket essentially works the same way, just with a LOT of particles all at once.
 
  • #11
resistance force is the force which an https://en.wikipedia.org/w/index.php?title=Effort_force&action=edit&redlink=1 [Broken] must overcome in order to do work on an object
 
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  • #12
Drakkith
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resistance force is the force which an https://en.wikipedia.org/w/index.php?title=Effort_force&action=edit&redlink=1 [Broken] must overcome in order to do work on an object
I'm unfamiliar with "resistance force" and "effort force" but it sounds similar to the force required to overcome static friction. This doesn't apply in this scenario. There is no friction that a rocket needs to overcome once in space.
 
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  • #13
the effort force is thrust expelled gasses
the resistance force is mass of the rocket and mass of expelled gasses

what is resisting the gas once it leaves the rocket nozzle?
what is resisting the rocket mass as the gas pushes on the rocket?
 
  • #14
A.T.
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  • #15
Drakkith
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the effort force is thrust expelled gasses
the resistance force is mass of the rocket and mass of expelled gasses
That's not correct. There is no effort force or resistance force here. After looking up those terms, they refer to scenarios where your applied force has to overcome another force in order to do work, such as lifting an object off of the floor. The force you apply to an object to pick it up off the floor has to exceed the force of gravity just to move the object. But we don't have that scenario here. ANY force, no matter how small, will accelerate an object in space (and thus perform work on that objet). A small force just won't accelerate it hardly at all.
 
  • #16
ok simplify the recoil force in a vacuum ....a gun and a bullet are two separate bodies and the gunpowder reaction the external force...it just happens to be in between the bullet and the gun
firing pin hits percussion cap making an expanding gas pushing on the gun(barrel) and the second body the bullet..
this system is two separate bodies and the external force....recoil pushes the bullet and pushes the gun equaly and they equally move away from each other in opposite directions ...
a rocket is not two separate bodies...a rocket is a gun with expanding gas but no bullet to push back against the expanding gas..gas cannot exist without a containing force so once it enters the vacuum how does it replace the mass of the second body the bullet?
 
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  • #17
A.T.
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  • #18
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In space, if you fire a gun, (gas + bullet ) goes one way and (gun) goes the other. Analogously, for a rocket, (gas) goes one way and (rocket) goes the other.

In both cases the explosion inside the gun or rocket led to the separation of all the masses in question.
 
  • #19
Drakkith
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a rocket is not two separate bodies...a rocket is a gun with expanding gas but no bullet to push back against the expanding gas..gas cannot exist without a containing force so once it enters the vacuum how does it replace the mass of the second body the bullet?
It pushes on the exhaust itself, and the exhaust pushes back. Think about it. In order to accelerate the exhaust from zero to the exhaust velocity, a force MUST be applied. There's only one source for this force, and that is the rocket. So, if the rocket is exerting a force on the exhaust, then by Newton's third law the exhaust MUST be exerting an equal and opposite force on the rocket. If there is a net force on the rocket, it MUST accelerate, and indeed it does.

Does that make sense? Don't worry about other scenarios right now. Just focus on this one.
 
  • #20
for the exhuast to push on its self there has to be some lag??how can there be a lag of gas if vacuum cannot apply drag to it...
 
  • #21
A.T.
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for the exhuast to push on its self
The exhaust pushes the rocket, not itself.
 
  • #22
what is pushing on what? mass flow is pushing off itself correct? which in turn is pushing the rocket? for the mass flow to push any thing there has to be resistance somewhere..? you guys say that the resistance is from the rocket itself and the gas pushing of it self? does there need to be resistance or not?
 
  • #23
it makes perfect sense to me..to say the shoe will not leave my hand....as i throw the shoe the mechanical force created in my arm equaly moves into the shoe and the opposite direction of effort cancelling each other..shoe no move...
 
  • #24
Mechanical force cannot be applied to a space vacuum...
Every machine that moves is mechanical: relies on friction, pressure, exchanging energy with objects external to it. Everything except space rockets, that is.
 
  • #25
Tom Wolfe's book "The Right Stuff" documents high altitude flight tests with rocket powered aircraft that would invariably fail in the thin air and plummet back to earth. Chuck Yeager almost died in a NF-104A rocket plane failure while attempting to set a height record. These planes were liquid fuel ROCKETS and not air-fed jets. Why would NASA claim to be able to send rockets into space when the USAF couldn't get the same technology into even the upper atmosphere?
 

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