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thunderchicken
2004-Sep-11, 01:10 AM
I know this is probably a stupid question, but it comes up in conversation with my friends every once in a while. If you were to fire an object into space would it accelerate forever (space = vacuum = no air friction)? I guess in reality space is not a vacuum and the projectile would eventually get sucked in by the gravity of some star or other body. But I am wondering more about the theoretical possibility of an object accelerating forever in a true vacuum. I know this can't happen, but why??

Captain Kidd
2004-Sep-11, 01:18 AM
I know this is probably a stupid questionNope.

Accelerate or travel? Travel yes, until, as you pointed out, it hits something. Accelerate, no. Only for as long as it has fuel onboard; however from your wording I think you mean like if you fire a gun and want to know what happens to the bullet.

So if you take a gun into space and fire it; whatever muzzle velocity the bullet has upon exiting the gun will be its velocity until it finally hits something. (Ignoring for now the occasional gas particle and other stuff itíll happen across.) Thereíll be some gravitational influence, but thatíd be mostly making it curve. However, it could accelerate if it is traveling towards something with significant gravity like a planet. But thatís an outside force acting on it to cause it to accelerate.

Brady Yoon
2004-Sep-11, 01:53 AM
Yeah. Newton's 2nd law is still valid in a vacuum. For acceleration to occur, there must be an unbalanced force acting on an object.

thunderchicken
2004-Sep-11, 11:15 PM
Hey, thanks for the replies.


Accelerate, no. Only for as long as it has fuel onboard

Even if there was a space ship with unlimited fuel and an extremely powerful engine I guess it can't accelerate forever. Is this because eventually the thrust from the engine will match the forward speed? If this is the case, why couldnít I say that a ship with unlimited fuel and engine power can accelerate forever and reach the speed of light?

PS: I know this question is dumb, but I can't find my old school physics text books.
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Dgennero
2004-Sep-11, 11:20 PM
You use photons as accelarator (they leave the ship at light speed), but the problem is, that as the ship approaches the speed of light, the relativistic effects let it gain mass, and to actually reach light speed, it would take an infinite acceleration force, because the ship's mass approaches an infinite value. But YOU, in the ship, would have the impression that you still accelerate more and more, because your time slows down, and you subjectively age so slow, that you seem to be faster than light and at 0.99999... c can reach distant galaxies within your lifetime.

worzel
2004-Sep-12, 12:15 AM
Hey, thanks for the replies.


Accelerate, no. Only for as long as it has fuel onboard

Even if there was a space ship with unlimited fuel and an extremely powerful engine I guess it can't accelerate forever. Is this because eventually the thrust from the engine will match the forward speed? If this is the case, why couldnít I say that a ship with unlimited fuel and engine power can accelerate forever and reach the speed of light?

PS: I know this question is dumb, but I can't find my old school physics text books.

As said above, you can keep accelerating forever feeling the same g-force, but you will never overtake a photon, or even measure one moving relative to you at anything other than c - that's what special relativity is all about.

The top speed you experience on earth is due to the resistance of the environment you're accelerating against - the faster you go the more resistance you feel. There is no resistance in a vacuum so a rocket in a vacuum would just keep accelerating. At any instant you have velocity v w.r.t your starting point but it would be equally valid to figure that you're not moving and your starting point was. So there's no difference between the accelerative force of the rocket if you were stationary or moving at .999c, they're just different ways of describing the same reality. But the same "trick" on earth would require you to view the environment moving towards you at an ever increasing rate until the resulting force equalled your accelerative force.

bigsplit
2004-Sep-13, 02:53 AM
I know this is probably a stupid questionNope.

Accelerate or travel? Travel yes, until, as you pointed out, it hits something. Accelerate, no. Only for as long as it has fuel onboard; however from your wording I think you mean like if you fire a gun and want to know what happens to the bullet.

So if you take a gun into space and fire it; whatever muzzle velocity the bullet has upon exiting the gun will be its velocity until it finally hits something. (Ignoring for now the occasional gas particle and other stuff itíll happen across.) Thereíll be some gravitational influence, but thatíd be mostly making it curve. However, it could accelerate if it is traveling towards something with significant gravity like a planet. But thatís an outside force acting on it to cause it to accelerate.

Certainly the bullet would accelerate forever, we justify our acceleration now with this mysterious dark energy to justify inflation....Yet, we know that we should not continue to accelerate, but inflation must be true...right?

But, you are right the bullet would not accelerate indefinately and neither would the Universe in the Big Bang.....the sooner we give up the inflation and spherical Universe model, the better off we will be. We could abandon the misconceptions of dark energy and figure out our Universe. It is a funnel and we are accelerating downward on an equiangular spiraling path. Galaxy Clusters have great attractors and likewise there is a Universal attractor. It is time to make the next great leap in our understanding of the Universe.

Ut
2004-Sep-13, 03:29 AM
Someone's getting a little off topic.

Without getting into controvercial issues, the bullet will travel at a constant velocity unless acted upon by an outside force. Some of these forces -- gravity, for instance -- may change either the speed, or the direction, or both, but in clear and open space, the bullet will essentially travel at constant velocity for all time.

Bringing in complaints or criticisms of some of the more wide spread ideas about space muddies the issue and complicates the answer beyond the level necessary for the question asked or the understanding sought. Besides, this is the wrong forum for such discussion.

drhex
2004-Sep-13, 10:46 AM
A ship with "unlimited fuel" would also have an unlimited mass and therefore be rather hard to accelerate.

bigsplit
2004-Sep-13, 12:10 PM
I did go off topic, the question just led me there, this is an astronomy board. The answer to his question was elementary physics and you answered it early on; it was too hard to resist. Sorry....no, no...as a Republican would say...I regret it.

worzel
2004-Sep-13, 12:43 PM
A ship with "unlimited fuel" would also have an unlimited mass and therefore be rather hard to accelerate.
Maybe it hoovers up the virtual particles in the vacuum.

Captain Kidd
2004-Sep-13, 01:49 PM
A ship with "unlimited fuel" would also have an unlimited mass and therefore be rather hard to accelerate.
Maybe it hoovers up the virtual particles in the vacuum.Bussard Ramjet (http://woodmansee.com/science/rocket/r-interstellar/r-interstellar-18.html)

Ilya
2004-Sep-13, 05:08 PM
Even if there was a space ship with unlimited fuel and an extremely powerful engine I guess it can't accelerate forever. Is this because eventually the thrust from the engine will match the forward speed?

All previous posters concentrated on the speed-of-light limit, but if I understand your question correctly, it is entirely possible for a rocket to move faster than its exhaust. In fact, EVERY rocket that ever went into orbit did so - Earth's orbital velocity is 7.9 km/sec, while best chemical fuel (hydrogen + oxygen) creates exhaust velocity of only 4.5 km/sec.

Correct me if I misunderstood the question, but "thrust" and "speed" are measured in different units and can not be compared (or "match").

worzel
2004-Sep-14, 12:03 AM
Correct me if I misunderstood the question, but "thrust" and "speed" are measured in different units and can not be compared (or "match").
Thrust = force = acceleration * mass

So for a constant mass, a constant thrust will give a constant acceleration, i.e. a constant increase in speed.

Hope I got that right 8-[

frogesque
2004-Sep-14, 12:51 AM
A rocket is essentially a reaction jet. Connect a rubber hose to the water main and you can feel the force on the nozel even when you are standing still. A rocket gets propelled because of the difference in velocity between the rocket motor and the mass ejected in the jet. Velocity is not the same as speed, it is a vector and has direction as well as speed. Because of this it is possible to steer a jet by tweeking the direction of the expelled mass in the jet. (It then has a sideways component to the vector as well as one parallel to the direction of flight)