Speed Of Objects In Space

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Speed Of Objects In Space
Posted by Muxman on Tuesday, August 11, 2009 10:45 AM
Please could somebody explain to me why objects travel at different speeds in space? What is the cause of this and what determines an objects speed?
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Posted by bogenj on Tuesday, August 11, 2009 12:25 PM

This is a difficult question to answer.  Perhaps you could tell us what education background you have had in science, and/or ask a more specific question.  Please do not take offense.

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Posted by Muxman on Tuesday, August 11, 2009 12:37 PM
No offence taken, as for my education in science regard it as non existent, so the more simple the explanation the better. To try to get more specific...Why does Earth travel at a different speed through space than Neptune. Does a meteor 5 Miles wide travel faster through space than a meteor 1 mile wide? If so why? Why does a comet travel at a different speed to a planet?
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Posted by bogenj on Tuesday, August 11, 2009 12:57 PM

O.K.  We are talking about orbital mechanics here.  First, let's distinguish between "speed" and "velocity."  Simply put, velocity implies speed AND direction.  Example: one can drive a car at a speed of 55 mph, but at a velocity of 55 mph headed north.  If we are talking about planets in our solar system, physics tells us that the farther away a planet is from the sun, the velocity required for it to remain in orbit around the sun decreases.  That's because the force of gravity is weaker the farther away two objects are from each other (twice the distance means one-fourth the gravitational attraction).  Earth's mean orbital velocity is 29.8 kilometers per second (km/sec) in a near circular orbit around the sun.  Neptune is slower (5.4 km/sec) because it is much farther away from the sun. 

Also, orbital velocity does not depend on the size or mass of an object, just the orbit itself.

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Posted by bogenj on Tuesday, August 11, 2009 1:07 PM

Comets generally have elliptical orbits around the sun (experts, I'm leaving hyperbolic orbits out of my discussion).  According to Newton's and Kepler's laws, the closer the comet is to the sun, the faster is it's velocity with respect to the sun.  This makes intuitive sense.  As the comet heads towards the sun, it is pulled by gravity, and speeds up.  Directionwise, the orbit is bent towards the sun.  After perihelion (closest approach to the sun), the comet slows down due to gravity.  At it's farthest point from the sun at aphelion, the comet is travelling the slowest.

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Posted by Muxman on Tuesday, August 11, 2009 1:14 PM
Thanks so much for the explanations, makes perfect sense now. The only thing which bothers me now is why is the force of gravity different on Earth to what it is in space? To elaborate a little I mean why is there terminal velocity on Earth and this does not exist in space. Or does it?
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Posted by bogenj on Tuesday, August 11, 2009 1:19 PM

One more example.  Let's talk about "relative velocity."  Suppose a red car is travelling north at 55 mph (with respect to the ground), and a blue car is right behind the red car in the same lane, and is going in the same direction at 65 mph.  No brakes are used.  The blue car rear-ends the red car at a relative velocity of 65 minus 55 = 10 mph.  But let's suppose the blue car was headed south, was in front of the red car, and there was a head-on collision.  The relative velocity would be 55 plus 65 = 120 mph, and the crash would be much more devastating.  Objects travelling through the solar system are going at all sorts of speeds and directions (i.e. velocities).  The way they interact, or collide, depends on both speed and direction.  In the example of a comet hitting the Earth, the relative velocity of the impact depends on both speed and direction of the orbits of the Earth and the comet.  Same explanation goes for space "junk" hitting spacecraft in orbit.  And remember to think in 3 dimensions.

I hope I helped.

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Posted by Muxman on Tuesday, August 11, 2009 1:23 PM
You certainly did help, once again thank you very much. I try to make a point of learning something new each day and this forum never disappoints in enhancing my knowledge.
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Posted by bogenj on Tuesday, August 11, 2009 1:36 PM

By force of gravity in space, I assume you ask why astronauts feel weightless.  Well, Earth's gravity is a little weaker up there, and is still pulling on the astronauts, but because they are in orbit, they are essentially in "freefall," but stay at about the same distance from Earth because the Earth is curved.  If you are in an elevator in freefall, you will also feel weightless, and probably terrified too.  The Tower of Terror ride at Disneyworld gives you this feeling (in fact, I remember being pushed down even faster than gravity).

Terminal velocity is a different issue.  It takes into account air resistance.  The terminal velocity of a human is about 120 mph at sea level.  If you pop open the parachute, the speed is much slower, even though the weight of the parachutist remains the same.  In general, terminal velocity occurs when the force of gravity downwards matches the force of air resistance / drag upwards, at which point the velocity of the object will not change.

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Posted by Ictinike on Tuesday, August 18, 2009 9:54 PM

 Other thing that should be mentioned... the distance between two objects, and the increase of that distance does not correlate directly with most peoples thinking of "speed". Space is expanding, so the objects are not really "speeding through space", but the space they exist in is making them farther apart.

 There was no bang in the big bang. It was less of an explosion and more of a rapid expansion.

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Posted by zachsdad on Wednesday, August 19, 2009 7:32 AM

Ictinike
Space is expanding, so the objects are not really "speeding through space", but the space they exist in is making them farther apart.

On a cosmological scale that is true, but on a more local scale everything is indeed in motion.  As you read this the Milky Way is moving toward the Andromeda galaxy at about 268,000 miles per hour.

 

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Posted by Ictinike on Wednesday, August 19, 2009 8:06 AM

zachsdad

On a cosmological scale that is true, but on a more local scale everything is indeed in motion.  As you read this the Milky Way is moving toward the Andromeda galaxy at about 268,000 miles per hour.

 

True true. Left that out as assumed. In fact, there it is now!

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Posted by Enfilade1 on Wednesday, August 19, 2009 2:22 PM

Is it gravity that is causing some galaxies to move towards each other even while the universe is expanding?

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Posted by chipdatajeffB on Wednesday, August 19, 2009 2:40 PM

Enfilade1

Is it gravity that is causing some galaxies to move towards each other even while the universe is expanding?

According to current thinking, yes. The same thing apparently happens in clusters: galaxies and their dark matter halos "hang together" locally via gravity.

It's important to think about the difference between "local" and "universal". The Milky Way and M31 are part of what we call The Local Group of galaxies. So there are gravitational effects between and among the galaxies in this group.

The Local Group itself is a part of a larger supercluster of galaxy clusters. And these are all streaming through space toward something we call The Great Attractor, and it lies in a different direction from the overall Hubble Flow (thought to be universal). So, there is also a supra-local, yet not universal, gravitational effect.

The same is likely true for other galaxy clusters, and superclusters.

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