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  1. #1 Mars. 
    Forum Ph.D.
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    Sending spacecraft to the planets must involve a great deal of top technical work backed by scientific knowledge,
    Some craft are sent to orbit other solar system bodies, particularly the planets. Others are even able to land on planets such as Mars.
    I believe I am right in saying the craft appear to cover tremendous distances.
    At the moment Mars is around 35 million miles away and this its closest approach!
    Given the ability to send spacecraft with a great deal of accuracy, to reach an object, such as a planet, at great distances, why are the craft not sent to the rendezvous, with the planet, when it is at its closest point.
    This might be seen as simpler and also involve a shorter travel time span.
    This does not seem to be the approach used so clearly there must be sound reasons to use the normal method of covering much larger distances.
    Anyone give an answer?


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    It has to do with orbital mechanics and the energies/velocities needed. The trajectory that takes the lowest launch velocity from Earth orbit and reaches Mars orbit starts on one side of the Sun and end 180 degrees on the other side of the Sun. This is half of an elliptical orbit around the Sun with it closest point to the Sun at Earth orbit and its furthest point at Mars orbit.
    Like this:


    A smaller launch velocity will have the object fall short of Mars orbit, and a higher one would end up further from the Sun than Mars.
    An example of a higher initial velocity trajectory would look like this.


    An analogy would be like standing at the bottom of a building and trying to throw a rock onto it roof. There is a minimum speed at which it leaves your hand where it will reach the roof. You can throw it harder, but then it goes higher than the roof and you've spend more energy than you needed to.

    At this point you might be thinking " So why do we bother with all that elliptical orbit stuff? Why don't we just launch the craft straight up away from the Sun at just the right speed to just reach Mars orbit? The catch is that the Earth itself has an orbital velocity of some 30 km/sec. The elliptical orbit trajectory uses this to it advantage so that we just need to add some extra velocity to it to achieve are required velocity. If we tried to launch into a straight up way from the Sun trajectory, we would lose this advantage and would even have to expend fuel in order to cancel out the Earth's orbital velocity.

    The other consideration is the relative velocity of the craft when it get to the destination. If the object is to land or orbit the planet rather than just a fly-by. You will have to match speed with it when you get there, so you want the difference between them to be small if possible.

    So let's imagine that you were to try a short distance "straight shot" to Mars when it was at its closest. First, you would have to give it enough initial velocity for it to reach a point much further than Mars orbit (or as above, use fuel to kill off the velocity the Earth has given it to put it into a "straight up" trajectory.) In either case,when it gets to Mars orbit, it would be moving at a pretty good clip relative to Mars, ( with the straight up approach it would be at rest with respect to the Sun, Mars would be moving at 24 km/sec) and you would have to burn fuel to match velocities.

    Now we use rockets to make our velocity changes. And the greater the velocity change you need to make, the more fuel you need to carry. But this is not a direct proportion. For example, a doubling of the total velocity change needed increases the fuel requirement by over a factor of 6. A factor of 3 in velocity increase requires 19 times the fuel. even a 10% increase in velocity change means doubling the fuel.

    So you can see why we prefer to use the slower, longer routes over the short direct ones and why sometimes we use circuitous routes that pass by planets other than the target ones in order to use them to sling shot around and pick up some added velocity for "free".


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  4. #3  
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    Thanks for the reply.
    I'm going to need some time to, hopefully, be able to get a decent grasp of your answer!
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    Both the Earth and Mars are moving targets. When we send a spacecraft to Mars or anywhere else, it's not on a straight line trajectory (like the diagram Janus posted).

    We use the forward momentum of the Earth to push the satellite is a slow arc away from the sun until it intersects with our target. Launch windows are determined by when the target will cross paths with a least-effort path away from Earth -- not when the target is closest to Earth. THAT would require many times more fuel than is practical.
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    I believe that we could decrease the time it takes to get to other planets in our solar system, such as Mars, or even other stars, for passengers aboard a spacecraft, by engineering near light speed spacecraft, due to time dilation. For example, if a spacecraft is traveling at near light speed the trip from Earth to Mars may seem like it took a small amount of time for the passengers aboard the spacecraft, while for the ground crew monitoring the ship back down on Earth, it would have seemed like it had taken a much longer time for the spacecraft and it's passengers to reach Mars, if I grasp time dilation correctly.
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    Quote Originally Posted by mmatt9876 View Post
    I believe that we could decrease the time it takes to get to other planets in our solar system, such as Mars, or even other stars, for passengers aboard a spacecraft, by engineering near light speed spacecraft, due to time dilation. For example, if a spacecraft is traveling at near light speed the trip from Earth to Mars may seem like it took a small amount of time for the passengers aboard the spacecraft, while for the ground crew monitoring the ship back down on Earth, it would have seemed like it had taken a much longer time for the spacecraft and it's passengers to reach Mars, if I grasp time dilation correctly.
    That's a bit of overkill isn't it? Mars at its closest is just 4.35 light min away. Even at 10% of c that just a 43 1/2 min trip as measured by the Earth, and 13 seconds less according to our astronauts. The distance just isn't far enough to require speeds that involve significant time dilation effect.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by Janus View Post
    That's a bit of overkill isn't it? Mars at its closest is just 4.35 light min away. Even at 10% of c that just a 43 1/2 min trip as measured by the Earth, and 13 seconds less according to our astronauts. The distance just isn't far enough to require speeds that involve significant time dilation effect.
    It would like taking Concorde London-Paris: you'd spend more time boarding and deplaning than you would travelling...
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    Quote Originally Posted by mmatt9876 View Post
    I believe that we could decrease the time it takes to get to other planets in our solar system, such as Mars, or even other stars, for passengers aboard a spacecraft, by engineering near light speed spacecraft, due to time dilation. For example, if a spacecraft is traveling at near light speed the trip from Earth to Mars may seem like it took a small amount of time for the passengers aboard the spacecraft, while for the ground crew monitoring the ship back down on Earth, it would have seemed like it had taken a much longer time for the spacecraft and it's passengers to reach Mars, if I grasp time dilation correctly.
    Yep. But the acceleration required would turn you into a thin layer of goo, and the energy required would take far more energy than humanity has ever generated. (The radiation would also kill you instantly, but since as a layer of goo you are already dead, that's not a big deal>)
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    I believe that we could decrease the time it takes to get to other planets in our solar system, such as Mars, or even other stars, for passengers aboard a spacecraft, by engineering near light speed spacecraft, due to time dilation. For example, if a spacecraft is traveling at near light speed the trip from Earth to Mars may seem like it took a small amount of time for the passengers aboard the spacecraft, while for the ground crew monitoring the ship back down on Earth, it would have seemed like it had taken a much longer time for the spacecraft and it's passengers to reach Mars, if I grasp time dilation correctly.
    That's a bit of overkill isn't it? Mars at its closest is just 4.35 light min away. Even at 10% of c that just a 43 1/2 min trip as measured by the Earth, and 13 seconds less according to our astronauts. The distance just isn't far enough to require speeds that involve significant time dilation effect.
    Sorry, my mistake. I guess near light space travel would be more practical for light year distant trips than to Mars.
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  11. #10  
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Janus View Post
    That's a bit of overkill isn't it? Mars at its closest is just 4.35 light min away. Even at 10% of c that just a 43 1/2 min trip as measured by the Earth, and 13 seconds less according to our astronauts. The distance just isn't far enough to require speeds that involve significant time dilation effect.
    It would like taking Concorde London-Paris: you'd spend more time boarding and deplaning than you would travelling...
    If it were physically and safely possible to accelerate almost instantaneously to near light speed and back down to a full stop, in orbit around Mars, or on the Martian surface, and the time and money it would take to engineer and operate such a spacecraft were somehow not an issue of any kind, why not get from point a to point b in 43 1/2 minutes, for people outside the near light speed spacecraft, or 13 seconds, for the near light speed spacecraft passengers?
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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by mmatt9876 View Post
    I believe that we could decrease the time it takes to get to other planets in our solar system, such as Mars, or even other stars, for passengers aboard a spacecraft, by engineering near light speed spacecraft, due to time dilation. For example, if a spacecraft is traveling at near light speed the trip from Earth to Mars may seem like it took a small amount of time for the passengers aboard the spacecraft, while for the ground crew monitoring the ship back down on Earth, it would have seemed like it had taken a much longer time for the spacecraft and it's passengers to reach Mars, if I grasp time dilation correctly.
    Yep. But the acceleration required would turn you into a thin layer of goo, and the energy required would take far more energy than humanity has ever generated. (The radiation would also kill you instantly, but since as a layer of goo you are already dead, that's not a big deal>)
    The energy involved to operate the spacecraft and the forces on the passengers would be extreme unless we could find a way to engineer around it.
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  13. #12  
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    Quote Originally Posted by mmatt9876 View Post
    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Janus View Post
    That's a bit of overkill isn't it? Mars at its closest is just 4.35 light min away. Even at 10% of c that just a 43 1/2 min trip as measured by the Earth, and 13 seconds less according to our astronauts. The distance just isn't far enough to require speeds that involve significant time dilation effect.
    It would like taking Concorde London-Paris: you'd spend more time boarding and deplaning than you would travelling...
    If it were physically and safely possible to accelerate almost instantaneously to near light speed and back down to a full stop, in orbit around Mars, or on the Martian surface, and the time and money it would take to engineer and operate such a spacecraft were somehow not an issue of any kind, why not get from point a to point b in 43 1/2 minutes, for people outside the near light speed spacecraft, or 13 seconds, for the near light speed spacecraft passengers?
    The 43 1/2 min Earth trip time would be for 1/10 c and the ship time would be 13 sec shorter than this (~43 1/3 min)
    Near light speed would be closer to 4. 35 min. But the energy difference between 1/10c and near c speed is enormous, and not really worth the time saved.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  14. #13  
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Janus View Post
    That's a bit of overkill isn't it? Mars at its closest is just 4.35 light min away. Even at 10% of c that just a 43 1/2 min trip as measured by the Earth, and 13 seconds less according to our astronauts. The distance just isn't far enough to require speeds that involve significant time dilation effect.
    It would like taking Concorde London-Paris: you'd spend more time boarding and deplaning than you would travelling...
    If it were physically and safely possible to accelerate almost instantaneously to near light speed and back down to a full stop, in orbit around Mars, or on the Martian surface, and the time and money it would take to engineer and operate such a spacecraft were somehow not an issue of any kind, why not get from point a to point b in 43 1/2 minutes, for people outside the near light speed spacecraft, or 13 seconds, for the near light speed spacecraft passengers?
    The 43 1/2 min Earth trip time would be for 1/10 c and the ship time would be 13 sec shorter than this (~43 1/3 min)
    Near light speed would be closer to 4. 35 min. But the energy difference between 1/10c and near c speed is enormous, and not really worth the time saved.
    Thank you. It seems like you only yield time dilation benefits for long distance travel.
    Last edited by mmatt9876; October 2nd, 2018 at 09:41 PM.
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    At what distance would near light speed travel make sense in terms of saving time spent traveling for the crew of the vessel due to time dilation? A trip to the nearest star? A trip to the nearest galaxy?
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  16. #15  
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    Quote Originally Posted by mmatt9876 View Post
    At what distance would near light speed travel make sense in terms of saving time spent traveling for the crew of the vessel due to time dilation? A trip to the nearest star? A trip to the nearest galaxy?
    Nearest star. Even Pluto would be just a bit over 2 1/4 days away at only 1/10 of the speed of light and time dilation would only save you ~ 1/2 hr.

    At near light speed it would take over 4.3 yrs Earth time to reach Alpha Centauri, and it would be advantageous to trim a significant portion of that time for the passengers.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  17. #16  
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    At what distance would near light speed travel make sense in terms of saving time spent traveling for the crew of the vessel due to time dilation? A trip to the nearest star? A trip to the nearest galaxy?
    Nearest star. Even Pluto would be just a bit over 2 1/4 days away at only 1/10 of the speed of light and time dilation would only save you ~ 1/2 hr.

    At near light speed it would take over 4.3 yrs Earth time to reach Alpha Centauri, and it would be advantageous to trim a significant portion of that time for the passengers.
    I believe that I had read somewhere that if you were able to construct a ship that could somehow travel at light speed the trip for the passengers from point a to point b, regardless of distance, would be or seem instantaneous. Is that true?
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    Quote Originally Posted by mmatt9876 View Post
    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    At what distance would near light speed travel make sense in terms of saving time spent traveling for the crew of the vessel due to time dilation? A trip to the nearest star? A trip to the nearest galaxy?
    Nearest star. Even Pluto would be just a bit over 2 1/4 days away at only 1/10 of the speed of light and time dilation would only save you ~ 1/2 hr.

    At near light speed it would take over 4.3 yrs Earth time to reach Alpha Centauri, and it would be advantageous to trim a significant portion of that time for the passengers.
    I believe that I had read somewhere that if you were able to construct a ship that could somehow travel at light speed the trip for the passengers from point a to point b, regardless of distance, would be or seem instantaneous. Is that true?
    It isn't possible for a ship to travel at light speed, even in theory. The best you could ever do is to get very near it.
    At 0.99 c it would take 1 year ship time to travel just under 7 light years.
    At 0.999 c you could travel 22.34 light years in one year of ship time
    At 0.9999c you could travel over 70 light years in one ship year.
    At 0.99999c it would be 223.6 light years in one ship year
    Etc.

    However, as you get closer to the speed of light, the energy cost increases rapidly.
    It takes more energy to go from 0.9999c to 0.99999c than it did to go from 0.999c to 0.9999c, even though the velocity change is smaller.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  19. #18  
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    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    At what distance would near light speed travel make sense in terms of saving time spent traveling for the crew of the vessel due to time dilation? A trip to the nearest star? A trip to the nearest galaxy?
    Nearest star. Even Pluto would be just a bit over 2 1/4 days away at only 1/10 of the speed of light and time dilation would only save you ~ 1/2 hr.

    At near light speed it would take over 4.3 yrs Earth time to reach Alpha Centauri, and it would be advantageous to trim a significant portion of that time for the passengers.
    I believe that I had read somewhere that if you were able to construct a ship that could somehow travel at light speed the trip for the passengers from point a to point b, regardless of distance, would be or seem instantaneous. Is that true?
    It isn't possible for a ship to travel at light speed, even in theory. The best you could ever do is to get very near it.
    At 0.99 c it would take 1 year ship time to travel just under 7 light years.
    At 0.999 c you could travel 22.34 light years in one year of ship time
    At 0.9999c you could travel over 70 light years in one ship year.
    At 0.99999c it would be 223.6 light years in one ship year
    Etc.

    However, as you get closer to the speed of light, the energy cost increases rapidly.
    It takes more energy to go from 0.9999c to 0.99999c than it did to go from 0.999c to 0.9999c, even though the velocity change is smaller.
    Are there a lot of star systems with planets within 223.6 light years of our sun? If so, a one year of trip time, for the passengers of a spacecraft travelling at 0.99999c, to one of those star systems with planets, to colonize them, would be a reasonable trip time for the spacecraft passengers.
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    Quote Originally Posted by mmatt9876 View Post
    If it were physically and safely possible to accelerate almost instantaneously to near light speed and back down to a full stop, in orbit around Mars, or on the Martian surface, and the time and money it would take to engineer and operate such a spacecraft were somehow not an issue of any kind, why not get from point a to point b in 43 1/2 minutes, for people outside the near light speed spacecraft, or 13 seconds, for the near light speed spacecraft passengers?
    Another point here is that you don't need to. If you are able to accelerate at a constant 1G, you can get places very quickly (and at no additional stress for passengers, other than launch.) You could get to the Moon in three and a half hours, Mars in about three days, Saturn in eight days. And such short travel times at one gravity greatly reduce issues with provisions, radiation exposure and atrophy due to zero-G.

    Unfortunately the only theoretical drive system we have that comes anywhere close to that is the Orion system, and there are a lot of reasons that will never get off the ground (no pun intended.)
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    Quote Originally Posted by mmatt9876 View Post
    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    Quote Originally Posted by Janus View Post
    Quote Originally Posted by mmatt9876 View Post
    At what distance would near light speed travel make sense in terms of saving time spent traveling for the crew of the vessel due to time dilation? A trip to the nearest star? A trip to the nearest galaxy?
    Nearest star. Even Pluto would be just a bit over 2 1/4 days away at only 1/10 of the speed of light and time dilation would only save you ~ 1/2 hr.

    At near light speed it would take over 4.3 yrs Earth time to reach Alpha Centauri, and it would be advantageous to trim a significant portion of that time for the passengers.


    I believe that I had read somewhere that if you were able to construct a ship that could somehow travel at light speed the trip for the passengers from point a to point b, regardless of distance, would be or seem instantaneous. Is that true?
    It isn't possible for a ship to travel at light speed, even in theory. The best you could ever do is to get very near it.
    At 0.99 c it would take 1 year ship time to travel just under 7 light years.
    At 0.999 c you could travel 22.34 light years in one year of ship time
    At 0.9999c you could travel over 70 light years in one ship year.
    At 0.99999c it would be 223.6 light years in one ship year
    Etc.

    However, as you get closer to the speed of light, the energy cost increases rapidly.
    It takes more energy to go from 0.9999c to 0.99999c than it did to go from 0.999c to 0.9999c, even though the velocity change is smaller.
    Are there a lot of star systems with planets within 223.6 light years of our sun? If so, a one year of trip time, for the passengers of a spacecraft travelling at 0.99999c, to one of those star systems with planets, to colonize them, would be a reasonable trip time for the spacecraft passengers.
    Even with 100% conversion of energy into velocity, it would require ~200 times the entire world's present energy consumption for a year to get just 1 kg of mass up to 0.99999c. Double that to slow back down again. Consider the size of ship it would take to support an colonizing effort. While it would be a reasonable trip, time-wise, for the passengers, it is unlikely to be a reasonable trip in terms of the resources expended to make it.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Quote Originally Posted by billvon View Post
    Quote Originally Posted by mmatt9876 View Post
    If it were physically and safely possible to accelerate almost instantaneously to near light speed and back down to a full stop, in orbit around Mars, or on the Martian surface, and the time and money it would take to engineer and operate such a spacecraft were somehow not an issue of any kind, why not get from point a to point b in 43 1/2 minutes, for people outside the near light speed spacecraft, or 13 seconds, for the near light speed spacecraft passengers?
    Another point here is that you don't need to. If you are able to accelerate at a constant 1G, you can get places very quickly (and at no additional stress for passengers, other than launch.) You could get to the Moon in three and a half hours, Mars in about three days, Saturn in eight days. And such short travel times at one gravity greatly reduce issues with provisions, radiation exposure and atrophy due to zero-G.

    Unfortunately the only theoretical drive system we have that comes anywhere close to that is the Orion system, and there are a lot of reasons that will never get off the ground (no pun intended.)
    Thanks, it is good to know we can get around the solar system in a reasonable amount of time with a spacecraft traveling at a constant speed of 1G. Considering what Janus said about the energy costs pf accelerating a spacecraft up to 0.9c or greater, and back down to a full stop, I believe that it would cost less in terms of energy, an of course money, to operate a spacecraft at a constant 1G rather than at a constant 0,9c or greater.

    Keeping travel time in space short is good for, as you pointed out, the threat of reducing provisions, radiation, and health concerns due to 0G are issues that come with prolonged space travel, unless you figure out a way to engineer around them.
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    Quote Originally Posted by mmatt9876 View Post
    to operate a spacecraft at a constant 1G rather than at a constant 0,9c or greater.
    No no no.
    1G is an acceleration, 0.9c is a speed.
    If you keep up 1G long enough you'll reach 0.9c anyway. But even (with current technology) a constant 1G would be prohibitively fuel-expensive.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by mmatt9876 View Post
    to operate a spacecraft at a constant 1G rather than at a constant 0,9c or greater.
    No no no.
    1G is an acceleration, 0.9c is a speed.
    If you keep up 1G long enough you'll reach 0.9c anyway. But even (with current technology) a constant 1G would be prohibitively fuel-expensive.
    Thank you for the correction. I made a mistake. If I am correct I believe that the difference between motion at a constant speed and motion due to acceleration is acceleration can keep building on itself unless the force or energy applied to the accelerating object is removed and it slows down due to friction, a collision, or another opposing force, if there are any, because without any friction, or an opposing force, acting on the moving, or an accelerating, object it will continue to travel at what ever speed it was moving at, or continue to accelerate, if it is still under the influence of a constant force, due to inertia.
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