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Can I go up with a constant speed?

  1. Nov 9, 2013 #1
    I'm trying to think about ascending in the atmosphere with constant speed, that means I will have no acceleration and that means that the net force on me will be zero, BUT, there is always gravity pulling me upward so i must have some force that's pushing me upwards, thus I must be going up with an acceleration!
     
  2. jcsd
  3. Nov 9, 2013 #2
    Every thing you said makes sense except the last sentence. You won't be accelerating since forces acting on you will cancel each other out producing a net zero force.
     
  4. Nov 9, 2013 #3
    yep , i just figured that out, cheers, net force is zero, gravity is downward, some force is upward, they're equal and opposite! :D
     
  5. Nov 9, 2013 #4

    phinds

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    Uh ... since you, by necessity, start off with zero velocity, how are you going to even get off the ground if you have no acceleration on the vehicle?

    Your question would be better stated if you said that you were going to accelerate to some velocity and then maintain that velocity.
     
  6. Nov 9, 2013 #5

    E_Q

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    It was all one sentence o.o

    I'd like to point one thing out: constant speed doesn't always mean no acceleration. For example, circular motion - there is always a centripetal force (and hence acceleration) on the body towards the centre, but as it is always perpendicular to the body's motion the speed doesn't change. (Its velocity does, though).

    Acceleration = Change of speed or direction (change in velocity)

    To rise from the earth into space at a constant speed you'd need to slowly decrease your thrust to match the gradual decrease in gravity so you didn't accelerate. Although as phinds said you have to start off with an acceleration...
     
  7. Nov 9, 2013 #6

    K^2

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    Typically, you have three forces acting on the rocket. Thrust, weight, and drag. It's much easier to get them to balance than you might think, because as you speed up, drag increases. So long as your thrust-to-weight ratio isn't too high, at some point, you'll be going fast enough that drag is going to cancel any excess thrust and the rocket will continue at a uniform speed.

    Of course, as you go up, you'll also encounter decreasing density. At some point, or maybe from the start depending on your thrust-to-weight and aerodynamical properties of your rocket, the density will be dropping faster than you can catch up to equilibrium speed, and the rocket will resume acceleration.
     
  8. May 24, 2015 #7
    In the long haul, you would need a decreasing upward force of G x mass earth x mass object/ [4000 + altitude in miles]^2. You have to look up mass of earth, and express mass of object in kilograms. Then the upward motion would be constant. If the force were constant, as the gravitational attraction between the object and earth would decrease as the object leaves the earth due to Newtonian Law.
     
  9. May 24, 2015 #8

    phinds

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    You are right of course, but you have necroposted on an 18-month old thread and the OP hasn't been here in a year so at this point it seems like a waste of time.
     
  10. May 24, 2015 #9

    sophiecentaur

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    Quite possible to do this but it would be a hugely inefficient way of launching into orbit. The shorter time a rocket is operating and transferring energy into gravitational potential energy, the less energy will be wasted. A rocket, hovering has zero efficiency, for instance.

    @phinds An aged thread but not an out of date idea and one doesn't need to read the posting dates to get a good message from a good thread.
     
  11. May 24, 2015 #10

    phinds

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    I agree, it just seems like a waste to answer the question of someone who hasn't been here in a year.
     
  12. May 25, 2015 #11

    sophiecentaur

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    I look at it as an OP just launching an idea. Many good threads are started by an OP who never contributes further to them, often scared off by an enthusiastic avalanche of ideas that are outside their knowledge.
    I never look at the dates on a thread. But that's just me being sloppy, perhaps.
     
  13. May 25, 2015 #12

    phinds

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    Fair enough, but I do see the mods often shutting down necroposts and asking that new threads be started if someone is now interested in the topic again.
     
  14. May 25, 2015 #13
    Hey, an idea is an idea. The date is immaterial. A new thread could be started, so be it. The late answers, such as mine, would still be up for consideration. My post brought in a concept which had not been presented on earlier replies. I love these threads in general as they really stimulate my old brain to think about concepts I learned in high school and college.
     
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