• Question: do things at a higher altitude travel faster than things at a lower one travelling the same distance and the same speed?

    Asked by 337spcq22 to Vinita, Rachel, Pam, Christopher, Brian, Andrew on 5 Mar 2019.
    • Photo: Pam Anderson

      Pam Anderson answered on 5 Mar 2019:

      If you have a satellite orbiting a planet (like Earth) the satellites at lower altitudes will travel faster than those at higher altitudes. For example, the International Space Station is at about 410 km altitude and travels at 27,600 km/h but Geostationary satellites (those that provide your satellite TV signals) are at an altitude of 36,000 km and travel at around 11,000 km/h.

    • Photo: Christopher Youens

      Christopher Youens answered on 6 Mar 2019:

      It can be a bit counter-intuitive to think about velocity in orbit. In order to get to a higher orbit you need to speed up (Hohmann Transfer). But because you are covering a larger distance (bigger circle) your orbital velocity would be faster but your speed over the ground would be slower! A good thing to remember when thinking about orbits is that you need to ‘speed up to slow down’!

    • Photo: Rachel Hudson

      Rachel Hudson answered on 7 Mar 2019: last edited 7 Mar 2019 12:21 pm

      Well, by definition, in your question you stated that they are travelling at the same speed so neither can be faster that the other…
      If you meant to say that they were starting at the same speed then that is different. (I’m also assuming you mean an object that is falling from a higher altitude rather than orbiting – see answers below if you meant about orbits instead)

      Two things are relevant here:
      1. The air is thinner at high altitude so there is less air resistance. If there are less resistive forces to slow the object down then it will have a larger acceleration, so long as the ‘falling force’ (aka weight aka force of gravity) is the same.

      2. The force due to gravity will actually be weaker at a higher altitude because of Newton’s formula for gravity (in it’s long form):
      F = GMm/r^2
      Here we can see that if Mm is the same (mass of earth x mass of object) and G is always constant, then as r (the distance from the center of the earth) gets bigger then the force gets smaller.
      We also know F=ma so if the force is smaller then so is the acceleration, so the object will not fall so fast.

      So overall it depends actually exactly how high of an altitude you mean as to which factor is most significant!

      (If you are wondering how this links with the force of gravity from your GCSEs, then you should know that little g = GM/r^2 = 9.81 N/kg on the surface of earth !)

    • Photo: Brian Weaver

      Brian Weaver answered on 15 Mar 2019:

      If you want to orbit (fall continuously around the earth) and use a little fuel as possible then the farther from earth the slower you must travel.
      *
      Imagine you are in a low earth orbit, “close” to earth. You feel a strong pull from earth’s gravity. Your speed must be “fast” or else you’ll fall back to earth.
      *
      Imagine you are in a high earth orbit, “far” from earth. You feel a weak pull from earth’s gravity. Your speed must be “slow” or else you’ll shoot off into space.
      *
      Of course Chris is right, the father away from earth the more distance you travel through space so orbital velocity must be faster.
      *
      It might help you to play with this online tool (link below). Notice how planets near the sun travel faster while planets far away travel slower!
      https://theskylive.com/3dsolarsystem

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