Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A math question if I may.

  1. Jan 30, 2008 #1
    I am reading " The Honor Harrington series" by author David Weber. It is pretty cool scifi with some intricate plots and massive space battles. Thru these books the refrence used for speed has always eluded me in a "frame of refrence" sence. The ships have "accelerations" refrences in gravities. If a ship were to accelerate at 525 gravities, How fast is this? How far will it travel in an hour? a day? I have tried to sus this out but I dont have the math skills for it i guess. Any help would be appreciated
  2. jcsd
  3. Jan 30, 2008 #2
    1 (earth) gravity is roughly 32 feet per second per second. 525 gravities would be around 15000 feet per second per second. That is roughly 3 miles per second per second. The formula for velocity is v = a times t, where v is velocity, a is the acceleration and t is the time. At that rate of acceleration, starting from rest, it would be going:
    roughly 10000 miles / second at the end of 1 hour
    roughly 240000 miles / second at the end of 1 day. That's faster than the speed of light, so that couldn't actually happen.
    Any humans in the ship would be crushed. The last thing to go through their minds would be the heels of their feet.
    Last edited: Jan 30, 2008
  4. Jan 30, 2008 #3
    Humans can only withstand several gravities (or g's) before passing out, and only a few more g's and you're dead.

    500 would be something like an explosion right next to you. Maybe even worse.
  5. Jan 30, 2008 #4
    Ok thanks. That is really really fast. Of course it is scifi. They have inertial compensators to deal with the splat factor. fwiw this is the slow mode of their ships. When they are in the Hyper bands they can cover 37 lightyears in 5 days.....lol I love science fiction!
  6. Jan 30, 2008 #5


    User Avatar
    Gold Member

    Yes, collisions are measured in hundreds or thousands of g's. So a ship that accelerates at 525g's would effectively be colliding with a planet continuously for an hour or a day.
  7. Jan 30, 2008 #6


    User Avatar
    Gold Member

    The upshot of this acceleration is that the ship will very rapidly reach within a very large fraction of c (and getting closer all the time) and spend almost the entire trip there.

    At this speed, a trip of 20 light years will take, like, 20 years + 1 day (or more rigorosly, a round-tip of 20 light years will take about 40 years + 2 days) from an external frame of reference.

    Note that no amount of acceleration will shorten the duration of the trip from an external frame of reference to less than 40 years.

    However the occupants will only experience a VERY short passage of time - something on the order of days to weeks.

    But every time they took a vacation on Gliese 581c and came back, the Earth would be 40 years older!
    Last edited: Jan 30, 2008
  8. Jan 30, 2008 #7


    User Avatar
    Gold Member

    If you really want to see what relativistic travel can do, read Joe Haldeman's 'The Forever War'.

    How do you cope with returning to an Earth that's a thousand years older than when you left? And how do you keep a relationship together when your partner isn't coming home for another two lifetimes?

    Of course, it's actually about a war, so lot's of battlesuit infantry stuff.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook