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

Train kept a-rollin'

  1. Nov 10, 2003 #1

    jimmy p

    User Avatar
    Gold Member

    My friend was in a ponderous mood and asked me the following question:

    If a train is travelling at 40mph and you are inside the train and running 10mph in the same direction as the train is travelling, what speed are you moving at?

    The answer i gave him was that

    1/ The overall system is travelling at 40mph
    2/ You are running at 10mph inside the train
    3/ To an outside observer you appear to be running at 50mph

    Was i right in my assumptions?

    Thanx
     
  2. jcsd
  3. Nov 10, 2003 #2

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You are correct. The critical point is that you have to define your "frame of reference" before you can give a velocity.

    All of the following are correct:

    The person is running at 10 mph to an observer sitting on a seat on the train.

    The person is running at 50 mph to an observer sitting still beside the railroad track.

    The person is running at 0 mph to an observer who is running along with him.

    - Warren
     
  4. Nov 10, 2003 #3

    jimmy p

    User Avatar
    Gold Member

    Ok thats cool then, glad i was right for once

    If the train were to travel at 40mph and you were inside the train running 10mph in the opposite direction, what would observers view?

    Am i right in assuming that again,

    1/ The system is moving at +40mph
    2/ You are running at -10mph to an inside observer
    3/ To an outside observer you are facing the wrong way travelling at +30mph

    Thanx
    [?]
     
  5. Nov 10, 2003 #4

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That is also correct. In general, you'll make your life easier if you never refer to velocities without mentioning the reference to which those velocities are measured.

    Instead of saying

    The system is moving at +40mph

    Say

    The system is moving at +40mph to an observer at rest on the ground.

    - Warren
     
  6. Nov 10, 2003 #5

    jimmy p

    User Avatar
    Gold Member

    Yeah thanks i was just thinking simple terms, my friend isnt the sciency type so its best not to confuse him!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?