Momentum of Freight Train

  1. I saw a freight train cruising by today and thought "The momentum of that thing must be mind-blowing". I only have high school physics and thus couldnt calculate the momentum including many external forces. Did some quick research and was wondering what the momentum of an 80 car freight train travelling 40km/h would be? The average car being approximately 110.2 tons or 100,000kg

    I came up with:
    p=mv
    p= (100,000*80)*(11.11...)
    p= 88,888,889 kgm/s
    Thats a lot of momentum but with friction, drag etc what would it actually be?
     
  2. jcsd
  3. Nugatory

    Staff: Mentor

    The momentum is exactly what you calculated it to be. Friction and drag would slow the train down if the engine weren't pulling... But as long as the train is moving at a constant speed the engine's pull is exactly balancing the friction and drag, thereby keeping the momentum constant.
     
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