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Himanshu posted an Question
April 19, 2020 • 07:15 am 30 points
  • IIT JAM
  • Physics (PH)

Mechanics conceptual

If train is moving with speed xkm/h and we start removing it's coaches the velocity of Train remains same "but why "the mass is countinosly decreasing and the engine is applying the same force

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    Abhishek singh Best Answer

    here is the correct demonstration. Thanks for pointing out. Do ask doubts if any.

    cropped119960200896155583.jpg
  • Himanshu Pandey

    Sir in your demostration how you cancelled out M with m2 for obtaining V2=V

    cropped5708246332272372807.jpg
    eduncle-logo-app

    Thanks for pointing out. I have attached the corrected one.

  • comment-profile-img>
    Ruby negi best-answer

    this is the case of conservation of linear momentum, so velocity of train will remain same even after the removal of it's coaches

  • comment-profile-img>
    Abhishek singh

    I would like to make two arguments: 1) if you are assuming no friction, then the velocity of train will remain same for infinite time. 2) if you are assuming friction, then also the velocity of train will remain the same just after the removal of coaches. After some time when the friction decrease the speed of removed coaches, then the velocity of train will increase for sure. The physics behind it is conservation of linear momentum. A cute demonstration is attached. satisfy yourself with this. take time to digest. but don't hesitate to ask further doubts, if any. Best regards.

  • comment-profile-img>
    Shanu arora best-answer

    Answer : Even after the coaches are being removed, the engine produces same force, actually this is a hypothetical condition which says that F to remain constant for the ease of question . thus resultant of this is that the speed of the engine is increased( as engine still applies same force ,but mass is less so acceleration is more , also due to reduction in mass ,the normal force is also reduced ,which reduces the friction force, hence speed is gone up). A question can also framed like where speed is maintained constant and thus find the change in Force.

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