1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: FBD Diagram

  1. Oct 27, 2011 #1
    1. The problem statement, all variables and given/known data

    A passenger is standing without slipping in a forward accelerating train. The coefficient of static friction between the feet and floor is 0.47

    a) Draw an FBD for the passenger in Earth's frame of reference
    b)Draw an FBD for the passenger in the train's frame of reference.

    3. The attempt at a solution


    Relative to the earth, we know the passenger in traveling with the same acceleration as the train. Therefore, the train has a force applied and a force of friction in the opposite direction. However, does this also apply to the person who is standing still in the train?

    b) For this, I am assuming that there is no force applied or force of static friction because the person is standing still. There is a force normal and force of gravity. Is this right? If it is, do I only draw the FBD with only Fg and Fn?
  2. jcsd
  3. Oct 30, 2011 #2
    Won't they be the same? In either frame the man accelerates so there must be a force in the direction of acceleration on the man to cause it to accelerate?
  4. Oct 30, 2011 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The force that is accelerating the train and passengers with respect to the earth has little to do with the force that is accelerating the passenger with respect to the earth
    If the person is standing still with respect to the train, then that person must be accelerating with respect to the earth, at the same acceleration of the train with respect to the earth.
    Now the use of non-inertial (accelerating) frames should be avoided like the plague except in certain problems such as this one, since it was asked, so you cannot avoid the question unless you choose not to answer it. FBD's in non-inertial frames require the use of pseudo forces (fictitious forces) and real forces. There is no acceleration of the passenger with respect to the train...since they are both accelerating together...so the real force F is balanced by the make-believe force F' = -ma, where 'a' is the acceleration with respect to the earth. Although they cancel, they both should be shown and properly marked, in the x direction. Fg and Fn still show in either case, since the pseudo force in that direction is 0, so those forces in the vertical direction result in the same FBD of forces in the y direction. Pseudo forces are not fun!:yuck:
  5. Oct 30, 2011 #4
    Thank you! :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook