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: Speed of two bodies that sticks together after collision

  1. Nov 24, 2013 #1
    1. The problem statement, all variables and given/known data

    Two railway trucks of masses m and 3m move towards each other in opposite directions with speeds 2v and v respectively. These trucks collide and stick together after the collision. What is the speed of the trucks after the collision?

    2. Relevant equations

    Conservation of momentum: m1v1 + m2v2 = (m1+m2)Vtotal

    3. The attempt at a solution

    m(2v) + 3m(-v) = (m+3m)V

    V= -4/v
    ∴ speed = 4/v

    Answer given: 5v/4

    Apparently they added the 2mv and 3mv together for the initial momentum. Is the book wrong or am I wrong?
  2. jcsd
  3. Nov 24, 2013 #2


    User Avatar
    Homework Helper

    Correct so far.

    Wrong, do it again.

    The book is wrong.

  4. Nov 24, 2013 #3
    From " m(2v) + 3m(-v) = (m+3m)V "

    2mv -3mv = 4mV

    -mv = 4mV

    V= -v/4

    Speed does not have a direction, so answer is v/4. Why wrong?
  5. Nov 24, 2013 #4


    User Avatar
    Homework Helper

    You wrote speed=4/v in the original post:biggrin:

  6. Nov 24, 2013 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That would be the right answer if m had speed v and 3m had speed 2v. Are you sure you quoted the question correctly?
  7. Nov 24, 2013 #6
    Yes. I quoted it correctly.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted