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!

Collision problem /what internal forces?

  1. Jan 21, 2010 #1
    1. The problem statement, all variables and given/known data
    A moves with momentum p into B at rest. The objects collide and move off with momenta p_1, p_2 at an angle (irrelevant but anyway).
    What are the internal forces in the system?

    2. Relevant equations
    F=p dot

    3. The attempt at a solution
    What's an internal force? Isn't it usually something like gravity (system of planets) or electromagnetic forces (system of charged particles)?

    The internal forces seem to be given by a pair of dirac delta functions centre time/point of impact. Though as it's a Newtonian pair, they cancel out.
    Then again, that's probably too advanced given the context of the question...

    What is the marker looking for?
    It could be nothing for two reasons for infinity for another reason.

  2. jcsd
  3. Jan 22, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Jerbearrrrrr! Welcome to PF! :smile:

    An internal force is any force that isn't an external force.

    An external force is anything from outside, such as gravity, wind, friction with the ground.

    Internal forces include the forces between two bodies in the system.

    And also forces within a body, such as springiness and chemical bonds.

    (of course, if the system includes the Earth, then the gravity will be an internal force :rolleyes:)
  4. Jan 23, 2010 #3
    So suppose the universe is just two particles, A and B.
    Then any force is internal, since our system is the entire universe?
    So there are no internal forces (unless A and B have a mass, in which case there is the Newtonian pair of attraction)?

    If two particles collide perfectly elastically, do they exert a force on each other? If so, how do we quantify this force?
    There's no time scale so the formulae break down. D:

    (This wasn't my homework problem, it's a friend's, who does physics. I do maths but I'm just curious.)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook