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: Momentum Problem;

  1. Nov 15, 2011 #1
    1. The problem statement, all variables and given/known data[/b]

    Two heavy frictionless carts are at rest. They are held together by a loop of string. A light spring is compressed between them. When the string is burned, the spring expands from 2 cm to 3 cm and the carts move apart. Both hit the bumpers fixed to the table at the same instant but cart A moved 0.45 m while cart B moved 0.87m. What is the ratio of:
    a) the speed of A to that of B after the interaction?
    b) their masses?
    c) the impulses applied to the carts?
    d) the acceleration of the carts while the spring pushes them apart?

    2. Relevant equations

    mava+ mbvb= mava' + mbvb'

    1/2 mv2= FfΔd

    3. The attempt at a solution
    First you will have to figure out everything you know about the carts before the string is burnt. You would then use the other part of the equation to solve after the string is burnt.
    You would use the second equation that is shown by filling in the knowns.
  2. jcsd
  3. Nov 16, 2011 #2


    User Avatar
    Homework Helper

    Part c should be easy.

    Impulse = force x time

    We don't know how strong the force was, and we don't know how long it lasted for, but with our understanding of Newton's Third Law, we do know that the spring was pushing on each cart with the same sized force at any instant.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook