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: Momentum problem?

  1. Dec 7, 2014 #1
    1. The problem statement, all variables and given/known data
    A large, uniform-density cube of styrofoam has a side length of 3.0 m and a mass of 90 kg. It is at
    rest on a horizontal, frictionless (ice) surface. An ice-skater with a mass of 60 kg is standing at rest,
    touching one face of the cube. You should consider all of the ice-skater's mass to be located exactly at
    the position of the cube's side. The ice-skater now pushes off against the cube. When the cube has
    moved 4.0 m from its starting position, how far has the ice-skater moved?

    2. Relevant equations
    hard to tell what other equations come into play

    3. The attempt at a solution
    tried to use conservation of momentum equation (mvi=mvf) to solve, but ended up falling short since I can't determine the velocities of the cube and the ice-skater. I know information about the side length of the cube and the ice skater's position on the cube will be important in figuring out the problem, but i just can't see how they are connected.
  2. jcsd
  3. Dec 7, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    The conservation of momentum equation will give you one equation with two unknowns - you need to find another equation (or two) in those unknowns.

    You have yet to use the distances - what is the relationship between the distance travelled and the velocity?

    Note: you can also do this in terms of ratios - what is the skaters speed as a fraction of the cube's?
    What does that suggest about the distances they travel in the same time?
  4. Dec 8, 2014 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What Simon said.

    Write your equations.
    Substitute Velocity = distance/time

    You might find that the time cancels :-)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted