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: Relative velocities - Velocity of impact

  1. Apr 2, 2013 #1
    1. The problem statement, all variables and given/known data
    https://dl.dropbox.com/u/51681259/physics_diag.png [Broken]

    Object C & P are moving at the same constant velocity to the right, in the configuration of the image. The friction between object C and the ground is 0. Suddenly the friction increases to μ=0.9 between the ground and object C. This makes object C decelerate to a stop over 4m. At what velocity does object P impact the right internal side of object C. There is no friction between C & P.

    d = 1m (distance between p right side and c internal right side)
    μ = 0.9
    s = 4m (stopping distance)

    2. Relevant equations
    I can fine object c's initial velocity by combining a couple of formulas:

    I find the initial velocity of C & P as being 8.4m/s.

    Then I can find the acceleration of C, relative to the ground, after the increase in fiction which turns out to be:

    And that means that relative to C, P accelerates forward at 8.82m/s/s. <IS THAT CORRECT??>

    Then I take p's relative acceleration and the initial velocity and put them through v^2=u^2+2as
    and find:

    So my answer is that P impacts C at 9.39 m/s. Is this even close to correct?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Apr 2, 2013 #2


    User Avatar
    2017 Award

    Staff: Mentor

    This looks fine.

    Both C and P are never moving faster than 8.4m/s, and they are moving in the same direction. There is no way the impact can occur with more than 8.4m/s.

    The calculation has two problems:
    - the initial relative velocity between P and C is zero, not 8.4m/s
    - the distance is just 1m and not 4m.
  4. Apr 2, 2013 #3
    Sorry! That was so stupid!!

    v^2 = 17.64
    v=4.2 m/s

    Is that correct?
  5. Apr 3, 2013 #4


    User Avatar
    2017 Award

    Staff: Mentor

    That looks good.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted