1. Not finding help here? Sign up for a free 30min 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!

The law of conservation of momentum

  1. Oct 8, 2015 #1
    1. The problem statement, all variables and given/known data

    a trailer with a mass of 150 kg happen to roll away with the speed 20 m/s. 450 kg cement fall staight down into the trailer! which speeds does the trailer get when the cement fall into it?

    2. Relevant equations

    I know how to solve it but I don't know why it works. Is it because this counts as a collision?

    3. The attempt at a solution

    150 * 20 = V(150+450)
     
  2. jcsd
  3. Oct 8, 2015 #2
    Where the situation is collision or merge, it satisfies
    [tex] p_i = p_f. [/tex]
    That's it.
     
  4. Oct 8, 2015 #3

    Doc Al

    User Avatar

    Staff: Mentor

    You can think of it as a collision. Or just think of "trailer + cement" as a system whose momentum (at least horizontally) is conserved.
     
  5. Oct 8, 2015 #4
    okay thanx :)
     
  6. Oct 8, 2015 #5
    Momentum is described by a vector, because velocity is a vector. You can bilance the momenta before and after the cement fell on the trailer in two perpendicular directions (x / horizontal & y / vertical) - in both directions momentum has to be conserved.

    x: mT ⋅vT1x + mC ⋅vC1x = mT ⋅vT2x + mC ⋅vC2x = 150 kg ⋅ 20 m/s + 450 kg ⋅ 0 m/s = 150 kg ⋅ vT2x + 450 kg ⋅ vC2x with T2x = vC2x

    The sum of the momenta of both objects in x-direction remains the same.

    y: mT ⋅vT1y + mC ⋅vC1y = mT ⋅vT2y + mC ⋅vC2y = 150 kg ⋅ 0 m/s + 450 kg ⋅ vC1y > 150 kg ⋅ 0 + 450 kg ⋅ 0 m/s

    As the momenta of the two objects can't be the same, a force must have affected them - the earth. Taking into account the momentum of earth (assuming it was standing still, when the cement hit the trailer):

    y: mT ⋅vT1y + mC ⋅vC1y + mE ⋅vE1y = mT ⋅vT2y + mC ⋅vC2y + mE ⋅vE2y = 150 kg ⋅ 0 m/s + 450 kg ⋅ vC1y + 5.6 ⋅1024 kg ⋅0 m/s = 150 kg ⋅ vT2y + 450 kg ⋅ vCy2 + 5.6 ⋅1024 kg ⋅ vE2y with vT2y = vC2y = vE2y

    and you can calculate what's the velocity of the earth (+ trailer + cement), due to this collision.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: The law of conservation of momentum
Loading...