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: Collision and using x and y coordinates.

  1. Nov 8, 2011 #1
    1. The problem statement, all variables and given/known data

    You and your friends are doing physics experiments on a frozen pond that serves as a frictionless, horizontal surface. Sam, with mass 72.0 kg, is given a push and slides eastward. Abigail, with mass 54.0 kg, is sent sliding northward. They collide, and after the collision Sam is moving at 36.0 degrees north of east with a speed of 5.80 m/s and Abigail is moving at 15.0 degrees south of east with a speed of 8.80 m/s.

    Part A:
    What was the speed of each person before the collision?
    Sam's speed:
    Abigail speed:

    Part B:
    By how much did the total kinetic energy of the two people decrease during the collision?

    2. Relevant equations

    The momentum on initial x = momentum on final x

    The momentum on initial y = momentum on final y

    m1 = Sam, m2 = Abby
    mv = momentum

    3. The attempt at a solution

    on x:
    (72.0 kg)(unknown) + (54.0 kg)(0 m/s) = (72.0 kg)(5.80cos36)+(54.0 kg)(8.80 cos15)

    on y:

    (72.0 kg)(0)+(54.0 kg)(unknown) = (72.0 kg)(5.80sin36)+(54 kg)(8.80sin15)

    Sam = 11.067 m/s
    Abby = 6.823 m/s

    I also need help for Part B.

    Am I approaching this problem correctly? Thanks.
  2. jcsd
  3. Nov 8, 2011 #2
    Alright, I know Sam's initial velocity is 11.1 m/s for sure.

    I am doing something wrong when calculating Abby's initial velocity, but I'm not sure what.

    I did do (72.0 kg)(11.1 m/s) = (54.0 kg)(initial velocity) and received 14.8 m/s.

    I think this is right, however, I do not know how to calculate the answer the way I was suppose to(using y-coords).
    Last edited: Nov 8, 2011
  4. Nov 8, 2011 #3
    Check your signs, but your equations look correct. Since you have two equations and two unknowns it is a solvable system of equations. If you need helping solving the system, I would use substitution or you could try elimination.
  5. Nov 9, 2011 #4
    When I re-did the y-coords, I found that if I took 90 degrees minus the degrees given, then I would get 14.8 m/s.

    For example, sin15 should have been sin75, and sin 36 should have been sin56.

    Does this seem alright?
  6. Nov 9, 2011 #5
    I changed Abigail's final velocity to negative since it has direction. It worked.

    Thanks therealnihl.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook