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Homework Help: Elastic and Inelastic Collision problem/lab

  1. Nov 8, 2011 #1
    1. The problem statement, all variables and given/known data
    Okay, we have a lab about elastic and inelastic collision, and we have to do it in an air table/spark table that generates dots (can be like ticker tapes). We have to determine Momentum (P and P') and Kinetic Energy (Ek and Ek')

    Please view my attachment.
    (' = after collision)

    our data:
    d1 = 0.21 m
    d1' = 0.34 m
    d2 = 0.21 m
    d2' = 0.32 m


    I get that everything is straightforward, but since its a lab and I have no partner, I wanted to make sure my equations and calculations are right (there's no handout to refer, or text book examples)

    I already solved the initial velocity, for the velocity after, do I use equation 1 or 3?

    For solving P, we use 2 right? Or no? and we find P' using 3?

    We won't be using the degrees, right?

    And we dont need 4 right?

    Lastly, for Ek, we use 5 to get the Ek? but what do we use/do to get Ek'? Do we use 5 or 6

    2. Relevant equations
    1 Velocity: V=d/t
    2 Momentum: P=mv
    3 Momentum(?)/Collision(?): m1V1 + m2V2 = m1V1' + m2V2'
    4 Work: W=F(cosθ)d
    5 Kinetic Energy: Ek=1/2mv2
    6 Ek=Ek'

    3. The attempt at a solution

    I already solved the initial velocity:
    V1 = 0.21/0.3
    = 0.7 m/s
    V2 = 0.21/0.3
    = 0.7 m/s

    for the velocity after, I tried doing both 1 and 3. But I got two different answers (When solving for V1' or V2', I used the V1' or V2' in the equation). So do I stick to 1?

    V = d/t

    V1'= 0.34/0.5
    = 0.7 m/s
    V2' = 0.32/1
    = 0.32 m/s

    The rest of the question, I am stuck. I want to continue solving but then, I realize I might screw up on parts and get zero on my lab. See, I'm getting confused on which equations to use.

    I am just looking for guidelines, not answers.

    I hope you guys can help me. Thanks a lot:)

    Attached Files:

    Last edited: Nov 8, 2011
  2. jcsd
  3. Nov 8, 2011 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    First you have to stop this focus on which equations to use and focus on the experiment itself.

    What was the aim of the experiment?

    For instance - if you want to see if momentum is conserved in the collision, you have to measure the momentum before the collision and measure it after the collision to see if they are the same. That's what "conserved" means.

    You did not measure momentum directly, but weighed the masses and measured their positions at different times. Now you have to turn that information into momentums to compare.

    You know that momentum, by definition, is mass times speed.
    -- you have the mass, but need the speed :( but:

    Speed, by definition, is distance over time.
    -- hey, you got those too! :)

    Now describe all that in math will give you: p=md/t ... expressing momentum in terms of what you know.

    Got it now?

    Note: you get better results if you use the maximum distances and times available.

    The most common problem people have in these forums stem from agonizing too much over getting the "right" equations instead of figuring out what actually happened.
    Last edited: Nov 8, 2011
  4. Nov 8, 2011 #3

    That's actually true. Thanks. :) The aim of the experiment is to calculate P, P', Ek, and Ek'.

    So given the masses of the two pucks of air/spark table, mpuck1 = 356.40g and mpuck2 = 180.57g

    For the momentum. I got the speed = d/t.
    So P1 = mv

    P2 = 0.7(180.57)

    am I going to the right track now?

    So now, I am solving for V' either 1 or 2.
    I used the elastic collision equation. 3. Is this right?

    This is what I got:
    356.40(0.7)2 + 180.57(0.7)2 = 356.40V1'2 + 180.57V2'2

    I ended up with:
    174.636 + 88.4793 = 356.40V1'2 + 180.57V2'2
    263.1153 = 356.40V1'2 + 180.57V2'2
    Then square root both sides? Is that right?

    and I got:
    16.22 = 18.88V1' + 13.44V2'

    Now I am stuck. I know I could do a simple system of equations (do use it?) but I don't know what equations to use now.
  5. Nov 8, 2011 #4

    Simon Bridge

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    Science Advisor
    Homework Helper

    But why? What is the point?

    Presumably you will need to compare the motion against the conservation laws, so you can decide if the collision is elastic or not.
    What do you expect to be the relationship between P and P', Ek and Ek'?

    So far so good.
    Why are you calculating the final speeds when you went to so much trouble to measure them?! What you measured is what is really happening in the real world. The formulas are just someone's theory.

    Did you read the bit where I told you stop thinking about finding the right equations?
    You are doing it again. Stop it!

    Focus on what is actually happening on the table in front of you.
    That's where the physics is.

    The whole point of doing experiments is to get the universe to tell you stuff about physics.
    By focusing on finding the "right" equations, you are actually telling the universe what the physics should be - this will never work.

    You have measured enough to find the momentum and kinetic energy, directly, both before and after the collision. Why not do that?

    If the total momentum before the collision is the same as the total momentum after the collision, what does that mean?
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