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Projectile problem (This is a quickie)

  1. Jun 1, 2014 #1
    Hello, I need help on my basic math

    This is the problem
    A projectile is launches with 200kg.m/s of momentum and 1000 J of kinetic energy. What is the mass of the projectile.
    (a) 20 kg

    This is my math:

    P = mv so
    m = P/v
    m = (200 kg.m/s)/v

    KE = 1000 J
    1000 J = 1/2 mv ^2
    1000 J = 1/2 (mv) v
    1000 J = 1/2 (200) v
    v = 10 J

    IF m = 200/v
    m= (200 kg.m/s)/ (10 J)
    m= (200 kg.m/s) / (10 NM)
    m = (200 kg.m/s) / (10 ma.m)
    m = (200 kg.m/s) / (10 kg.m/s.m)
    m = (20/m) WHY IS THAT I HAVE THIS ANSWER?? 20/M? Instead of 20kg?
     
  2. jcsd
  3. Jun 1, 2014 #2

    NascentOxygen

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    Staff: Mentor

    Both sides of every equation has units, including this one:
    1000 J = 1/2 (200) v

    You wrote v = 10 J
    but v does not have units of Joules.
     
  4. Jun 1, 2014 #3

    Nathanael

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    Homework Helper

    You forgot that the 200 has units of kg*m/s so when you divide Joules by kg*m/s you get m/s
    So the answer is v=10 m/s

    Now try it with 10 m/s instead of 10 J and see if it works out.
     
  5. Jun 1, 2014 #4
    when I divide J/(kg*m/s), I get m/s??? how so??

    isn't J = NM?

    so it would be [(nm) / (kg*m/s )] ???
     
  6. Jun 1, 2014 #5

    Nathanael

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    Homework Helper

    Yes, and a Newton is [itex]kg*m/s^{2}[/itex] so a Joule is [itex]kg*m^{2}/s^{2}[/itex]
    So divide [itex]kg*m^{2}/s^{2}[/itex] (Joules) by [itex]kg*m/s[/itex] and what do you get?
     
    Last edited: Jun 1, 2014
  7. Jun 1, 2014 #6

    NascentOxygen

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    Staff: Mentor

    This is where mastery and easy recall of equations is important.

    W=Fs
    But F=ma

    So W=mas
    This tels you that the Joule is a kg m m / sec2
     
  8. Jun 5, 2014 #7
    @nascent Oxygen

    I am sorry , I do not understand ......what is fs? what is mas?
     
  9. Jun 5, 2014 #8
    @nathaneal,
    thanks!
     
  10. Jun 5, 2014 #9

    NascentOxygen

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    Staff: Mentor

    These are equations you must commit to memory. They are your tools of trade.

    W=F.s where F is force & s is displacement (both F and s are vectors).

    F=m.a where F is force & a is acceleration (both a anf F are vectors).
     
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