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Homework Help: Conservation of EnErGy Problem help please

  1. Nov 3, 2007 #1
    [SOLVED] Conservation of EnErGy Problem help please :(

    1. The problem statement, all variables and given/known data

    If a Saturn V rocket with an Apollo spacecraft attached has a combined mass of 2.3 105 kg and reached a speed of 11.2 km/s, how much kinetic energy would it then have?

    2. Relevant equations

    k= m(vsquared)/ 2

    3. The attempt at a solution

    I multiplied 11.2km by 1000m to get 112000m and i squared that. then i multiplied it by 105kg and then divided by two but i got the wrong answer. please help! what am i doing wrong! thank you soo much.
  2. jcsd
  3. Nov 3, 2007 #2
    please help me with this problem .
  4. Nov 3, 2007 #3
    Please Please Please Help!
  5. Nov 3, 2007 #4
    well.. 11.2 * 1000 = 11200 and not 112000!!
    what is the combined mass?? 105kg? or, 2,3105kg, or what??
    what is the answer?
    Last edited: Nov 4, 2007
  6. Nov 4, 2007 #5
    the equation is half of mass times velocity squared. square the velocity, then multipy that by half of the mass. Thant is your kinetic energy. (1/2)m(v^2) Where in the equation say mulitiply by 1000m?
  7. Nov 4, 2007 #6
    saket: i meant 11200, i put 11200 in my calculator not 112000 sorry it was a typo.
    and where did you get the number 2,3105kg from?????

    and antineutron i multiplied it by 1000m so i could have common units you know-- like you should use km rather than m. so that is why i multiplied it by 1000.
    and i used that equation you told me but i am still getting it wrong.
  8. Nov 4, 2007 #7
    tell me all the units that are in energy.

    what units are included in Newtons? Nm=J which is energy right?

    N=kg(m/s^2) , so kinetic energy should be in units of Nm=kg(m^2/s^2) first, convert to the correct units, then use the formula to get the result.
    Last edited: Nov 4, 2007
  9. Nov 4, 2007 #8
    I think your statements have flaws.
    Have you directly quoted the question or have you tried to put it up in your own words?
    And, when you say, "i got the wrong answer" .. you should be knowing the correct answer?
    Anyways, as far as classical Kinematics go:
    Kinetic Energy = m*(v^2)/2.

    {From where did I get 2,1305 kg? Look at what have I quoted in last post in this thread.}

    Anyways, if mass is 105kg, and speed is 11.2km/s.. KE = 6.58 x 10^9 J.

    Another probabiltiy is the question being put up in an inappropriate fashion.
    Looking at the speed with which they are projected, (if from Earth), they will escape from Earth... and if your question is referring to this, final speed should be zero => KE = 0, but.. finally!
  10. Nov 5, 2007 #9
    thankyou for your help, but my friend explained it to me.
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