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Kinetic Energy question

  1. Jan 27, 2012 #1
    1. The problem statement, all variables and given/known data

    assume you are taking a trip on a train. you carry a filled suitcase that weighs 170N, and during the course of your trip you are told that the train is moving at 140 km/hr.
    (a) what do you observe the kinetic energy of the suitcase to be, before you are told the velocity?
    (b) what do you interpret the kinetic energy of the suitcase to be after knowing the velocity?

    2. Relevant equations

    part B would be utilizing the equation ke= 1/2mv^2

    3. The attempt at a solution

    kE= 1/2(170/9.18)(140)^2
    KE= 169834

    Im not sure about how to go about part a of the question, how would you determine the kinetic energy if no velocity is given? any information would be highly appreciated thank you.
  2. jcsd
  3. Jan 27, 2012 #2
    I get the feeling part a is some kind of trick question. The phrase that popped out to me was "what do you observe the kinetic energy to be"? It makes me think that, since you and the suitcase are in the same reference frame, and if you assume that you're at a steady velocity (i.e. the train is not accelerating) that the suitcase has no kinetic energy; it is as if the train is not moving at all.

    In part b, they ask a similar question since you now know the train is definitely moving. Now, with what you know, what is the kinetic energy of the suitcase in relation to the earth's reference frame? It definitely won't be zero in that case.

    Hope that helps.
  4. Jan 27, 2012 #3


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    Homework Helper

    Velocity, and therefore kinetic energy is always measured relative to a defined inertial frame of reference. People in different frames of reference see things differently. This was known well before Einstein's theory - the classical version is called Galilean relativity (after Galileo).

    The train is an inertial frame (since it is not accelerating relative to the ground). You are an observer within the train. The suitcase is at rest with respect to you. What is its kinetic energy?

    Part b) is just a trick question. The suitcase is still at rest relative to you and the train. If the question were posed from the perspective of someone outside the train stationary on the ground, it would be different, but they didn't do that, did they?
  5. Jan 27, 2012 #4
    Im understanding those factors but the example is looking for the kinetic energy and I dont know how it would be possible to find the kinetic energy with only giving you the mass of the suitcase. so would the kinetic energy therefore be 0 before you are told the velocity of the train?
  6. Jan 27, 2012 #5


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    0 after too. Relative to the train frame of reference. You must always state your frame of reference.

    Relative to a person standing still on the ground outside, you can calculate it as you did. But you made a mistake with your units. You should convert 140 km/h to m/s.
  7. Jan 27, 2012 #6
    so would the ke for B = ke=1/2(17.33)(38.89)^2?

    getting the mass by converting 170 n to kg

    and getting the velocity by converting 140 km/hr to m/s
    therefore, giving us the solution = 13105kj? thank you
  8. Jan 27, 2012 #7


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    The answer (depending on how many significant figures they require) is 13100J = 13.1kJ. Again, be careful with your units. Remember to state clearly that this is the KE measured by a an observer stationary relative to the ground. You would still observe a KE of 0.
  9. Jan 28, 2012 #8
    My attempt :
    For (B) , K.E. possessed by the suitcase = (1/2)(170/9.8)(140/3.6)^2 = 13120 J, g = 9.8 m/s^2.
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