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Physics by the Proportion

  1. Apr 20, 2015 #1
    1. The problem statement, all variables and given/known data
    If a particular spring is compressed 10 cm, it can launch a marble horizontally at 75 cm/s. How much must it be compressed to launch the marble horizontally at 150 cm/s?

    2. Relevant equations

    3. The attempt at a solution
    My guess is that the spring force needs to be doubled in order for the marble to reach double the height. Is this right? I feel like this is way too simple for my physics professor to put on an exam. I feel like there has to be something else like equations to try and solve this.
  2. jcsd
  3. Apr 20, 2015 #2


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    It's always simple to guess an answer. What about trying to work it out?
  4. Apr 20, 2015 #3


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    There are several different ways to work this out. For example, if you know the force applied by the spring at each amount of compression, x, you can calculate the acceleration and so the speed as it leaves the spring. Or, if you know the potential energy of a spring at a given compression then you can calculate the kinetic energy when all that energy is converted from potential to kinetic energy and so get the speed.
    Last edited by a moderator: Apr 21, 2015
  5. Apr 20, 2015 #4


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    The question asks about doubling the speed, but in your 'attempt' you mention doubling the height. If you think one implies the other then you are wrong. Also, it does say horizontally, so I'm not sure how height would come into it.
    As Halls suggests, think about energy. How do doubled compression, doubled speed and doubled height relate to energy?
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