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Homework Help: Help on Spring and conservation of energy problem!

  1. Dec 1, 2007 #1
    1. The problem statement, all variables and given/known data
    I have been trying this question for close to two hours now and have gotten it incorrect three time. I would appreciate any help!

    The spring has a constant of 30 N/m and the frictional surface is .3 m long with a coefficient of friction μ = 1.02. The 4 kg block depresses the spring by 17 cm, then is released. The first drop is 1.4 m, then it is on the .3 m lonf frictional surface, and the second drop is 1 m. How far from the bottom of the cliff does it land? Answer in units of m.

    I posted the same question on cramster so I could draw you a picture. http://answerboard.cramster.com/physics-topic-5-151971-0.aspx [Broken]

    2. Relevant equations

    3. The attempt at a solution

    I tried to find the energy stored in when it is compressed at distance 1 by doing
    w = Fd
    w = (30)(.17)
    After that, I tried to find the time it took to fall at both place, but I fell short of information.
    And then I tried to use the formula Wf = cf*M*g*d2, but I wasn't sure what went where. Now I am at a dead end and am extremely confused. Will someone please help?!?!!?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Dec 1, 2007 #2
    The diagram helps a lot in clearing that up... but it's still slightly tricky working out quite what is going on here! It may be worthwhile writing out questions word for word in future.
    Firstly, you have the right idea in working out the energy stored in the spring, but you're using the wrong equation. W=Fs is the work done on an object by a force over a distance s. The energy stored in a spring is a different equation. (Think about what the force exerted by a compressed spring is, and you'll be nearly there!)
    Secondly, what is this equation?
    I think you're trying to work out the frictional force, or the work done by it, or something similar, but you haven't defined your variables, even on your diagram. I'm just hoping c means something different here to it's usual meaning :rolleyes:

    The really important thing you haven't mentioned is the independence of the horizontal and vertical components of the velocity. Work out what happens to each of these components at the various stages of your block's descent.
  4. Dec 1, 2007 #3
    I just pulled out the Wf equation from my book because it was in the same section as springs. I don't know what some of the variables even stand for. For the components of the velocity, do you mean just use kinematics?
  5. Dec 1, 2007 #4
    Rule of thumb: if you don't know what an equation is, find out before assuming it's the one you want! If you don't know what the variables mean then how can you use the equation???
    I mean: gravity doesn't affect how fast an object goes sideways; it just gives you the time it takes to fall a certain distance. Time is a useful variable to know if you want to work out a distance and can work out a horizontal velocity ;)
  6. Dec 1, 2007 #5
    I know you are probably going to be very frustrated with me, but I am completely lost. We have never even worked problems of this caliber before, so I am very confused.
  7. Dec 1, 2007 #6
    It's ok.
    Break the problem down into separate stages. Your spring gives you an initial horizontal velocity. Your mass then accelerates downwards whilst still travelling with your initial velocity. Work out where it lands on the frictional surface, and work out how much it slows it down. Then repeat...
  8. Dec 1, 2007 #7
    First thing

    F = kx Where k is the spring constant and x is the compression or extension.F is force and not work.

    Now this froce is apposed by the friction whose value is (mu)n

    Now you know the forces acting so find acc and then carry on with the kinematics equation and find out what would be the velocity of the bloack when it is just on the edge of the side.And then follow the projectile motion.

    Try to do this much and then we may proceed.
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