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Homework Help: What is the minimum amount that the spring must be compressed

  1. Apr 14, 2012 #1
    1. The problem statement, all variables and given/known data
    A 0.25 kg projectile is launched across a frictionless surface by a spring of spring constant k=1200 N/m. The block is then redirected up a 25 degree incline and sent through the air with the intent of clearing a 1.2 m high wall that is 4.0 m away from the end of the incline. The last 1.0 m along the incline is not frictionless, and has a coefficient of kinetic friction of 0.60. If the launch point (from end of incline) is 0.50 m above the horizontal surface, what is the minimum amount that the spring must be compressed for the projectile to clear the wall? (note: the velocity vector upon leaving the ramp will be parallel to the incline.

    I'm not sure how to go about solving for the next step of the problem. Any help would be greatly appreciated!
    2. Relevant equations

    3. The attempt at a solution

    I tried solving for the velocity needed to clear the wall from the end of the incline by using kinematic equations in the x and y direction. Just can't seem to grasp what heights to use and such. I am assuming once I can get the velocity needed from that point I can use energy conservation and account for the work done by friction. Starting from immediately after the mass has left the spring. Then from there I could possibly see what distance the spring must be compressed to give me that value. Is this correct at all?
  2. jcsd
  3. Apr 15, 2012 #2
    Since our goal is to clear the wall, we want to use the trajectory of the projectile which will have its highest point coinciding with the location of the wall. If the top of the ramp is 0.5m off the ground and the wall is 1.2m tall, how high must the projectile rise after leaving the ramp to clear the vertical length of the wall? Using this height, can you determine what the initial vertical velocity of the projectile must be at the point which it leaves the ramp?
  4. Apr 15, 2012 #3
    I ended up getting around 3 something for the vertical component of velocity at the end of the incline and an overall velocity of 8.76. After using that for energy conservation from beginning to the end of the ramp I got that the spring needed to be compressed 0.14 meters. I just don't feel real comfortable with the answer for some reason.
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