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Why does a projectile sitting on the ground travel further than a

  1. Jul 12, 2011 #1
    Why does a projectile sitting on the ground travel further than a projectile launched on a cliff? As per HyperPhysics it said that sitting on ground a projectile at 15m/s and 40 degrees above the horizontal would travel +22 m but if you put on top of a 3 m high cliff, the range would be shorter. Can someone explain this?

    This is the site:

    http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html#tracon
     
    Last edited: Jul 12, 2011
  2. jcsd
  3. Jul 12, 2011 #2

    Doc Al

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    Re: Projectiles

    That doesn't sound right. Can you give the exact link where it states that?
     
  4. Jul 12, 2011 #3
  5. Jul 12, 2011 #4

    Doc Al

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  6. Jul 12, 2011 #5
    Re: Projectiles

    I just compare the two scenarios using the simulations.

    The first one was the box right above "Will it clear the fence?"

    The second one was "Where will it land?" a couple of boxes down.

    I used Vi of 15 m/s and angle of 40 degrees and a height of 3 m for the second one.

    Now I just noticed that if you add the X1 and X2 together then it matches the answer from the first one but shouldn't it still be further?
     
  7. Jul 12, 2011 #6
    Re: Projectiles

    I used the one above "Will it clear the fence?" and then "Where will it land?" for the cliff scenario.

    initial velocity of 15 m/s at 40 degrees and then a cliff height of 3 m.

    Now I did just notice that if you add the two x components, it equal the answer from the first one but shouldn't it still be further coming off a cliff?
     
  8. Jul 12, 2011 #7

    Doc Al

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    Re: Projectiles

    The "Will it clear the fence?" problem is different than the "Where will it land?" problem. If you want to compare how far something goes when fired off of a cliff to how far it goes on flat ground, use "Where will it land?" for both. The difference is that for a cliff you'll set y = 3 m; for flat ground set y = 0.
     
  9. Jul 12, 2011 #8
    Re: Projectiles

    They simulations give the same answer. My concern is that whether it is off the ground or off a cliff, you still get the same range. Is that right? Shouldn't it be further for the scenario off the cliff?
     
  10. Jul 12, 2011 #9

    Doc Al

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    Re: Projectiles

    The simulations give different answers. (Note: As stated in the simulation, you must use y = -3 for the cliff example.)

    For flat ground (y = 0): x2 = 22.6 m

    For cliff (y = -3 m): x2 = 25.7 m

    As expected, the range from the cliff is greater.
     
  11. Jul 12, 2011 #10
    Re: Projectiles

    thanks...otherwise I was totally confused.
     
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