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Homework Help: Projectile physics problem without the angle given! HELP

  1. Nov 6, 2011 #1
    projectile physics problem without the angle given! HELP!!

    You accidentally throw your car keys horizontally at 6.0 m/s from a cliff 80 m high. How far from the base of the cliff should you look for the keys?

    Okay so I honestly don't know how to figure this problem out, but I think that I need to use the equations vf=vi+at and Δx=vit

    For the second equation, I know that Δx=(6.0)t. I also know that for the first equation I gave, vf=vi+(-9.8)t. I don't know how to figure it out because I feel like I need the angle in order to find out the rest of the information.

    I know it sounds like I am a complete slacker, but I really need help with this as soon as possible! Please help me! Thanks!!
     
  2. jcsd
  3. Nov 6, 2011 #2
    Re: projectile physics problem without the angle given! HELP!!

    The angle is given, it is just given indirectly. Reread the first statement.
     
  4. Nov 6, 2011 #3
    Re: projectile physics problem without the angle given! HELP!!

    well I know that if the horizontal velocity is 6.0 then 6.0=vi(cosθ), but it doesnt actually give me any other information, and I need to know the vertical velocity in order to solve the problem.
     
  5. Nov 6, 2011 #4
    Re: projectile physics problem without the angle given! HELP!!

    Hi,
    Just saw your post and though I guess the answer may come a bit late here it is anyway:

    You need to work separately the two relevant coordinates (vertical and horizontal).

    Calculate the time the keys will take to fall the 80m of the cliff (vertical acceleration of g (due to gravity) with zero initial velocity.

    Once you have this time you work the horizontal direction: a constant speed of 6m/s times the time calculated on the previous calculation.

    Hope it helps.
     
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