Projectile physics problem without the angle given HELP

  • #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

The angle is given, it is just given indirectly. Reread the first statement.
  • #3

well I know that if the horizontal velocity is 6.0 then 6.0=vi(cosθ), but it doesn't actually give me any other information, and I need to know the vertical velocity in order to solve the problem.
  • #4

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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