Kinematics question on a rock

  • Thread starter Intr3pid
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Homework Statement


A rock is dropped from a sea cliff, and the sound of it striking the ocean is heard 3.2s later. If the speed of sound is 340 m/s, how high is the cliff.



Homework Equations


d= v1t + 1/2at^2
v2^2 = v1^2 + 2ad



The Attempt at a Solution


I tried using d = vt since we're given the speed of sound and the time taken for the sound to reach our ears. Since sound is unaffected by gravity, i thought i could straightaway use that formula, but I guess I was wrong. This is how far I got up to. Can someone give me a hand?

Thanks
 

Answers and Replies

  • #2
cristo
Staff Emeritus
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You can use d=vt for the sound. However, note that we are told that 3.2s after the rock is dropped, we hear it hit the ocean, not that the sound takes 3.2s to reach our ears.

Use d=vt to obtain the time taken for the sound to reach out ears iafter the rock hits the ocean(in terms of d). We then know that the rock takes (3.2-t)s to fall. Can you write another equation for the rock as it falls?
 

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