Low Level Physics Problem...1D Motion [Archive]

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Maskkkk

Sep14-04, 07:26 PM

47.

A rock is dropped from a sea cliff and the sound of it striking the ocean is heard 3.4s later. If the speed of sound is 340 m/s, how high is the cliff?

Could somebody map this one out for me, it isn't making any sense.

Leong

Sep15-04, 04:06 AM

Solve the equations.

HallsofIvy

Sep15-04, 07:42 AM

First, do you know a formula for distance an object falls in t seconds?
Solve that for t1 (the time until the rock hits the water) as a function of d (the height of the cliff).

Second, you are given that the speed of sound is 340 m/s so you can calculate the time, t2 neccessary for the sound to come back up a distance d.

Since t1+ t2= 3.4 seconds, you can add those two formulas, involving d, to get an equation. Solve the equation for d.

joyful55

Sep15-04, 10:38 AM

Umm....Another question. Where in the world are you standing to hear the splash?

OsiriS^

Sep17-04, 12:33 PM

Since the acceleration of the rock is uniform i.e under the affect of gravity, couldn't you use the equations of motion?

u=0, a = 9.81, t = 3.4

s=ut+0.5xat^2
s=0.5*9.81*3.4^2
s=56.7m

I know that's not the information he was given but would it work? The only assumption I've made is that the rock was dropped from rest.

Leong

Sep17-04, 08:09 PM

The time given in this question is not the time the rock takes to reach the ocean but is the total time the rock reaches the ocean plus the time for the splash sound to travel back to the dropper. So i don't think it would work here.

OsiriS^

Sep18-04, 04:12 AM