How high is the bridge?

Question

4. A boy standing on a bridge and threw a stone into the water below. He heared the sound 2 seconds later. How high is the bridge?

Answer 1

Acceleration, velocity, and position, versus time (in seconds) for free fall, on Earth.

A = -9.8
V = -9.8T
P= -4.9T^2

so we fill in 2 for T, in the position equation -4.9 x 4 = -19.6. The bridge is 19.6 meters high.

In response to THETANK, I HIGHLY doubt it was intended that the speed of sound should be taken into consideration.

Answer 2

the two answers previously given are off by a little amount...
the OP obviously wanted us to factor in the speed of sound into the equation, if not, he would not have stated "he heard the sound 2 seconds later"
with the above information, and the given that the speed of sound is 344m/s we can now attempt to answer the question....

A=4.9 ms/s
v(speed of sound)=34m/s

bear with me here, as i attempt to explain how i got to my equation...

you will agree that the distance the stone traveled is the same as the distance the sound traveled. therefore, we will equalize between the two equations

the only tough part about the rest of the problem is the "distance the sound traveled" side... it is obviously 344*the amount of time...but how much time you ask? the answer is 2 seconds minus the time the stone was in free fall.

THEREFORE- the correct equation is
.5at^2=344(2-t) >

4.9t^2= -344t + 688
t=~1.94 Sec

plug it back into one of the sides of the equation and you get total distance= 18.576

i hope that was understandable considering i took physics in another language namely Hebrew.

Peace out! THETANK

Answer 3

x=1/2at^2
x=1/2(-9.8)(2)2
x= 19.6m