I do not understand this at all!!!!!

Question

my teacher did not explain this to me very well and i am having a hard time understanding this... can someone explain this to me ??

1.

A rifle is fired along a valley with parallel vertical walls. The echo from one wall is heard 1.8 s after the rifle was fired. The echo from the other wall is heard 1.8 s after the first echo. How wide is the valley?

___km

2.

A certain instant camera determines the distance to the subject by sending out a sound wave and measuring the time needed for the echo to return to the camera. How long would it take the sound wave to return to the camera if the subject were 2.80 m away?

___ s

Answer 1

In dry air at 20 °C (68 °F), the speed of sound is 343 m/s (1235 km/h, or 770 mph, or 1129 ft/s, or approximately 5 seconds per mile).

1) 343 m/s x 1.8s = 617 m but that is round trip, so one wall is 308 m away, and twice that for the other. total is 926 m.

2) 2.8m / 343 m/s = 0.00816 second for one way, twice that for round trip or 0.0163 sec, or 16.3 ms

.

Answer 2

1) you are between two walls. and sound will go both walls and come back to you. one took 1.8s and other took 3.6 to come back to you. total of 5.4 seconds for sound to travel between the walls. You need to find distance that sound can travel in 5.4 s. half of that distance will be your answer. because 5.4 is both ways.

2) this is similar you need to calculate the time for a sound to travel 2.80 m

speed = distance / time

time = distance/ speed

distance = speed x time

Answer 3

You need to know the speed of sound. (S)  

1. Distance to wall A = S/(1.8 * 0.5)

Distance to wall B = S/(1.8)

Total width = Distance to A + Distance to B

2. Distance from camera to subject, round trip = S * 2.8

Distance to subject = 2.8/2

Answer 4

This is a little confusing.  I guess you have to figure that you are in the valley shooting parallel to the walls.

XXXXXXXXXXXXX w  

gun--->              i

                        d

                        t

XXXXXXXXXXXX   h

An echo is heard after the sound traveled to the wall and back.  The first shot took 1.8 seconds to make the round trip so it took 0.9 seconds to go one way.  Sound traveles about 330 m/s and 330m/s x .9s = 297 meters.  The second echo took the first 1.8 s plus another 1.8 seconds so one way the sound traveled 1.8 seconds.  1.8 s X 330 m/s = 594 m so the total width of the valley is 891 m

2) speed is distance divided by time

s = d/t

solve that for t = d/s = 2.8 m / 330 m/s = 0.00848s

Answer 5

Q1: Sound travels at 730 ft/sec, so you would have to calculate how far the sound traveled from each wall.

Q2: same as above, except now you're dealing with miles (I hope that you're not being asked to measure meters, unless your course is being taught in the metric system--which, by the way, I find much easier and intuitive than the so-called "standard" system we have in the States).

Whoops, got my numbers wrong. Go with the answer above, which cites Wikipedia data.