Legit Reviews

Ok ive need this physics question done.... (urgent)
k the question is as follows.
A park has a circular fence around it. The top rail of the fence is a metal pipe. A physics Teacher sets a group of students the task of finding the radius of the fence by using their knowledge of sound. One member of the group hits the pipe with a hammer giving a sound of 350Hz. A second student, standing directly opposite on the other side of the park, detects two sounds, 0.30s(seconds) apart. If the speed of the sound is 330ms^-1(meters per second) in air and 1310ms^-1( meters per second) in this metal pipe, what is the radius of the fence?

This question is from the textbook Physics Problems in Context - Year Twelve (the book is a STAWA)

Can i please please please i not just the answer some brief working in how to do it?



P.S the correct answer is 82 m

The answer can't be 82 m, it takes less than 0.30 s even for the slower sound to travel 82 m.

I just got up, and am going to go eat breakfast, but I'll solve it for you when I get back.

I can guarantee the answer is 82m and that the values are listed Correctly.. Heres a scanned image, i tried a few times but the paper is slightly glossy so it didn't scan too well.. My teacher said it is one of the harder sound questions in the set ive managed to do everything else fine but this one has got me stumped....

oh yea did u go by the fact that the 'm' stands for Meters??
This is how i see it and im pretty sure my diagram is right.
Any help is almost always Helpful, its probably some little thing im not taking into account or noticing.. Thanks for your help so far!

I had a similar question last semester. Only i forgot what the equation was.

You know that 82 is the radius. I think zelig might has been thinking diameter which would be 164m.

its been too long since i did stuff like this, so i cant help you with the equation. what i can help you with is that diagram! you know there is an ellipses tool in paint right?

Oops, yeah, was thinking diameter, the answer is in fact 82.

This should be enough for you to work through it:

Known:

V1 = 330 ms^-1
V2 = 1310 ms^-1
t2 - t1 = 0.3 s

Then, given that the circumference of a circle is C = 2*pi*r, you can rearrange to find that the distance around half a circle is pi/2 times the diameter of the circle.

So you can divide V2 by pi/2 to get the a "relative" velocity as if it was moving in a straight line across the circle.

This gives you:

d = 330 * t2
d = (2620/pi) * t1
t2 - t1 = 0.3 s

Here you've got 3 equations and 3 variables, you shouldn't have a hard time solving for d (which is the diameter, don't forget to divide by 2 for the final answer) and plugging the numbers back in to verify they work.

Zelig seems to have hit the nail on the head, I've done it slightly differently (check but verify) hope Zelig doesn't mind.

First, we need the distance traveled by the sound waves and because the air wave is a straight line the distance will be 2r, For the metal wave the distance is half the circumference C/2=2πr/2=πr

Now to find the time it takes each sound to reach the second student, we divide the distance by the velocity (remember D=V*T so T=D/V) so it takes t1=2r/330 s for the sound to travel through the air and t2=πr/1310 s for it to travel through water.

Now that we know both times, and we know the difference between the times, we can setup a simple equation: t1-t2=.30 s
Substitute the equations for t1 and t2 and solve for r! Easy peesy lemon cheesy!


BTW, not quite sure the General Computer Forum is the right place for this...just saying ya know.

Good thing the website isn't Ethical Reviews.

There are a lot of smart people in this forum, consider them Tutors!

vicaphit wrote:Good thing the website isn't Ethical Reviews.

There are a lot of smart people in this forum, consider them Tutors!

You all are way smarter than I am. I haven't had physics for over 10 years.

Waay over my head ..........but moved to a more appropiate forum ;)

Thank you VERY VERY much i went through the working on both this morning and i got about 81.98 for both so That is a big IOU need a favour just ask

But for now


Drinks on me

martini161 wrote:its been too long since i did stuff like this, so i cant help you with the equation. what i can help you with is that diagram! you know there is an ellipses tool in paint right?

lol!