• Support PF! Buy your school textbooks, materials and every day products Here!

Properties of sound

  • Thread starter Tereza
  • Start date
  • #1
5
0
Hello,

I'm calculating a physics problem and I'm supposed to calculate the height of a building given that:

rock is dropped from the building,
the sound of the rock hitting the ground is heard 4.8s later,
the speed of sound is 350m/s,

and I don't know how do I calculate the time since the rock hit the ground until it reached my ears.

Thank you!
 

Answers and Replies

  • #2
Simon Bridge
Science Advisor
Homework Helper
17,848
1,645
how do I calculate the time since the rock hit the ground until it reached my ears.
distance over speed.
 
  • #3
5
0
  • #4
Simon Bridge
Science Advisor
Homework Helper
17,848
1,645
That's right - distance is what you are trying to find.
You do, however, have the total time, the speed of sound, and the acceleration due to gravity.
 
  • #5
Drakkith
Staff Emeritus
Science Advisor
20,801
4,544
Is it 4.8 seconds from the time you drop the rock until you hear the sound? Or 4.8 seconds from the time the rock hits the ground until you hear the sound?
 
  • #6
Simon Bridge
Science Advisor
Homework Helper
17,848
1,645
I'm interpreting that at t=0, rock is dropped, t=T=4.8s hear the sound.
There are four equations and four unknowns ... waiting for OP to realise ;)

Rock accelerates at g=9.8m/s, falls a height h, and hits the ground in time T1 at speed v making a "crack" noise.

From this we see that OP can work out:
v in terms of g and T1 ...that will be eq(1)
h in terms of v and T1 ...that will be eq(2)

The noise returns the same distance at speed c=350m/s in time T2:
height of the building by the speed of sound and T2 ... (3)

The total time between drop and sound is:
T=T1+T2 ...(4)

Four equations and four unknowns.
 
  • #7
5
0
Thank you.

I have : -1/2aT12 = c(4.8-T1)

I still can't find the answer.
 
  • #8
Simon Bridge
Science Advisor
Homework Helper
17,848
1,645
Well - technically T1 is one of the variables you want to eliminate, you want to keep h because that's what you want to find. I think you may have misplaced a minus sign too.

Please list your four equations... number them.
You will probably instinctively combine the first two... that's fine, you then have three equations and three unknowns.

However: notice that you know a and c, so you can get T1 from that equation. Substitute that number into all your equations where T1 appears and you now have three equations and three unknowns.

But it is better to to use the first three equations to eliminate all the unknowns except for h. Have you solved simultaneous equations before?
 
Last edited:
  • #9
5
0
You give me wonderful ideas on how to start solving physics problems effectively, but I need so much more practice.

This time I got: T2= 2h/a + h2/c2

however, I still can't get it. It's always so close.

I don't know what more to ask you, I think you've told me all that you could.

Thank you!
 
  • #10
Simon Bridge
Science Advisor
Homework Helper
17,848
1,645
That does not look right to me - I need to see your working or I cannot help you :(

I suspect you actually had $$T=\sqrt{\frac{2h}{g}}+\frac{h}{c}$$ and you squared both sides. Is that the case?
If so then that was a mistake.

But look at the form of your final equation ... you've seen it before.
What do you call it when the variable is squared in a function: what sort of function is it?
It has form ##Ax^2+Bx+C=0##
 
Last edited:
  • #11
5
0
I think I have it now.

1/2at12=ct2

and t1= t-t2

Then solve quadratic equation with a=1; b= -71.4; c=23.04

Thank you so much!
 
  • #12
Simon Bridge
Science Advisor
Homework Helper
17,848
1,645
No worries.
 

Related Threads on Properties of sound

  • Last Post
Replies
7
Views
3K
  • Last Post
Replies
4
Views
3K
  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
2
Views
1K
Replies
1
Views
4K
  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
7
Views
3K
  • Last Post
Replies
2
Views
4K
  • Last Post
Replies
7
Views
3K
Top