Help me with this problem plz.

[Gens]

1. A hammer taps on the end of a 4 m long metal bar at room temperature. A microphone at the other end of the bar picks up two pulses of sound, one that travels through the metal and one that travels through the air. The pulses are separated in time by 11.0 ms. What is the speed of sound in this metal?

 

I know that sound travels faster in metal than in air. What does 4m metal bar supposed to represent?

 

 

Does pulses of sound mean "Beats"? I am confused here.

 

 

[TonyMcC]

The longer the bar, the greater the time difference. The pulses refer to the short burst of sound that results from the hammer blow. The question could better start with the words "A hammer taps ONCE on the end.........". It seems to me that you should have been given (or should know) the speed of sound through air.

Edited September 23, 2011 by TonyMcC

[Externet]

Time needed for the sound to travel 4 metres of air as picked by the microphone MINUS time needed for the sound to travel trough the metal as picked by the microphone EQUALS 0.011 seconds.

 

The first term of the equation above has to take in account the sound constant speed of 340 m/s on air; find out the time it takes to travel 4 metres and then the above equation can be solved for time.

 

The speed will then be distance / time.

[Gens]

The first term of the equation above has to take in account the sound constant speed of 340 m/s on air; find out the time it takes to travel 4 meters and then the above equation can be solved for time.

Time= distance/speed

so 4m/340m/s=0.011s (time needed to travel 4m in air)

 

Time needed for the sound to travel 4 metres of air as picked by the microphone MINUS time needed for the sound to travel trough the metal as picked by the microphone EQUALS 0.011 seconds.

0.011s - x=0.011s ??? (x=time needed for the sound to travel trough the metal?)

-x=0 ???

Edited September 24, 2011 by Gens

[Daedalus]

Time= distance/speed

so 4m/340m/s=0.011s (time needed to travel 4m in air)

 

0.011s - x=0.011s ??? (x=time needed for the sound to travel trough the metal?)

-x=0 ???

 

First of all you need to check your math:

 

[math]\frac{4\, m}{340 \, m/s} \ne 0.011 \, s[/math]

 

Then, you will need to know how to set up the proper equation:

 

• You have the distance - [math]4 \, m[/math]
• You have a value for the speed of sound through air (use the value the homework specified) - This is what Wikipedia gives [math]343.2 \, m/s[/math] at [math]20^\circ C[/math]
• You know how to derive the time in seconds given a distance and a velocity - [math]s=\frac{m}{m/s}[/math]
• you know that it takes longer for the sound to travel through air.
• So, you would then know that [math]Air_{(time)} \left(\frac{m}{m/s}\right) - Material_{(time)} \left(\frac{m}{m/s}\right)=\Delta t[/math].

Next, you are looking for the speed of sound through the material where the change in time between pulses is [math]\Delta t=11 \, ms=0.011\, s[/math]:

 

[math]\frac{4\, m}{343.2 \, m/s} - \frac{4\, m}{X \, m/s} = 0.011 \, s[/math]

 

Finally, it is up to you to solve for [math]X[/math].

Edited September 27, 2011 by Daedalus