How Do You Calculate the Speed of Transverse Waves in a Vibrating Wooden Bar?

AI Thread Summary
A wooden bar vibrates as a transverse standing wave with three antinodes and two nodes, producing a fundamental frequency of 43.6 Hz. The speed of transverse waves on the bar can be calculated using the relationship between frequency, wavelength, and velocity. The correct wavelength for this configuration is equal to the length of the bar, leading to a velocity calculation of 24.15 m/s. The confusion arises from the interpretation of nodes and antinodes, where three antinodes and two nodes indicate the second harmonic rather than the fundamental frequency. Visualizing the wave pattern can clarify the relationship between nodes and antinodes in this context.
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A wooden bar when struck vibrates as a transverse standing wave with three antinodes and two nodes. The lowest frequency note is 43.6 Hz, produced by a bar 55.4 cm long. Find the speed of transverse waves on the bar.


I assumed that 3 antinodes and 2 nodes means the eigenfrequency f=3/2(v/L). I also assumed that 43.6 Hz was the fundamental frequency. Since I want f3, I multiplied 43.6 by 3 and got 130.8 Hz.

From here I plugged into the first equation 130.8=(3/2)(v/.554) and solved for v.
v=48.3088 m/s.

But this answer was wrong, so I am not sure what I did wrong.

I would appreciate any advice, Thanks,
Jason
 
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For a wooden bar with anti-nodes on both sides, the formula for wavelength is:
Wavelength = 2L/n, where L is the length of the bar and n is the harmonics number.

For three antinodes and 2 nodes, the bar is in its second harmonic and so wavelength is:
2L/2 = L

Since Frequency*Wavelength = Velocity,
Velocity = (43.6)(0.554) = 24.15 m/s

**Its been a while since I did this, so i may be wrong.
 
Thanks, that was correct.

I guess i still don't understant how it is the second harmonic though. I thought the antinodes were the max points, and three max points means 3/2 of a wavelength. For example, two positive maximums, and one negative maximum.
 
Ah but think about it. Say we're looking at one interval of a cosine curve (0 to 2pi). How many max points are there? How many points cross y = 0? The points that cross y = 0 are like the nodes (when you multiply the cosine curve by -1 they remain invariant) while the antinodes are the points where y = +/- 1. (This results in 3 antinodes and 2 nodes which means 3 antinodes and 2 nodes = 1 wavelength of a cosine curve).

Its a weird way of thinking about it. But try drawing a picture, it might help.
 
You're right, I was picturing a sine function. I didn't notice that the sine is opposite, three nodes and two antinodes.

Thanks again
 
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