Learning objective 6

1. Learning objective 6
Question #1:
Jonny Cash is on stage and in order to create the perfect tune
out of his guitar he knows (based on his extensive education in
the field of physics) he needs to create a wave speed somewhere
between 200cm/s and 215cm/s in the string he plucks. The
distance between the third and the sixth nodes in a standing
wave on Jonny Cash’s guitar is 32cm.
A)
If the frequency is 10.0 Hz after he plucks the string,
calculate the wavelength.
Solution:
λ = 2/3 x 32cm = 21.333cm
B)
Did Jonny Cash pluck the string properly and create the
right tune? Or has he disappointed his many fans?
Solution:
v = λf
= 21.333 cm x 10.0Hz = 213.33cm/s
213.33cm/s is in between 200 and 215cm/s. therefore the wave
speed is good.
Question #2:
A skipping rope tied to a post is being moved up and down
rapidly by a young girl creating vibration of 100Hz. Her older
sister stands back and notices that there are nine nodes
(including one node at each end) in the whole skipping rope,

2. which is 150cm long. What is the wavelength of the waves
traveling in the string?
Solution:
Nine nodes, therefore there are a total of 4 wavelengths in
the skipping rope.
150cm x 1/4 = 37.5cm