Video Transcript
In the diagram, a wave traveling
along the spring is shown. Parts of the spring are displaced
from the horizontal line between points A and B. The parts of the spring shown
within the red lines are compressed, and the parts of the spring shown within the
blue lines are extended. Which of the following types of
wave motion corresponds to the displacement of the spring’s sections perpendicularly
to AB? Is it (A) longitudinal, (B)
transverse, (C) both transverse and longitudinal, or (D) neither transverse nor
longitudinal.
Let’s begin by clearing some room
on screen and briefly recalling the defining properties of both types of wave
motion. Longitudinal waves, sometimes
called compression waves, are characterized by the medium oscillating along the same
direction as the propagation, or travel, of the wave as a whole. One typical example of a
longitudinal wave is a spring that’s been wiggled along its axis, so that the whole
spring stays straight but still sends waves down its length.
For transverse waves, on the other
hand, the medium oscillates perpendicular to the overall direction of
propagation. A typical example of a transverse
wave is a rope that’s being wiggled up and down. So the medium, which is the rope,
oscillates vertically, but the wave travels horizontally.
Now let’s look at the diagram given
to us. We might be used to associating
springs with longitudinal waves. But this spring actually isn’t
oscillating in that way. The areas of compression and
extension are simply due to the spring being bent into a wavy shape. If we look closely, we see that
there is no compression wave being sent along the changing direction of the spring’s
length. Besides, in this question, we’ve
been asked specifically about the type of wave motion corresponding to the spring’s
displacement perpendicular to the horizontal line AB. The perpendicularity is the key
here.
We can draw some arrows on the
diagram to help us focus on the spring’s vertical displacement. While the spring wiggles up and
down, the waveform along it will travel horizontally. This describes a transverse wave,
so answer option (B) is correct. The displacement of the spring’s
sections perpendicularly to the line AB corresponds to a transverse wave motion.
