4 mistakes learners make with waves, each one named and explained.
A wave carries energy without carrying the material it travels through, and its speed is set by that material.
Air is described as moving from the loudspeaker to your ear, or particles as shuffling along from one compression to the next.
Something plainly travels across the room, and the particles are the only thing there to point at.
Question. A loudspeaker plays a note across a room. What travels from it to your ear?
A common answer. the air particles
The answer. the energy, carried by particles that only vibrate about one spot
Why. If the air itself travelled with the sound there would be a wind, and the room would run out of air.
A wave carries energy without carrying the material it moves through.
The gap between two crests on a displacement against time graph is called the wavelength.
A wave graph looks exactly like a wave, so the axes stop being read.
Question. Two crests on a displacement against time graph are 0.2 s apart. What is that gap?
A common answer. the wavelength
The answer. the period, so the frequency is 5 Hz
Why. Time runs along the bottom, so a gap measured along it is a time and not a length.
Read the label on the bottom axis before reading anything off the graph.
A sound wave is described as the sound itself, so a wave with nobody there to hear it is said not to exist.
One word does both jobs in ordinary speech, where a sound is simply the thing you hear.
Question. A tree falls where nobody can hear it. Is there a sound wave?
A common answer. no, because nobody heard anything
The answer. yes. The air vibrates whether or not an ear is there to turn it into hearing
Why. The wave is the vibration of the air, and hearing is what an ear and a brain make of that vibration.
Separate the vibration from the sensation. A physics question is always about the vibration.
Doubling the frequency is said to double the speed, because frequency sits on the right of speed = frequency x wavelength.
The equation holds three quantities and two of them are on the right, so changing one of those looks as though it has to change the answer.
Question. The frequency of a sound in air is doubled. What happens to its speed?
A common answer. it doubles
The answer. it stays the same, and the wavelength halves
Why. Speed is set by the material the wave travels through, so raising the frequency shortens the wavelength instead.
Ask what the wave is travelling through first. That sets the speed, and the equation then settles the other two.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.