A ringing alarm bell hangs by threads inside a sealed glass jar on a lab bench. Dilys switches on a pump that draws the air out of the jar. The hammer keeps striking, but the ring fades until the room is silent.
The bell still vibrates. Something the sound needed is gone. This lesson names what that something is.
The bell's sound wave traveled through the air inside the jar, then through the glass, then through the room's air.
Pump the jar's air out, and the first link is gone: around the bell there is almost nothing left to disturb.
No material, no traveling disturbance. The sound cannot leave the bell.
Earlier in the course, 'medium' named the water or air a boat or a plane travels through.
Waves use the word the same way. The material substance a wave travels through is called the medium.
Sound is one example: the silent jar shows sound needs a medium. Ripples need water. A rope wave needs the rope. All of these are mechanical waves.
On the Moon there is no air. Anselm asks: if an astronaut strikes a bell held at arm's length, will a suited astronaut standing nearby hear it through the space between them? Explain.
Sound is a mechanical wave, so it needs a medium.
Between the bell and the nearby astronaut there is no air: no medium.
No. With no medium between bell and listener, the sound wave cannot travel across the gap.
A whale's call travels 10 km through the sea to another whale. Name the medium.
The medium is the material substance the wave travels through.
The call travels through sea water.
The medium is the sea water.
A ripple crosses a fish tank from one glass wall to the other. Which medium does the ripple travel through?
A sealed chamber has had all its air pumped out. Can a sound wave travel across the empty chamber?
Cosmo says: 'A wave on a jump rope is a mechanical wave, because the wave cannot exist without the rope.' Is Cosmo correct?
A music box hangs by threads inside a display case. A pump removes the case's air, and the room falls silent, though the box's drum keeps turning. Why does the sound stop reaching the room?
Which of these is a mechanical wave?
Which statement about a medium is correct?