Fenna flicks the end of a long rope tied to a fence post, and a hump races away down the rope. A strip of tape stuck to the rope's midpoint rises, falls, and stays exactly where it was.
Two motions just happened on one rope. Which one is the wave's?
The hump is the wave pattern. It raced along the whole rope, from Fenna's hand to the post.
The tape rides a piece of the medium. That piece moved up and down about its rest position, and it ended the trip exactly where it began.
So there are two speeds on this rope, and they belong to different things.
The wave's speed is how fast the pattern travels along the rope. That is the speed from the last definition: distance the crest covers, divided by time.
A particle's speed is how fast one piece of the medium moves up and down about its rest position. It says nothing about travel along the rope.
The two are different measurements, and they are usually different numbers. The pattern can cross the whole rope while each piece only bobs through a few centimeters.
The wave carries energy down the rope; it does not carry the rope.
A ripple crosses a pond while a floating leaf bobs up and down as it passes. Which motion shows the wave's speed?
The ripple pattern travels across the pond: following it measures the wave's speed.
The leaf rides one patch of water surface, which only moves about its rest position.
The ripple pattern's travel shows the wave's speed; the leaf shows how a piece of the medium moves.
Hale sends a pulse along a garden hose lying straight on the grass. A chalk mark on the hose swings sideways and back as the pulse passes. Did the mark travel with the pulse?
The pulse pattern traveled along the hose.
The chalk mark is a piece of the medium: it swung about its rest position and returned.
No. The pulse traveled along the hose; the mark only moved about its rest position.
A pulse travels along a clothesline while a clothespin clipped to the line wiggles. Which timing finds the wave's speed?
Hale says: 'While a ripple crosses a pond, each bit of the water surface it passes only moves about its rest position.' Is Hale correct?
Eirik watches a ripple cross a pond and says: 'The water under the crest travels all the way across the pond with it.' Is Eirik correct?
Which statement tells the wave's speed apart from a particle's speed?
A ripple spreads across a pool and passes a floating ring. What does the ring do?
A wave pulse crosses a trampoline's surface while a shoe resting on it hops up and down in place. Which motion gives the wave's speed?