Idea 8of 9Builds onThey add, point by point
Crest meets trough
Crest meets trough; the rope goes flat. Where's the energy?
Today you'll discover
- what adding does when two crests line up
- what it does when a crest lines up with a trough
- the two names for building up and tearing down
Think firstA 1 cm crest heads for a 1 cm trough
At full overlap the rope is flat. Where has the energy of the two pulses gone?
Decide first, then press Next. Watch the bottom line at full overlap, both ways.
Three rows: a pulse travelling right, a pulse travelling left, and the rope itself beneath them. After a prediction, two crests overlap and the rope's line is twice as tall; then a crest and a trough overlap and the rope's line is flat; then both pulses walk on unchanged. The last step asks what happens at a quiet spot between two speakers.
See the whole ideathe big idea and the key word, in one place
The big idea
Crest on crest builds up; crest on trough tears down.
Compare themConstructive: the sum is bigger than either wave. Destructive: the sum is smaller, even zero. The same rule, adding point by point, gives both.
Key words
- constructive interference
- waves adding to a larger displacement, as when crest meets crest
- destructive interference
- waves adding to a smaller displacement, as when crest meets trough; the energy remains, in the medium's motion
About this modelThe pulses never bounce. They add while they overlap, then walk on unchanged.
The pulses never bounce. They add while they overlap, then walk on unchanged.
Ready to use it?
- Say whether two described overlapping waves interfere constructively or destructively, and what remains afterward.