Waves that add up and stand still

Science · Waves, Light & Sound · ages 16-18

Name ______________________ Date ____________

Nodes are points that never move.
Circle one: True False
Two identical waves arrive crest to crest. What results?
- Double amplitude, same wavelength
- Zero amplitude
- Half the wavelength
Where do antinodes sit?
- Exactly on nodes
- Halfway between nodes
- Outside the string
A 1.2 m string sounds its second harmonic: one full wave. Give the wavelength in metres.
Answer: ______________
Neighbouring nodes are 0.3 m apart. What is the wavelength?
- 0.6 m
- 0.3 m
- 0.15 m
Two pulses meet crest to trough, then move on. What happens?
- They stick together forever
- They cancel for an instant, then pass through unchanged
- They double permanently
A string of length L carries waves of speed v. What is the second harmonic frequency?
- v over 2L
- v over 4L
- v over L
A 1.2 m string sounds its third harmonic: one and a half waves. Give the wavelength in metres.
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Cancelling waves always meet at nodes.
Double amplitude, same wavelength · In-phase disturbances add, doubling the height.
Halfway between nodes · The swing is widest midway between still points.
1.2 · The second harmonic fits exactly one wavelength on the string.
0.6 m · Nodes split each wave in half, so double 0.3 to get 0.6.
They cancel for an instant, then pass through unchanged · Opposite displacements cancel briefly, then each wave continues.
v over L · The second harmonic wavelength is L, so frequency is v over L.
0.8 · Divide 1.2 by 1.5 to get 0.8 m.