Practice question · Multiple choice
Two identical wave pulses — one upright, one inverted — travel toward each other on a string. At the instant they perfectly overlap, the string is completely flat. Where did the energy go?
Hints
- A flat string is not necessarily a still string. What are the string elements doing at that instant?
- Displacements cancel; velocities add. Check the direction each pulse makes the string move at the overlap.
Show the answer
A. Into the string's motion: flat but moving
Why
At perfect cancellation, every point has zero displacement but maximum speed, the pulses’ velocity fields reinforce. All the energy is momentarily kinetic, which is why the pulses re-emerge unchanged and keep going. Superposition erases nothing.
Practise Superposition, Interference, and Standing Waves
The app has 9 more questions on this lesson, and keeps your place in the course. Physics I is free to start.
More questions on Superposition, Interference, and Standing Waves
- Sort each standing-wave feature into Node or Antinode.
- A standing wave has nodes that never move and antinodes that swing hardest. Why does a travelling wave have…
- Order these milestones in the history of wave physics.
- A guitar string 0.65 m long vibrates in its fundamental mode with wave speed 260 m/s (fundamental: λ = 2L).…