Waves that add and cancel
Grade 8 Science Worksheet
Circle the best answer, or write your answer on the line.
Each wave repeats every 4 s. Two waves pass the same spot. What is the amplitude of the combined wave there, Ari?
Each ripple repeats every 6 s. Two ripples pass the same spot in Bo's pond. What is the amplitude of the combined ripple there?
Each wave repeats every 8 s. Two waves rock Esme's boat at the same time. What is the amplitude of the combined wave at the boat?
Jun graphs two ripples at one spot in a pond. What kind of interference do they make there?
- Constructive: both ripples repeat with a 4 s period, so they must be in step and add up
- Partial: the crests are 2 s apart, a small shift next to the 8 s the graph spans
- Destructive: B is at a trough whenever A is at a crest, so the water stays flat there
- Constructive: the ripples have the same size and the same period, so they build a taller ripple
Read this first
Two waves at one spot (example data)
wave crest times at the spot (s) A 1, 5, 9 B 1, 5, 9 Each wave repeats every 4 s.
Two waves cross at one spot, and the table lists their crest times there. What kind of interference is it, Kira?
- Constructive: the crests arrive together, so the combined wave is taller
- Destructive: waves with matching crest times are the same wave, so one removes the other
- Neither: waves arriving at a spot at the same moment cannot combine with each other
- Destructive: two crests arriving at the same moment push on each other, so they cancel
Two waves cross at one spot, and the table lists their crest times there. What happens, Luz?
- Fully destructive: crests that arrive at different times meet troughs and cancel each other out
- Fully constructive: both waves repeat with a 4 s period, so their crests must arrive together at the spot
- Fully constructive: B's crests come just 1 s after A's, which is close enough to count as in step
- Partly: B's crests come 1 s after A's in a 4 s period, so the waves are not fully in or out of step
Seat and two speakers (example data)
first speaker (m) second speaker (m) wavelength of the note (m) 5.0 6.5 0.5 Room echoes are ignored.
Two speakers play one note in step. By how many wavelengths do Milo's distances to the speakers differ?
Two speakers play one high note in step. By how many wavelengths do Pia's distances to the speakers differ?
The note has a wavelength of 0.5 m. Room echoes are ignored. Two speakers play one note in step. Which seat is quiet, Ren?
- Seat C, where the paths differ by 0.5 m, a whole wavelength, so the waves cancel
- Seat B, where the paths differ by 0.25 m, half a wavelength, so crests meet troughs
- Seat A, where the paths differ by 1.0 m, the biggest gap of the three, so the waves cancel there
- No seat: two speakers playing in step add up everywhere, so the room stays loud
Room echoes are ignored. Two speakers play one note in step. Seat D is 4.0 m from both speakers. What does Zee hear at D?
- A quiet note: equal distances leave no path difference to add anything to the sound
- A loud note: the waves travel equal distances, so crests arrive together and add
- A normal note: two speakers playing in step cannot add up to a louder note
- A quiet note: waves that arrive at the same moment from two speakers cancel