Waves that add and cancel
Grade 8 Science Worksheet - Building Confidence
Circle the best answer, or write your answer on the line.
Heights above the rest line are positive. Two waves overlap on Ari's rope, and their heights add. What is the height of the combined wave at point S?
Heights above the rest line are positive. Two ripples cross in Bo's pond tank, and their heights add. What is the height of the combined wave at point U?
Heights above the rest line are positive. Two waves overlap on a pond, and their heights add. What is the height of the combined wave at point T, Esme?
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Two waves on a rope (example data)
point wave A (cm) wave B (cm) A + B (cm) R 3 −3 0 S 2 2 4 T 4 −1 3 U 0 0 0 At which point do the two waves cancel completely, Jun?
- U, where the sum is 0 cm because both waves are at rest
- R, where equal and opposite heights add to 0 cm
- T, where a trough meets a crest, so the two cancel there
- S, where the two waves are equal and give the largest sum
At which point is the combined wave tallest above the rest line, Kira?
- S, where two crests of 3 cm and 2 cm add to 5 cm
- T, where wave A is the biggest single wave at 4 cm in size
- R, where the two crests are equal in height at 1 cm each
- U, where a 4 cm crest meets a 4 cm trough, the strongest meeting
At point S, how does wave B change wave A's height, Luz?
- B cancels it completely, since a trough meets the crest there
- B raises it from 4 cm to 5 cm, since B's size of 1 cm gets added to A's height
- B lowers it from 4 cm to 3 cm, since B's height there is negative
- B changes nothing, since 1 cm is much smaller than 4 cm
Two pulses on a rope (example data)
time (s) 3 cm pulse at (m) 2 cm pulse at (m) tallest point on the rope (cm) 0 1 9 3 1 5 5 5 2 9 1 3 Milo films two pulses on a rope. What do the readings show about the pulses?
- They bounced: at 1 s they met and each went back toward the end it started from
- They merged: the 5 cm bump at 1 s replaced both, so one pulse was left after that
- They passed through: they added to 5 cm at 1 s, then the 3 cm pulse reached 9 m
- The 3 cm pulse absorbed the 2 cm pulse, so the readings after 1 s show a single pulse at the far end
Two ripples at one spot (example data)
time (s) ripple A (cm) ripple B (cm) 0 2 2 1 −2 −2 2 2 2 3 −2 −2 Ripples A and B meet at one spot in Ren's pond. What kind of interference does the table show there?
- Constructive, but the combined ripple is still 2 cm tall, since equal ripples do not add
- Destructive: the two ripples are the same size at each time, so they must cancel
- Destructive: the ripples rise and fall together, so they cancel out at that spot
- Constructive: the ripples rise and fall together, so the combined ripple is 4 cm tall
Two ripples at another spot (example data)
time (s) ripple A (cm) ripple B (cm) 0 2 −2 1 −2 2 2 2 −2 3 −2 2 Ripples A and B meet at another spot in the pond. What kind of interference does the table show, Tala?
- Constructive: the ripples are the same size, so they build up a combined ripple that is 4 cm tall
- Neither: the sums are 0 cm, so no ripples ever reached this spot from either side
- Destructive: the sums are 0 cm, so these ripples are destroyed for good everywhere
- Destructive: the ripples rise and fall opposite each other, so the water stays flat there
Two ripples at a third spot (example data)
time (s) ripple A (cm) ripple B (cm) 0 3 1 1 −3 −1 2 3 1 3 −3 −1 Ripples A and B meet at a third spot in the pond. What does the table show, Zee?
- Destructive: the 1 cm ripple weakens the 3 cm ripple to a combined ripple of 2 cm
- Constructive, with a combined ripple 3 cm tall, since the larger ripple wins the meeting
- Constructive, with a combined ripple 2 cm tall, found by subtracting 1 cm from 3 cm
- Constructive, with a combined ripple 4 cm tall, since the ripples rise and fall together