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?
- R, where equal and opposite heights add to 0 cm
- U, where the sum is 0 cm because both waves are at rest
- S, where the two waves are equal and give the largest sum
- T, where a trough meets a crest, so the two cancel there
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 changes nothing, since 1 cm is much smaller than 4 cm
- B lowers it from 4 cm to 3 cm, since B's height there is negative
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 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
- They bounced: at 1 s they met and each went back toward the end it started from
Two crests on a rope (example data)
time (s) 4 cm crest at (m) 2 cm crest at (m) rope height at 5 m (cm) 0 1 9 0 2 5 5 6 4 9 1 0 Why is the rope 6 cm high at 5 m when the pulses meet at 2 s, Pia?
- The crests bounced off each other at 5 m and stacked up for a moment before moving back
- The crests pushed against each other at 5 m, and the push raised the rope above both
- Both crests were at 5 m, so their heights added, 4 + 2 = 6 cm, and then each moved on
- The 4 cm crest absorbed the 2 cm crest and stayed 6 cm tall as it moved on to 9 m
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?
- Destructive: the ripples rise and fall together, so they cancel out at that spot
- Constructive, but the combined ripple is still 2 cm tall, since equal ripples do not add
- Constructive: the ripples rise and fall together, so the combined ripple is 4 cm tall
- Destructive: the two ripples are the same size at each time, so they must cancel
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?
- Neither: the sums are 0 cm, so no ripples ever reached this spot from either side
- Constructive: the ripples are the same size, so they build up a combined ripple that is 4 cm tall
- Destructive: the ripples rise and fall opposite each other, so the water stays flat there
- Destructive: the sums are 0 cm, so these ripples are destroyed for good everywhere
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 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
- Constructive, with a combined ripple 3 cm tall, since the larger ripple wins the meeting