Q4. A wave on a string can be described in two ways: by fixing an instant, or by fixing a point. What does each show?
Fixing an instant shows the shape of the whole wave in space at that moment, like a photograph. Fixing a point shows how that one element of the string oscillates in time.
Q5. With the wave speeds of Example 3 (P waves 6.0 km/s, S waves 3.5 km/s), how far away is an earthquake if the S waves arrive 20 s after the P waves?
d(1/3.5 − 1/6.0) = 20, so d = 20 × 21/2.5 = 168 km.
Q6. Deeper in the Earth, P waves travel at about 8.0 km/s and S waves at about 4.5 km/s. Taking these speeds over a 600 km path, what is the gap between their arrival times?
P waves take 600/8.0 = 75 s and S waves 600/4.5, about 133.3 s, so the gap is about 58.3 s.