2020 A/L Physics Past Paper · Question 07 (English Medium)
Earthquake waves — which is not correct?
Which of the following statements about earthquake waves is not correct?
Answer and explanation
Answer: (4) S waves can travel through both liquid and solid media.
What the question asks
We want the one incorrect statement among five.
We compare each statement with the properties of P and S waves in the book.
We rule out the correct ones and pick what is left.
Relevant theory
Seismic waves are produced by intense vibrations or explosions taking place inside Earth.
It has been confirmed that a material medium is essential for the transmission of mechanical waves.
These waves propagate longitudinally as sound waves.
Hence P waves can travel through all three media, solid, liquid and gas.
The main difference between these waves and the P waves is that these waves are transverse.
The velocity of these waves is about 60% of the velocity of P waves.
Since these are transverse waves they can travel only through solid (rock) media.
The rules to match against:
(i) Seismic waves are mechanical waves. They need a medium.
(ii) P is longitudinal, S is transverse.
(iii) .
(iv) P travels through solid, liquid and gas.
(v) S travels only through solids. A liquid cannot carry a transverse vibration.
Reading the options
Each option is a statement.
Because we want the one that is "not correct", the four statements that match the rules cannot be the answer.
The statement that breaks a rule is the answer.
Reasoning and calculation
Checking each one:
Statement (1) matches rule (i).
Statement (2) matches rule (ii).
Statement (3) matches rule (iii): .
Statement (5) matches rule (iv).
Statement (4) says "S waves travel through liquids too". This breaks rule (v).
A boundary check:
An S wave is a transverse wave.
For particles to move sideways to the direction of travel, the medium must resist a change of shape.
A solid does this.
A liquid flows instead, so a sideways vibration cannot travel through it.
So an S wave stops at a water boundary.
Why the other options fail
Statement (1) is correct.
Seismic waves are mechanical waves carried by the vibration of rock particles.
They need a medium.
Statement (2) is correct.
P is longitudinal, S is transverse.
Statement (3) is correct.
, so S is slower.
Statement (5) is correct.
P waves travel through solid, liquid and gas.
Conclusion
S waves are transverse.
So they travel only through solid (rock) media.
Statement (4) is not correct.
The answer is (4).
If asked "which wave travels through a liquid?":
A longitudinal wave (P) can.
A transverse wave (S) cannot.
This is why S waves are never recorded on the opposite side of the Earth's liquid outer core.
Exam tip