AQA A Level Physics Waves Question

The markscheme says that the answer is A, and I got that via process of elimination, but would someone be able to explain how P and R are in antiphase? Is it just because when the wave oscillates, they're on opposite sides of the 'equilibrium line?'

Reply 1

sigmagrindset49

P and R are 180 degrees or pi radians out of phase with each other. Let's define the point when P or R are at their maximum heights to be the 'start' of a wave - then this would also make this position the end of each wave too. Half way through a cycle (or 180 degrees through its phase) would be the minimum height which the points reach. It happens to be the case that when P is at its maximum height, R is at its minimum height (i.e. 180 degrees out of phase). Similarly, when R is at its maximum height, P is at its minimum height. (Here I am counting the 'minimum height' as being the physically lowest point which is reached).

The same logic can be applied to any other point in time. For example, when P is on the equilibrium line, but moving downwards, R is also on the equilibrium line, but moving upwards. This again indicates a phase difference of 180 degrees because moving either point by 180 degrees in its cycle would make the points perfectly in phase with each other.