The diagram below shows a wave on a string with particles P, Q, R, S and T on the wave. The wave is moving in the direction as shown.
a) What can you say about the motion of the particles the next moment?
b) What can you say about the motion of the particles at this instant?
a) P: down, Q: up, R: up, S: down, T: downb) P: at rest, Q: up, R: at rest, S: down, T: down
b) Take note of P and Q at crest and trough respectively. Hence at this particular instant, P is at its highest point and R is at its lowest point, hence both momentary at rest.
View video tutorial to learn how to draw the wave in the next moment in order to better visualise.
At one glance, both questions seem to be the same. But in actual fact, they are asking about different thing.
Firstly, you have to know that the wave above is transverse wave (wave in which the direction of the vibration is perpendicular to the direction of wave motion). Hence, particles will only vibrate up and down only.
For (a), the question asks about the motion of particles the next moment. You can consider it as in the coming next few seconds.
For (b), it asks about the motion of particles at this particular time as shown.