particle velocty ocr physics g482

June 2013- OCR Physics A G482 question 4d- why cant you use v=f*lambda because isn't the wave velocity same as the particle velocity.?

The question is not about wave velocity. It doesn't say it specifically, but I'm pretty sure the question is about the velocity that the point W moves perpendicular to the direction of the actual wave - it's just a question about a point on the string rather than about wave velocity, so it's really just a logic question.

Paper and mark scheme for those interested:

http://www.ocr.org.uk/Images/175458-question-paper-unit-g482-01-electrons-waves-and-photons.pdf

http://www.ocr.org.uk/Images/176278-mark-scheme-unit-g482-electrons-waves-and-photons-june.pdf

But wouldn't the wave velocity equal the particle velocity because its the movement of particles that causes the wave to move so shouldn't it be the same speed?

The questions is asking about the speed that the point W is moving up and down. None of the points on the cord are actually moving along the cord, which is what the wave velocity formula calculates - they're all just moving up and down (on the diagram), and they give you the speed that the point W has at 0 seconds.

The question isn't about wave or particle velocity.

but just in general is wave velocity equal to particle velocity.

In general, I think they describe different things. I don't actually know the OCR Physics spec (I do AQA), but I don't think they'd be the same, no.

Wave velocity is the speed that a point on a wave (not a particle in a wave; that's the wrong way to think about it) moves in the direction of the wave's propagation.

Particle velocity is the speed that a particle is moving at in a direction, in the same way that a ball or a vehicle has a velocity.

Using the De Broglie wavelength formula, you can work out the wavelength of a particle, but I don't think you can work out the wave's velocity from that, because it's not the same thing. They could equate if you work it all out, but I can't say any more than I have because I don't actually know lol :P

In my opinion, just don't try and combine the two when they describe different things that are easily dealt with by looking at the problem in context, and applying your normal methods to them.

ok thanks and could also try and answer my other post on January 2011 8c. Thanks.