# Waves-displacement vs movement

1. Feb 11, 2013

How can a particle move downwards and be displaced upwards? Sounds paradoxical. I understand the part about movement but not displacement. What's the difference? Isnt displacement like the amplitude so when a particle in a transverse wave move down doesnt it mean its displaced downward?

2. Feb 11, 2013

### Simon Bridge

The same way a skiier can be displaced a long way up a mountain but be moving (skiing) down it.
You have already met this sort of thing in your work on Newton's laws of motion.

When you displace a string in the +y direction, and let go, which direction does it move in? What physical quantity describes "movement"?

3. Feb 11, 2013

I'm sorry i still dont get the skiing part too :( hmm movement the physical qty is distance??

4. Feb 11, 2013

### Simon Bridge

for movement, the physical quantity is "momentum" but I'd accept "velocity".
for position, the physical quantity is "distance", I'd accept "displacement".

something can have a positive displacement and a negative velocity if it is headed back to the origin.
have you not covered Newton's laws yet? kinematics? v-t graphs?

5. Feb 11, 2013

That's right - in a wave of form $y(x,t)=A\sin k(x-vt)$ each point x will be oscilating about y=0 as $y(t)=A\sin \omega t$ the plot will give you a position-time graph. The velocity time graph is the derivative of this: $v(t)=\omega A \cos \omega t$ ... if you plot them above one another (so the time axis coincides) you'll see the relationship.