# Question regarding wave motion

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1. Jun 27, 2015

### toforfiltum

http://www.elateafrica.org/elate/physics/waves/traqnsversewaves.jpg
Note that :

• Leaf. A has attained maximum displacement and is about to start going down .

• Leaves B, C and D are still going up. Each of them will finally attain its maximum displacement and then move downwards to complete the cycle.

• Leaf E is at the lowest displacement and it is about to start going upwards.

• Leaves F, G, and H are still going downward and so on.
I have a problem with point 2 and 4. How can leaves B,C and D go up when the crest is at point A? I thought that in a wave, there are only certain points in waves that are crests and troughs, and these are separated by some distance called wavelengths? If so, how could there be so many crests, at leaves B,C, and D, and so many troughs at F, G and H? Could someone point out my fault in my understanding?

2. Jun 27, 2015

### phinds

You are likely looking at this as a standing wave, but it's a moving wave like and ocean wave. The crests (and troughs) are moving to the right so for example, as the crest marked A moves to the right, the thing bobbling at point B rises.

3. Jun 27, 2015

### toforfiltum

So, to help me picture this, the whole wave motion is constantly moving to the right or in other words, the leaves are constantly travelling through the wave to the left?

4. Jun 27, 2015

### theodoros.mihos

For moving to the right wave equation is $y(x,t) = A\sin(\omega{t}-kx)$ and the opposite is $y(x,t) = A\sin(\omega{t}+kx)$. Make some graphs to see this.

5. Jun 27, 2015

### phinds

yes
no, the leaves are just moving up and down. Do you have trouble picturing how an ocean wave works?

6. Jun 27, 2015

### toforfiltum

Uhm,so the leaves are constantly moving up and down while travelling through the shape of the wave to the left?

7. Jun 28, 2015

### Staff: Mentor

In this simplified picture the leaves only move up and down. However the pattern of the up and down motion gives rise to a 'wave' that travels to the right. You can look at one of the crests and simply pan your view to the right at the same speed as the crest to follow it without ever having to move your eyes up or down.

8. Jun 28, 2015

### phinds

Have you never seen something bobbling on an ocean wave? It doesn't change horizontal position at all, just bobbles up and down.

9. Jun 28, 2015