# Homework Help: Phase difference of 2 points in a same graph

1. Jun 7, 2014

### kelvin macks

1. The problem statement, all variables and given/known data

since point P is ahead of point O, why the displacement , y , of P shouldnt be Asin(wt + 2pi/(x)) ?

2. Relevant equations

3. The attempt at a solution

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2. Jun 7, 2014

### Jilang

Be careful, the graph is against x not t.

3. Jun 8, 2014

### kelvin macks

sorry, stil dont unseratand . can you explain further?

4. Jun 8, 2014

### Staff: Mentor

Think of the figure as a snapshot photo of an ocean swell.

Imagine O and P are corks floating on the ocean, and they rise and fall as the wave gently passes. At the moment captured in the photo, cork O is falling and crossing the zero line. How far must the wave move along (sliding along to the right) before cork P in turn finds itself falling through the zero line?

So P is lagging O by ....?

5. Jun 8, 2014

### kelvin macks

so are you assuming falling downward dispalcement is positive? floating upward dispalcement is negative since you said P is lagging behind by O. how do we know which direction is positive and which is negative (falling downward or floating upward) ?

Last edited: Jun 8, 2014
6. Jun 8, 2014

### Staff: Mentor

Defintely not assuming anything. I described how you could see things were you to imagine this as an ocean swell; I used an analogy.

You are told that movement is described as A.sin(ωt), so that's where you might choose/assume '+' direction if it's not explicitly indicated.

Any and every motion of O will be emulated at a time Δt later by a point which lags O by that amount of time, regardless of what that instantaneous motion of the sinusoid may be. Whatever O is doing will be copied by P a short time later.

Last edited: Jun 8, 2014
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