What does this mean? SHM.

[MedPR]

For any pendulum or spring in simple harmonic motion:

The displacement is 90deg out of phase with velocity and 180deg out of phase with the acceleration

Isn't velocity 0 when displacement is maximal? And if velocity is 0, isn't acceleration 0? So why wouldn't they both be 180deg out of phase?

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[chiddler]

For any pendulum or spring in simple harmonic motion:


Isn't velocity 0 when displacement is maximal? And if velocity is 0, isn't acceleration 0? So why wouldn't they both be 180deg out of phase?

Nono when it is at either maximum, then velocity is 0 because acceleration is pointing in the opposite direction of velocity.

Like,

Object moving
o
...o (slowing down)
.......o (slowing down)
...........o (maximum)

ask yourself: which way is velocity? (to the right)
if it's slowing down, which way must acceleration be? (to the left)

So even if velocity is 0, when at maximum, acceleration is to the left so that it can reverse directions and continue SHM

I've not heard of it worded like that, "out of phase 90 degrees." I guess it makes sense superficially, but, if you can, can you explain what is meant by that?

 

[MedPR]

Nono when it is at either maximum, then velocity is 0 because acceleration is pointing in the opposite direction of velocity.

Like,

Object moving
o
...o (slowing down)
.......o (slowing down)
...........o (maximum)

ask yourself: which way is velocity? (to the right)
if it's slowing down, which way must acceleration be? (to the left)

So even if velocity is 0, when at maximum, acceleration is to the left so that it can reverse directions and continue SHM

I've not heard of it worded like that, "out of phase 90 degrees." I guess it makes sense superficially, but, if you can, can you explain what is meant by that?

Can't explain it. I saw it on wikipremed :/

I understand what you wrote about velocity and acceleration though, thank you. I guess velocity and displacement are 90 out of phase because when displacement is maximal, velocity is minimal (0). If you consider a sin or cos graph, everytime 90deg (pi/2 radians) you alternate between a maximum and a minimum. So if I'm at a max and you are at a min, you and I are 90deg out of phase. I guess then I understand also why acceleration is 180deg out of phase. As the displacement reaches its maximum in the rightward direction it is at one of the maxima, lets say a crest. Since acceleration is pointing in the opposite direction, but also at a maxima, it must be at a trough.

Does that make sense?

 

[chiddler]

Yes! I was not thinking of it in terms of sin/cos graphs. This was drilled a lot in my physics class and I understand this well.