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# Physics A2 help (simple harmonic motion)

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1. I'm stuck on a question from the aqa physics year 2 textbook and i need some help on it.

heres the question:
An object oscillates in simple harmonic motion with an amplitude of 12mm and a time period of 0.27s. Calculate its:

b) its displacement and direction of motion
(ii) 0.20s after its displacement was +12mm

I got the displacement right but i don't understand how i got the direction of motion wrong.
So i wrote that the displacement is -0.7mm towards the maximum negative displacement but how is it towards the maximum positive displacement? Clearly the object came from its maximum positive displacement, so how is it its going back towards maximum positive displacement again??? Shouldn't it move towards the maximum negative displacement?

someone pls surprise me by proving me wrong. thanks
2. When you work out the new displacement, that doesn't tell you anything about which direction it is moving in. Is it at -0.7 mm on the way out or the way back? The value itself doesn't tell you.

A complicated way to do this would be to work out its velocity at the same moment and see if it is positive or negative. But I'm pretty sure the expressions for this are not in this version of the A Level.

Think about it like this - you know the period of the oscillation. If the object is released from rest at it's positive amplitute, then one period later, it will be back in the same place. For the first half of the period, it will be heading towards the negative amplitude, and for the second half of the period it will be heading back again towards the positive amplitude. Now - how far through the period is the time you have been given?
3. (Original post by Pangol)
When you work out the new displacement, that doesn't tell you anything about which direction it is moving in. Is it at -0.7 mm on the way out or the way back? The value itself doesn't tell you.

A complicated way to do this would be to work out its velocity at the same moment and see if it is positive or negative. But I'm pretty sure the expressions for this are not in this version of the A Level.

Think about it like this - you know the period of the oscillation. If the object is released from rest at it's positive amplitute, then one period later, it will be back in the same place. For the first half of the period, it will be heading towards the negative amplitude, and for the second half of the period it will be heading back again towards the positive amplitude. Now - how far through the period is the time you have been given?
Ah yes I get it now. What the question was simply telling you was that after it reaches maximum positive amplitude (+12mm), the object would already be facing the maximum positive amplitude after 0.20s because of the time period of the oscillation. So for example if it reaches maximum negative displacement from maximum positive displacement, the time would be half the time period which is around 0.135s . When it is at maximum negative displacement, the object would change directions and continue until t = 0.20s facing the maximum positive displacement but right now it is t = 0.135s.

thanks for the help i understand it now.

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