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# Need help with a question regarding SHM

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1. I am stuck on part di) of this question where it asks to draw a curve of KE/time

The things I want to clarify:
1. When it refers to negative cosine graph, although it doesn't specify, I assume the curve should be above/touching the x axis (i.e. doesn't go below) because energy is a scalar quantity?
2. Why is the amplitude 0.6mJ? (as oppose to 1.2mJ)

Any help is appreciated. Thanks in advance!

(I've attached the graph, question and mark scheme)
Attached Images

2. but it does say "negative cosine wave touching the x axis"

what the rest of it means it means is
Minimum KE = 0 (at the points where it touches the x axis)
Maximum KE = 1.2 mJ
Median KE = 0.6 mJ

the amplitude is the magnitude of the difference between the extremes and the median (which is not necessarily zero)... so amplitude is 0.6 mJ
3. (Original post by Spectral)
I am stuck on part di) of this question where it asks to draw a curve of KE/time

The things I want to clarify:
1. When it refers to negative cosine graph, although it doesn't specify, I assume the curve should be above/touching the x axis (i.e. doesn't go below) because energy is a scalar quantity?
2. Why is the amplitude 0.6mJ? (as oppose to 1.2mJ)

Any help is appreciated. Thanks in advance!

(I've attached the graph, question and mark scheme)
Look at the diagram closely. There are two identical springs. When one is in compression, the other is in extension. The elastic P.E. must therefore be shared equally in both.

The energy of the system must be conserved and is +ve when the P.E. is maximum (K.E and velocity are both zero) and P.E is zero when the K.E. and hence velocity of the mass is maximum. i.e. it follows a -ve (inverted) cosine wave. The displacement of the mass w.r.t. the rest position (central) swings +ve to -ve.
4. (Original post by Joinedup)

the amplitude is the magnitude of the difference between the extremes and the median (which is not necessarily zero)... so amplitude is 0.6 mJ
Thank you for clearing up my misunderstanding of what amplitude actually refers to!

(Original post by uberteknik)
The energy of the system must be conserved and is +ve when the P.E. is maximum (K.E and velocity are both zero) and P.E is zero when the K.E. and hence velocity of the mass is maximum. i.e. it follows a -ve (inverted) cosine wave. The displacement of the mass w.r.t. the rest position (central) swings +ve to -ve.
Thanks, I was unsure how having two springs would affect the energy transfers but you helped clear that up!

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