# SHM helpWatch

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#1
Its question 2di on this doc

http://fdslive.oup.com/www.oup.com/o....pdf?region=uk

Im really confused as to what it means by a negative cos graph because i thought the kinetic energy couldn't be negative would someone be able to draw it out for me? also why is the amplitude 0.6mJ not 1.2mJ?
any help would be great
0
4 weeks ago
#2
If you read the mark scheme it says a negative cos graph oscillating about 0.6mJ ie translated up by 0.6mJ so the graph will always be positive. The amplitude is defined as the maximum displacement from the mean position - in this case the mean position is 0.6mJ so if the amplitude is 0.6mJ you still get a maximum of 1.2mJ. Hope this helps
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#3
omg thank you also the answer says that there are two oscillations on the graph why is that because i swear on the graph thats in the question theres only one oscillation
(Original post by in a fish bowl)
If you read the mark scheme it says a negative cos graph oscillating about 0.6mJ ie translated up by 0.6mJ so the graph will always be positive. The amplitude is defined as the maximum displacement from the mean position - in this case the mean position is 0.6mJ so if the amplitude is 0.6mJ you still get a maximum of 1.2mJ. Hope this helps
0
4 weeks ago
#4
Odd - I think the markscheme is horribly wrong.
The original graph is velocity, so KE should look like velocity squared - not a cos graph, but a sin squared graph. As such, it'll never go negative and it will have two peaks (one at t = 0.3 one at t = 0.9s).
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#5
the thing is its from an exam paper so idk?
(Original post by old_teach)
Odd - I think the markscheme is horribly wrong.
The original graph is velocity, so KE should look like velocity squared - not a cos graph, but a sin squared graph. As such, it'll never go negative and it will have two peaks (one at t = 0.3 one at t = 0.9s).
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4 weeks ago
#6
Markscheme is strangely wrong - if I'd noticed before I'd complained, 2008 is a bit long ago though!
I'm a bit pedantic when it comes to Physics, I like things to be correct, and I would be very ashamed of that answer - even though I know they'd wave their hands and say 'cosine style'. I expect the person who wrote the answer has retired now, so we should probably move on!
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