Advanced higher physics help please? thanks :)

could someone please explain question 4a)ii) and 4b) from here
http://mrmackenzie.wikispaces.com/file/view/AHpastpaper2001.pdf

the answers are here

http://mrmackenzie.wikispaces.com/file/view/01miPhysicsAH.pdf

thankyou v much in advance I really appreciate any help :smile:

Reply 1

Joinedup

Original post by ah4p

4b is just restating the requirement for SHM - a restoring force proportional to displacement from the equilibrium point.
f=-kx

if the force is zero when x is not zero then you can't have SHM

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do you get given formula sheets for scottish exams?

if not you'll just have to remember them...

for circular and SHM
ω=2πf

for SHM
speed=2πf sqrt(A^2 - x^2)

since max speed occurs at x=0

max_speed=2πf A = ωA

(can't attach a picture of the formula today :frown:

)

Reply 2

ah4p

Original post by Joinedup

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thank you very much for replying :smile:

I have a prelim on Wednesday and I'm very worried :/

I thought it was just me who was having problems uploading pictures :P

I still don't really understand 4b because there's no froe mentioned in the question?

we do get a formula sheet :biggrin:

how would you sketch the graph of velocity?

Thanks again :smile:

Reply 3

Joinedup

Original post by ah4p

thank you very much for replying :smile:

how would you sketch the graph of velocity?

Thanks again :smile:

force is mass x acceleration
constant velocity means acceleration = 0
therefore there is no force in those parts of the motion where velocity is constant

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The instantaneous velocity is the gradient of the displacement time graph... so if the displacement graph looks like a cosine, the velocity graph looks like a negative sine graph

(if doing any calculus you might notice something about the above but if not, don't worry about it)

Reply 4

ah4p

Original post by Joinedup

Ah ok so does that mean objects performing simple harmonic motion are never at a constant speed they're always accelerating? (Even neglecting air resistance)

Haha yes I do
I'm doing advanced higher maths too :frown:

But can you assume that even if you're not specifically told its a cosine graph?

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Reply 5

ah4p

Original post by Joinedup

Oh wait you're supposed to know the displacement of an object performing is SHM is always sinusoidal

Is that right?

Thank you :smile:

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Reply 6

Joinedup

Original post by ah4p

But can you assume that even if you're not specifically told its a cosine graph?

Posted from TSR Mobile

it's pretty clear the displacement graph starts at +1A with a gradient of 0 so to my way of thinking it's cosine... the first thing the gradient does is turn negative
your dv graph needs to start from 0 and turn negative

but in general for SHM you could get the zeroes and max/min points and join them up with an approximately sinusoidal shaped line.

Reply 7

ah4p

Original post by Joinedup

thank you
yeah that's a good method to try :smile:

A d.c circuit consists of a switch, lamp and inductor.
The inductor is replaced with an inductor with twice the number of turns in the coil of wire, nothing else about it is changed.
State the effect on
A) maximum current B) the time to reach max current

I thought increasing the turns would increase the current
and also increase the time taken since the back emf would also be greater

but the answer is
max current decreases
and time to reach it increases

can anyone explain why?

for question 8b)
http://mrmackenzie.wikispaces.com/file/view/AHpastpaper2006.pdf
answers are here

http://www.sqa.org.uk/files_ccc/06miPhysicsAH.pdf
I don’t understand what they’ve done in the answers because I thought
back emf = supply voltage – voltage across inductor
so from the graph I used when 6V is across inductor the back emf is 3V and dI/dt is 8 (from graph)
then I used back emf = -L dI/dt
subbing in the values above I got L = 0.375H

but the answers have just used values straight from the graph – when 9V is across the inductor then dI/dt = 12 and subbed these in
I don’t understand this

any help is appreciated

thanks v much in advance :smile:

(edited 9 years ago)