Shm (1 Viewer)

[yeh]

with SHM, do you start with x=asin...... and differentiate your way down or start with the acceleration equation and integrate your way up. Or can you just use the equations:

acceleration = -n^2x
velocity = n^2(a^2 - x^2)
displacement = asin(nt + alpha)

 

[yeh]

for example: A particle moves in simple harmonic motion with n = 2. If its rest position is distant 4m from the origin, find its velocity through the origin.

Do you start with the displacement equation and differentiate to get the velocity.

or

Can you just go to the velocity equation and make a straight sub?

 

[esther]

iif u can sub in the no. using the formula u can use the formula instead of diff x....
well thats wot we do at skool

 

[Minai]

in addition, depending on how much the question is worth, i'd make sure u can derive the velocity = n^2(a^2 - x^2) from acceleration, i.e from a= d/dx(1/2v^2), rather than just knowing the formula

 

[yeh]

alright. thanx for your help peoples! i just wanted to know if it was similar to projectile motion. coz with that, you need to start with acceleration and integate.

yeh i've got the derivation of acceleration to velocity in my notes but i didn't know why it was there! i gotta pay attention in class more...

 

[Big Willy]

Yeah, u have to derive.
Its 3unit man - everything in 3unit u must know how to derive - even circle geometry.
You just cant state that this angle is such and such according to so and so theorm.

 

[Nelly]

Exactly, they could ask you to prove the theorom: "Angle at the centre is twice the angle at the circumference, standing on the same arc" So you can't just recite to formula.

 

[Minai]

im growing less confident about actually finishing the 3U paper...

 

[Nelly]

You don't need to finish the 3U paper, just make sure you get everything right that you know.

 

[Lazarus]

My exam mark last year for Maths Ext 1 was 46/50 and I didn't answer any of Q7 - I certainly didn't answer every other question correctly either.

 

[yeh]

and they scaled you up so that you just lost 4 marks? i hope 4u's like that!

 

[Morgues]

Its all a ranking though
so just because he lost marks, means most lost more marks then Lazarus

 

[Nelly]

Hexactly, if someone constantly comes ranked 1st for the entire year, even if he gets say 70%, he will still get 50/50 in the report

 

[Lazarus]

The top mark isn't always set at 100%, the student who comes first in the state may not necessarily get 50/50.

It's usually fairly close though.

 

[Minai]

yeah, so if u wanna get good marks, within your class, its a good idea to help one another out after the trials, to ensure everyone in your class does well, which pushes your mean up, therefore a higher range of moderated assessment marks
(is that right?)

so u were ranked first at school for 3U lazarus?

 

[Lazarus]

For 3u maths?

I was ranked first out of the guys that were just doing 3u, but fourth when you included the guys doing 4u. My moderated assessment mark was only 41/50 because of that ranking (the rest of the class weren't so great at maths). The top exam mark for our class (which had about 23 students) was 47.

 

[Nelly]

In our school, if someone comes 1st in all the assessment tasks for whatever course, they get 100% no matter what mark they actually get, and everyone else gets marked accordingly.

 

[Lazarus]

Not bad.

 

[Nelly]

Yeah it's a pretty good system, however, no-one comes first for all the subjects.

 

[Morgues]

but marks dont matter so its just for looking good on a report