Maths Extension 2 thoughts (1 Viewer)

apurba · Oct 21, 2015

Librah said:
lmao, i like how for the probability of q16 b. pretty much gives you the answer to a.

I worked that out in the last 2 minutes. I ended up getting (i)
but I didn't get enough time to do (ii) when it was the same thing again ahah

Physicklad · Oct 21, 2015

InteGrand said:
$It's looking like in the banked turn question, $\mu$ is to be treated as a variable (a function of the speed), which confused me for a bit, because normally $\mu$ is a constant (a coefficient of friction).$

EXACTLY they fucked up the physics real hard i actually wrote an essay in the exam telling them how badly they fucked up

Librah · Oct 21, 2015

That balloon q is a mid difficulty 3u q at best lol

InteGrand · Oct 21, 2015

Physicklad said:
EXACTLY they fucked up the physics real hard i actually wrote an essay in the exam telling them how badly they fucked up

$They should have made it clearer, maybe by writing $\mu(v)$ and $\mu (V)$ and stating it's a function of the speed. But somehow I'm guessing most people weren't confused, since no-one's mentioned it yet I think. Maybe because most people don't pay much attention to the physics behind it.$

$Edit: actually, they should have avoided $\mu$ if they wanted to use a variable representing the ratio of the friction at that speed to the magnitude of the normal force; they should have just used something like $k(v)$ and $k(V)$ (letter $k$ instead of $\mu$). Because their $\mu$ isn't really related to the coefficient of friction, which has conventional symbol $\mu$.$

Carrotsticks · Oct 21, 2015

Physicklad said:
EXACTLY they fucked up the physics real hard i actually wrote an essay in the exam telling them how badly they fucked up

InteGrand said:
$It's looking like in the banked turn question, $\mu$ is to be treated as a variable (a function of the speed), which confused me for a bit, because normally $\mu$ is a constant (a coefficient of friction).$

Where did you guys get the idea of mu being a variable from?

InteGrand · Oct 21, 2015

Carrotsticks said:
Where did you guys get the idea of mu being a variable from?

Well if it's the coefficient of friction, then little v would have been the maximum speed the car can travel without slipping up (i.e. a fixed speed for the given radius). Unless we are told that V is this speed (which we weren't), why are we letting v = V to do the question?

$So basically, just because at speed $V$ there's a tendency to slip upwards, that doesn't mean that the frictional force is $\mu N$ (if $\mu$ is constant (note $N$ is constant, given by $\frac{mg}{\cos \theta}$)). This would imply that any speed that has a tendency for the car to slip up has frictional force $\mu N$, which is constant if $\mu$ is constant. But actually, the frictional force should vary with the speed.$

Physicklad · Oct 21, 2015

Carrotsticks said:
Where did you guys get the idea of mu being a variable from?

no its not a variable. but i was thrown off by the equation we had to prove because in physics the static coefficient time the normal represents the maximum frictional force that can be provided, so the v they used in the first part was clearly the 'breaking point' past which the car would slide off the track. but then we had to use a different value of v in the next part but still with the same equation. THATs wrong. i ended up just gritting my teeth and believing their bullshit in the end

deboiz · Oct 21, 2015

OH HAHAHA yeah mu is a variable, because it's static friction - the friction only exists as much as needed to keep it from moving. But you didn't need to know that realllly to solve the quesiton.

Mikes · Oct 21, 2015

Would a 71 align to 85, considering the test was easier than previous years?

Physicklad · Oct 21, 2015

deboiz said:
OH HAHAHA yeah mu is a variable, because it's static friction - the friction only exists as much as needed to keep it from moving. But you didn't need to know that realllly to solve the quesiton.

dude. how is mu a variable? you of all people should know

Yeungster · Oct 21, 2015

Librah said:
That balloon q is a mid difficulty 3u q at best lol

HELL YEAH. had to flip back to the booklet cover to make sure i was doing the right test xD

tywebb · Oct 21, 2015

I have combined beanbean's and WhoStanLeee's into 1 pdf now:

deboiz · Oct 21, 2015

Physicklad said:
dude. how is mu a variable? you of all people should know

bro wellllllll we might be getting into definitions here, what i'm saying is that frictional force varies with speed HAHAH you can say that muN is the max friction and that would mean its not a variable LOL

P: P:

InteGrand · Oct 21, 2015

deboiz said:
OH HAHAHA yeah mu is a variable, because it's static friction - the friction only exists as much as needed to keep it from moving. But you didn't need to know that realllly to solve the quesiton.

Coefficient of static friction is a constant for two given surfaces. If you knew about this, I think the question as given could confuse you.

What you are referring to by ''the friction only exists as much as needed to keep it from moving'' is the frictional force; yes, this depends on the speed. I think the question was just interested in the ratio between the friction (variable depending on speed) and the normal force. This is not what coefficient of friction is, so it would have been best to avoid μ as the symbol in my opinion.

Physicklad · Oct 21, 2015

deboiz said:
bro wellllllll we might be getting into definitions here, what i'm saying is that frictional force varies with speed HAHAH you can say that muN is the max friction and that would mean its not a variable LOL

P: P:

good boy

Jacquischafer · Oct 21, 2015

Mikes said:
Would a 71 align to 85, considering the test was easier than previous years?

I would have thought 71 would definitely be up there in band 6. Do people agree?

Librah · Oct 21, 2015

Mikes said:
Would a 71 align to 85, considering the test was easier than previous years?

nah should be around 87-90

deboiz · Oct 21, 2015

Physicklad said:
good boy

wait but in the q didn't they just always assume static friction, not moving kinetic friction? so like you didn't need to know there was two types of friction to do it anyway ahh

mrpotatoed · Oct 21, 2015

the complex number stuff had to be all graphed on the same diagram right? the z, u and v stuff

Librah · Oct 21, 2015

Librah said:
nah should be around 87-90

Probably only just 90, but prepare for the worst lol

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