* Physics HL 2011 * HL predictions / discussion / aftermath * (1 thread only please)

JMcG92

Hey guys. I was really happy with the paper today! The particle physics question (involving 140 MeV) made me think the most, but once I got out the tables and jumbled around a few figures it seemed to do itself.

shunter07

hey guys.. that merry go round q made me think.. if there is no friction im guessing the answer they wer looking for is that the kid would fly out bang there head and die.. or what i wrote.. a friend of mine said that he wrote the child would just stay while the thing spins beneath him.. my reply to this was the kid would break his legs off those bars in the picture. what do you think the peril of this kid was?? broken skull or legs?!

ElvisChrist6

B_Fanatic wrote: »

The fcuk? I thought the paper was a little harder than usual, but nothing crazy like maths paper 1. But Q7 A, A metal spoon is then placed in the hot drink causing the temperature of the hot drink to drop... What the hell is the mass of the spoon???

This was my problem also!

LilMissCiara

ElvisChrist6 wrote: »

This was my problem also!

even my teacher couldn't get that, but somebody on here said it wasn't Specific heat capacity that they wanted and just heat capacity.

owenthorp7

For a thermocouple would it be ok to put down the increase in tempeture causes an increase in resistance experiment? And i didnt get the mass of the spoon because it only said heat capacity and could anyone honestly see a spoon with a mass of 2kg

electrictrad

One quick Q, did anyone get Fmax=0.05N for i) of 12a

Gyaradose

stealinhorses wrote: »

I wrote that Millikan was known for calculating the mass of the electron (which I remembered reading in my chemistry book). Any chance that they will accept charge or mass in the marking scheme? :P

Overall, I thought the paper was difficult in parts. I didn't get the final calculation in the particle physics question and tripped up on some other things, but it was OK in general.

The experiment questions were really handy though, as straight-forward as you will get.

Since F1 already has newtons in it the force 4/3F1 is in terms of newtons already. Writing 4/3f1 N would give you the answer with the unit wing newtons squared which is incorrect

Gyaradose

electrictrad wrote: »

One quick Q, did anyone get Fmax=0.05N for i) of 12a

a friend and I both got 0.2 m/s2

electrictrad

Gyaradose wrote: »

a friend and I both got 0.2 m/s2

?

ride-the-spiral

Geog ariphic wrote: »

Unfortunately, untrue.

F in this case is standing for some unknown constant (i think you'd call that a variable), however in the equation in the log tables it is a symbol, in which case units are implied.

If 'F' stood only for the concept of 'Force' then newtons would be implied. However, it is standing for some specific value of force, a number, therefore the units are required.


Well you gave a second attempt, with the correct answer, and the corrector must give the marks for the best attempt.
So, i think you get away with it......just this once

You don't need the to put in the units, because you're expressing one force as a multiple of another. In word form, you would say that F1 is 4/3 times larger than F2, therefore F1=4/3F2. It's just as valid as saying X is 4 times heavier than Y, or longer or whatever. Units not necessary.

Gyaradose

electrictrad wrote: »

?

My humblest apologies, 0.2 newtons

electrictrad

shunter07 wrote: »

hey guys.. that merry go round q made me think.. if there is no friction im guessing the answer they wer looking for is that the kid would fly out bang there head and die.. or what i wrote.. a friend of mine said that he wrote the child would just stay while the thing spins beneath him.. my reply to this was the kid would break his legs off those bars in the picture. what do you think the peril of this kid was?? broken skull or legs?!

The Biology Thread is about 4 threads down as I post, perhaps this question would be more relevant there. . .

I said he'd go off at a tangent to the circle. . .If you wanted a physics element to the aftermath, perhaps suggest that he hit another child, either producing pair annihilation, or producing a new child from the K.E.. . .

Geog ariphic

B_Fanatic wrote: »

[.....]But Q7 A, A metal spoon is then placed in the hot drink causing the temperature of the hot drink to drop... What the hell is the mass of the spoon???

LilMissCiara wrote: »

even my teacher couldn't get that, but somebody on here said it wasn't Specific heat capacity that they wanted and just heat capacity.

Reading the question: that was key to this exam. Correctly and thoroughly, i mean.

Physics_2011_SEC wrote:

A metal spoon, with an initial temperature of 20degreesC is then placed in the hot drink, causing the temperature of the hot drink to drop to 68degreesC. What is the heat capacity of the spoon? You may ignore other possible heat transfers.

For a second i was like wtf is, there's no mass for this spoon..then there was no specific heat capacity for any metal either, so an idea came to me, i checked my Tables to be double sure, and there it was, right next to specific heat capacity: Heat Capacity big c (C).
Energy to change temp of a body of some unknown mass, by 'd' degrees in difference, is = C*d



Note, d is 'delta theta' in log tables, i just cant write it here

Gyaradose

Am I the only one who though that without friction the merry go round would move independently of the child? I.e. it would move under him so nothing would happen, it will move he will not. Seems to be too simple to be true

LilMissCiara

Gyaradose wrote: »

Am I the only one who though that without friction the merry go round would move independently of the child? I.e. it would move under him so nothing would happen, it will move he will not. Seems to be too simple to be true

I thought he would move forward..

conorod

What was the principle protecting the person in the full-body metal foil suit from the high voltage power lines?

Gyaradose

conorod wrote: »

What was the principle protecting the person in the full-body metal foil suit from the high voltage power lines?

charge always resides on the outside of a conductor therefore he cannot be harmed.

Digits

Gyaradose wrote: »

charge always resides on the outside of a conductor therefore he cannot be harmed.

Nicest question in the paper IMO.

Geog ariphic

Gyaradose wrote: »

Am I the only one who though that without friction the merry go round would move independently of the child? I.e. it would move under him so nothing would happen, it will move he will not. Seems to be too simple to be true

Didn't do it, but just read it there... if it were possible to have zero friction, then he wouldn't move. Simple as.
They should no way have said 'i what direction' would the child move.

Digits wrote: »

Nicest question in the paper IMO.

Easy but long...writing out two demos/exprmnts? Took too long
For ppl who couldn't digure out A, or got it wrong, there were far too many marks going for the work involved. It was about 5 or 6 lines, - 18 marks?? (minus coulombs law, that still minimum gonna be 12 marks).

owenthorp7

I think everyone has to admit the new log tables are great

Digits

Geog ariphic wrote: »

Easy but long...writing out two demos/exprmnts? Took too long
For ppl who couldn't digure out A, or got it wrong, there were far too many marks going for the work involved. It was about 5 or 6 lines, - 18 marks?? (minus coulombs law, that still minimum gonna be 12 marks).

Not really, I took time to look over the second part of a, gave lengthy answers and a good few diagrams and still had time for an extra question.

BL1993

I'd just like to clarify something for q.6, part c. The centripetal force is in fact the force of friction so all the diagram was was a horizontal arrow towards the centre of the circle for this force, and two vetrical arrows in opposite directions acting as the weight and reaction. If the force of friction stopped working, the boy would fall off the merry-go-round at a tangent to the direction of the force of friction.

Geog ariphic

Digits wrote: »

Not really, I took time to look over the second part of a, gave lengthy answers and a good few diagrams and still had time for an extra question.

Took too long for that one question. I didn't run out of time my any means.

Geog ariphic

BL1993 wrote: »

I'd just like to clarify something for q.6, part c. The centripetal force is in fact the force of friction so all the diagram was was a horizontal arrow towards the centre of the circle for this force, and two vetrical arrows in opposite directions acting as the weight and reaction. If the force of friction stopped working, the boy would fall off the merry-go-round at a tangent to the direction of the force of friction.

But the question specifically asks/states that if there was no friction to begin with, what direction would the boy go as the merry-go-round starts to rotate. He was never moving, in the straight line (tangent) or otherwise, so how could he fall off (excluding those leag-breaking bars)?

MLM1

How the f**k do you do Q.12 (A) by the physics course? I got it out perfect but I had to use differential equations which are applied maths. I said

{between x and y} means the integral with limits x and y

v dv/ds= -16s
{between 0 and v}v dv= -16 {between 0.05 and 0} ds
v*2/2 {0 and v}= (-16)(0.05)
v*2= 1/25
v= 0.2 ms*-1

this gave me the frequency after a bit of fooling around but there must be an easier way. And will I get the marks for this like it is perfectly valid mathimatics and shows a huge understanding of the topic IMO. The equations of motion are no use for a variable acceleration and this is the only way I could think of to do it.

BL1993

@Geog: Sorry, I mis-read the question there. I think it is safe to say that he will not remain on the merry-go-round. The only thing I could say that until the merry-go-round starts moving, the kid will not move. When it starts moving, he will start moving or accelerating at a tangent to the circular path and will fall off in that direction. It is the only thing covered on the course so it seems like the only possible answer. I know what you mean but thats the only way i see around the question.

@MLM1: a=16s=w^2s so w=4 and then you use the t=2pi/w and get the reciprocal for the frequency However, you should get full marks because the marking scheme shows the most common way of solving it. There are always other ways to solve thewse type of questions and you will get full marks.

Geog ariphic

BL1993 wrote: »

@Geog: Sorry, I mis-read the question there. I think it is safe to say that he will not remain on the merry-go-round. The only thing I could say that until the merry-go-round starts moving, the kid will not move. When it starts moving, he will start moving or accelerating at a tangent to the circular path and will fall off in that direction. It is the only thing covered on the course so it seems like the only possible answer. I know what you mean but thats the only way i see around the question.

Ah but your trying to accomodate presumed conditions of the question (i.e. to find some direction), i'm trying to think of it in terms of real life (ehm....zero friction.yah...).
Say he's on a smooth-ish disc, that is free to rotate, and is at rest. Lets says he's wearing roller blades and the whole thing covered in an oil slick (thats as close as we'll get i'd say!). It starts moving...there is no friction to pull him along with it. And due to his inertia (Newton), he won't move.
There are zero unbalanced forces acting on him, therefore he will not move.

Unless the icecream man comes along, then he'll try to fun, slip sideways and crack his head on a bar and smash his nose on the disc. He'll cry, mommy will come over, wipe his oily schnoz before getting some icecream.

The unbalanced force was the force of attraction between the fat kid and the food, which is directly proportonal to the mass of the food and the sweetness of the food, and inversely proptional to how much effort it'd take him to get it.:D

Brian123M

I said the child travels backwards pure guess , was 50:50 with that and sayin it would travell towards the centre due to the centripical force

Dongl

Gyaradose wrote: »

Am I the only one who though that without friction the merry go round would move independently of the child? I.e. it would move under him so nothing would happen, it will move he will not. Seems to be too simple to be true

I said this, then mentioned the handlebars would hit him/her and they'd be knocked forward.

Honestly I'm discounting most of those 23 marks ;') sure you cna lose 46 and still get the A1

krisityfer

Paper's 400 marks, so you can only lose 40 for an A1 I thought?