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

RP77

krisityfer wrote: »

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

You could lose 42 marks and come out with 89.5% which would then be rounded up to 90% for your A1!

BrendaN_f

RP77 wrote: »

You could lose 42 marks and come out with 89.5% which would then be rounded up to 90% for your A1!

lol, that's not how it works.

anything less than 360/400 won't be an a1

Cardor

Percentages don't get rounded off on the leaving cert..

TopOfTheRight

Surely the answer they are looking for on the merry-go-road question is that the person wont move at all, showing your understanding of friction, what happens after that is irrelevant imo

deiseguuy93

TopOfTheRight wrote: »

Surely the answer they are looking for on the merry-go-road question is that the person wont move at all, showing your understanding of friction, what happens after that is irrelevant imo

no you have that completely wrong!! it says if the was no friction (meaning restricting friction 50N) then the answer would be that seen as there was no friction acting inwards then the person would move outwards!!

what.to.do

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.

I thought it was a short enough paper!
I did an extra question and I still left at noon.

My tummy started rumbling, figured I'd done enough anyway. :P

BL1993

I can confirm that the answer for q.7 part (a) is 0.25 kg and 69.19 J per kelvin

what.to.do

TopOfTheRight wrote: »

Surely the answer they are looking for on the merry-go-road question is that the person wont move at all, showing your understanding of friction, what happens after that is irrelevant imo

What about inertia though?
I didn't do the question, so I'm really not sure.

Alcohol_MAN

Can anyone just breakdown key bits of 7c.

I said that the difference in temp is directly proportional to the voltage and drew a shawdy little sketch graph of the curve but didn't give the theapparatus to use. I.e thermometers, beakers etc. I did draw a vague diagram of a thermo couple with an unlabelled voltmeter. Looking at a possible 10 marks I think :-/

Geog ariphic

Alcohol_MAN wrote: »

Can anyone just breakdown key bits of 7c.

I said that the difference in temp is directly proportional to the voltage and drew a shawdy little sketch graph of the curve but didn't give the theapparatus to use. I.e thermometers, beakers etc. I did draw a vague diagram of a thermo couple with an unlabelled voltmeter. Looking at a possible 10 marks I think :-/

Like that but simpler. Simple put ice in one beaker, a heat source under the other (literally red arrow up saying heat source), just a circle with a V in it for the voltmeter, make sure its obvious they are two different metals
Only need the junctions in the two liquids, not all four.
A thermometer in each beaker is probably necessary to get the marks aswell.

szybki

Does anyone have LC physics and chemistry book for sale ?

Mr. Maths

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

The friction is acting as the centipetal force, so without that the child would move away from the center and fall off. It's not a common sense question, you had to apply your knowledge of circular motion.

Gumbi

RP77 wrote: »

You could lose 42 marks and come out with 89.5% which would then be rounded up to 90% for your A1!

No, lol. The cut off point for an A1 is 90%. 89.5 is not 90%. therefore, no A1. I think it's a bit ridiculous to expect an A1 when you don't meet the criteria.

TopOfTheRight

Mr. Maths wrote: »

The friction is acting as the centipetal force, so without that the child would move away from the center and fall off. It's not a common sense question, you had to apply your knowledge of circular motion.

Centripetal force only applies to a body moving with circular motion though right? So if there is no friction between the child and the merry-go-round when it begins to move it will have no affect on him, he might as well be floating above it

magicianz

TopOfTheRight wrote: »

Centripetal force only applies to a body moving with circular motion though right? So if there is no friction between the child and the merry-go-round when it begins to move it will have no affect on him, he might as well be floating above it

Yeah the only reason that the merry-go-round affects the child is because of friction. With no friction, no movement

electrictrad

magicianz wrote: »

Yeah the only reason that the merry-go-round affects the child is because of friction. With no friction, no movement

Friction is the force which opposes motion. . .the reason the child doesnt fall off the merry-go-round is because of friction. . .but the merry-go-round still exerts a force on the child, as the child is standing on the merry-go-round. . .think about it, when you stand motionless on ice (almost no friction) the ground exerts a force equal and opposite to your weight. . .but if you move, and have little speed, you will fall. . .

. . .so the child will fall off the merry-go-round. . .and will do so at a tangent to the merry-go-round. . .as a ball at the end of a rope would if released from circular motion. . .

BL1993

electrictrad wrote: »

Friction is the force which opposes motion. . .the reason the child doesnt fall off the merry-go-round is because of friction. . .but the merry-go-round still exerts a force on the child, as the child is standing on the merry-go-round. . .think about it, when you stand motionless on ice (almost no friction) the ground exerts a force equal and opposite to your weight. . .but if you move, and have little speed, you will fall. . .

. . .so the child will fall off the merry-go-round. . .and will do so at a tangent to the merry-go-round. . .as a ball at the end of a rope would if released from circular motion. . .

I already had a big discussion with Geo and we concluded that the boy does not move at all. This friction you speak of is the centripetal force. The question says that if there is no friction i.e. no centripetal force, then what direction would he go. By newton's 1st law, the boy does not move because the resultant forces acting on the boy is zero. However, if the disc moves and the boy doesnt then the velocity of the boy relative to the merry-go-round is directed in the opposite direction in which the merry-go-round is spinning.

electrictrad

BL1993 wrote: »

I already had a big discussion with Geo and we concluded that the boy does not move at all. This friction you speak of is the centripetal force. The question says that if there is no friction i.e. no centripetal force, then what direction would he go. By newton's 1st law, the boy does not move because the resultant forces acting on the boy is zero. However, if the disc moves and the boy doesnt then the velocity of the boy relative to the merry-go-round is directed in the opposite direction in which the merry-go-round is spinning.

The boy doesn't move when the disc is stationary. . .that much is true. . .

But when the disc moves, the boy cannot remain stationary, because the floor beneath him is moving. . .the floor is like a piece of string from circular motion, and the boy is the bung or ball at the end of the string. . .the friction was the force connecting them, and now it's gone. . .

BL1993

electrictrad wrote: »

The boy doesn't move when the disc is stationary. . .that much is true. . .

But when the disc moves, the boy cannot remain stationary, because the floor beneath him is moving. . .the floor is like a piece of string from circular motion, and the boy is the bung or ball at the end of the string. . .the friction was the force connecting them, and now it's gone. . .

yes, but in your case, the string is already mvoing, in this case, the disc starts from rest, i.e the friction did not exist to begin with. In theory, the disc should just move under his feet.

larkin1993

electrictrad wrote: »

Friction is the force which opposes motion .

Friction opposes relative motion between two objects. If there is no friction there is no force keeping the boy from moving relatively to the disc, so when the disc starts to spin there is no force causing the boy to move with it.

electrictrad

BL1993 wrote: »

yes, but in your case, the string is already mvoing, in this case, the disc starts from rest, i.e the friction did not exist to begin with. In theory, the disc should just move under his feet.

I see your point. . .and if the ground were moving straight, i would agree with you. . .but this is a circular motion question, and as such there are other forces acting on the child. . .for example, the bit directly above it, the force acts on the child independant of friction. . .friction is just the force which opposes motion between the child and the floor. . .as soon as it starts to move, mv^2/r comes into effect, and off goes the child. . .

BL1993

No no. Mv^2/r is the friction. Centripetal force is just a nmae for a resultant force which in this case is the friction. Think about it, if you swing a string tied to a mass in a circle, then the force towards the centre is the tension, i.e. the centripetal force. In this case, if the merry go round moves, the boy tends to move away from the circle, but the friction i.e the centripetal force prevents him from doing so. I'm sure it says it somewhere in the book but just take note that centripetal force is not a real force, just a resultant.

Turtwig

BL1993 wrote: »

In this case, if the merry go round moves, the boy tends to move away from the circle, but the friction i.e the centripetal force prevents him from doing so. .

So no friction would mean he'd fly off?

Geog ariphic

BL1993 wrote: »

I already had a big discussion with Geo and we concluded that the boy does not move at all. This friction you speak of is the centripetal force. The question says that if there is no friction i.e. no centripetal force, then what direction would he go. By newton's 1st law, the boy does not move because the resultant forces acting on the boy is zero. However, if the disc moves and the boy doesnt then the velocity of the boy relative to the merry-go-round is directed in the opposite direction in which the merry-go-round is spinning.

We also concluded this is not the answer the SEC will give, at least in their first marking scheme, because of the way the question was asked.
We're just saying they are wrong xD

Geog ariphic

Malty_T wrote: »

So no friction would mean he'd fly off?

No, his weigth keep him down, and the result of the horizontal force from the disc keeps him up. Friction doesn't keep us stuck to the ground - in most cases, we can think of friction as a 'flat' force, only acting if there is motion between two surfaces (and any two REAL surfaces will experience friction between them). However this is no friction, i.e. not real. The kid will not be start moving when the disc does, as only friction from the disc would pull him along. Since there is no friction, it's like he's hovering - the disc just moves under him. He will feel neither centripedal, centrifugal of any other forces relating to motion, circular or otherwise.

electrictrad

Geog ariphic wrote: »

We also concluded this is not the answer the SEC will give, at least in their first marking scheme, because of the way the question was asked.
We're just saying they are wrong xD

So, we'll probably get the marks for it. . .

:D:D. . .I'll declare myself satisfied with that. . . .you can be as technically correct as you like. . .:D:D:D

Turtwig

Geog ariphic wrote: »

No, his weigth keep him down, and the result of the horizontal force from the disc keeps him up. Friction doesn't keep us stuck to the ground - in most cases, we can think of friction as a 'flat' force, only acting if there is motion between two surfaces (and any two REAL surfaces will experience friction between them). However this is no friction, i.e. not real. The kid will not be start moving when the disc does, as only friction from the disc would pull him along. Since there is no friction, it's like he's hovering - the disc just moves under him. He will feel neither centripedal, centrifugal of any other forces relating to motion, circular or otherwise.

Friction can be many things, but that's mostly irrelevant here. In the question you are given friction of 50N acting on the child. This is all that's relevant. Remove this from your system and there's only going to be one result. Once the merry go round starts to move the child will be "pushed" outward tangent to the motion of the merry go round. The force of friction is all that was keeping him travelling in a circle. To put it another way, the child does not move the merry go round does and as a result the child is no longer on it.:D

The only other option is to assume the child is not part of the system in the first place. Which is very difficult to do when the question tells you there is a child standing on it. Friction or no friction, when you're inside something that is moving in a circular motion it always appears as if there is something pushing you out of that something you're inside.

deiseguuy93

I asked my teacher and he said that they meant the friction in the question not all friction in the world! Meaning that you disregard the 50N of friction! and without this friction the child will move out from the center of rotation.

and if you were disregarding all friction in the question then the merry go round would not move at all!

Geog ariphic

I may not have a great grasp of circular motion, but what unbalanced force pushes him outwards once the thing starts moving? Centrifugal, obviously, but how does it act on him? What actually causes the child to move? Saying it was 'The circular motion' isn't good enough for me, becasue there is no friction so in my 'applied maths mind' (ps we didnt do circular motion in that xD) there is nothing to start him moving.

I mean in my mind's eye he'd start moving outwards, but mathematically i have no evidence for this, so i trust the maths.

irish_man

With all this talk i'm glad i didn't do question 6... :P