OCR Physics A G484 - The Newtonian World - 11th June 2015 Watch

BecauseFP
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Just reading over the spec, what are people saying for "describe how a mass creates a gravitational field in the space around it"?
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buxtonarmy
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(Original post by sagar448)
Use that equation when the pendulum or whatever is in SHM is released from the max amplitude.
Thank you, what sort of questions would involve it though? Would you be calculating displacement then?
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sagar448
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(Original post by buxtonarmy)
Thank you, what sort of questions would involve it though? Would you be calculating displacement then?
Well since it's released from max amplitude it can ask you to calculate anything from displacement to frequency. (All the variables present in the equation can be asked to be calculated)

Don't worry though something like that hasn't come up, and if it does it won't be too complicated.
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Hilton184
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(Original post by AwesomeSauce#1)
I still dont get it :cry2:
A question like that better not come up.
Don't worry it took me a good 2 days to figure it out haha. It's really strange. And don't worry, everyone can afford to drop marks on a question like that even when aiming for max ums.


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Leechayy
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(Original post by BecauseFP)
Just reading over the spec, what are people saying for "describe how a mass creates a gravitational field in the space around it"?
I don't know relate it to the formulae F= GMm/r^2 and g=F/m?
So:

> gravitational force acts between two objects of distance r
> objects are everywhere at varying distances
> So gravitational forces act radially
> gravitational field strength = force per unit mass
> So mass creates gravitational field in space all around it.

This is probably wrong and its pretty much a 'blagged' answer. But maybe you could expand on this

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Elcor
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(Original post by Hilton184)
Okay so your satellite is going over the Earth from pole, to equator, down to other pole, back up other side of earth to equator, and back to starting pole.

Photograph width is 3000km.

Circumference of Earth is greatest at equator. At the pole there is a lot of overlap between adjacent segments. As you move down to equator, there is less and less overlap between the photographed segments. At the equator your segments are just touching without overlap, because the circumference is greatest at the equator.
Hence the circumference of the earth at the equator divided by width of photograph gives you the number of times you need to go over the equator.

And as I explained above, each orbit passes over the equator twice, so you half your answer to get the number of orbits needed.


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But why are the segments touching? I feel like this is a perfect scenario, where in reality you'd most likely get gaps between the equator-segments.
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Hilton184
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(Original post by Elcor)
But why are the segments touching? I feel like this is a perfect scenario, where in reality you'd most likely get gaps between the equator-segments.
Yeah it is a perfect scenario I believe. Like the circumference at the Earth won't be exactly 2*pi*6400km, because our radius won't be exactly 6400km. The photograph width won't be exactly 3000km. There's always some percentage error.


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BecauseFP
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Just to check, when do we use the words "atoms", "molecules" or "particles"?

For gas laws, it seems that particles are interchangeable, but for stuff like the meaning of n in pV=nRT, I've seen OCR punish the use of "particles" but allow "atoms" or "molecules".

Can someone explain?
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Elcor
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(Original post by BecauseFP)
Just to check, when do we use the words "atoms", "molecules" or "particles"?

For gas laws, it seems that particles are interchangeable, but for stuff like the meaning of n in pV=nRT, I've seen OCR punish the use of "particles" but allow "atoms" or "molecules".

Can someone explain?
Particles can mean smoke particles which are made of billions of molecules

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[email protected]
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(Original post by BecauseFP)
Just to check, when do we use the words "atoms", "molecules" or "particles"?

For gas laws, it seems that particles are interchangeable, but for stuff like the meaning of n in pV=nRT, I've seen OCR punish the use of "particles" but allow "atoms" or "molecules".

Can someone explain?
When OCR decide you can
Cant remember papers but it was the same question worded differently but one allowed particles and a few years later particles wasnt allowed
Same issue with atoms and nuclei sometimes you can use both, sometimes only on is accepted
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0lds0uls
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(Original post by buxtonarmy)
Can anyone explain when to use x=Acos(2pi ft) equation please??

Thanks
Only just sussed this myself, literally when i saw someone reply use when released from max amplitude

I think its because if you were to plot Displacement - Time, and you released from max amplitude then it follows a cosine curve

So similarly if something is initially at 0 displacement and you push it into SHM then its following a sinusoidal curve
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0lds0uls
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(Original post by BecauseFP)
Just to check, when do we use the words "atoms", "molecules" or "particles"?

For gas laws, it seems that particles are interchangeable, but for stuff like the meaning of n in pV=nRT, I've seen OCR punish the use of "particles" but allow "atoms" or "molecules".

Can someone explain?
Kinetic theory is molecules, Anything nuclear size is atoms, anything to do with something that isn't in the nucleus and you cant definitively say is a single atom and is relatively large would be a particle

For example
water molecule, but not atom because it is made of more than one single atom and not particle because its not big enough to be a particle

Oxygen atom however this is perhaps when Molecule would be allowed as mark as Oxygen naturally exists in pairs, but still not a particle

Smoke particle because you can easily see it with a conventional microscope so its too big to classify as an atom, too big to be a molecule plus smoke can be any sort of collection of atoms so its easier to just quantify as a smoke particle

That's my reasoning anyway, could be wrong but I hope it helps you
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BecauseFP
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^Seems to work.

For Brownian motion, can someone explain to me why in June 2014 they only accepted "Molecules (of the liquid) are in random / haphazard motion (AW)" and "Molecules (of liquid) are smaller than pollen grains" but earlier they required you to mention at least one of "smoke particles are continuously moving because the air molecules are continuously moving"or "small movement of smoke particles is due to the large numbers of air molecules hitting from all sides" (which would not have been accepted in June 2014)?

Is there something special about the questions that I'm not seeing? It's Q6ai of June 14 and Q5aii of Jan 11.

EDIT:
I'm tempted to ask the same about the ideal gas assumptions question, Q6bi.

In June 11, they had (5aii, emphasis theirs):
"Any 3 from:
Volume of particles negligible compared to volume of vessel OR molecules much smaller than distance between them
No intermolecular forces acting (other than during collisions) OR molecules only have kinetic energy (and no PE)
Particles travel in straight lines/at uniform velocity between collisions OR force of gravity on molecules is negligible time of collisions much smaller than time between collisions
gas consists of a large number of molecules moving randomly (both needed for the mark)"
(the elastic thing was given in the question)

But in June 14 they had (emphasis theirs, my comments in []):
"Any three from:
Collisions with the walls/container/sides are elastic
[...I would've just said "all collisions are perfectly elastic", would that get me the mark?]
Force between molecules is negligible / zero except during collisions
[for a reason I can't divine, "molecules only have kinetic energy (and no PE)" is no longer accepted]
Volume of the molecules is negligible compared to the volume of the container (AW)
[they've mysteriously switched between molecules and particles, again - the emphasis is NOT mine, they actually underlined it in both schemes]
Time within a collision is negligible compared to time between collisions"

Notice particularly that "Particles travel in straight lines/at uniform velocity between collisions OR force of gravity on molecules is negligible" and "gas consists of a large number of molecules moving randomly" are both no longer accepted.

Can anyone explain this?

EDIT 2: I'm a fool for not reading the next paragraph of the examiner's report. Here's what they had, though it doesn't fully answer my question:
"Given that the random motion of the molecules had been established in (a) it was decided not to include this in the list of critical assumptions detailed in the markscheme. The mark scheme required specific references to the 'only collisions'considered at this level and comparisons in the references to volumes and times.Candidates were not penalised for giving extra details or additional assumptions.As a result the question discriminated well with the more able candidates scoringat least 2 marks while most were able to score 1 mark"
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Elcor
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(Original post by BecauseFP)
x
Basically OCR don't even know what they're doing themselves mate.

Bare in mind that we're probably too hard on ourselves and mark very strictly to the mark scheme, while the examiner is a human being with a brain (and probably has a degree in physics or a science subject), so they'll understand what you mean if you phrase things a bit differently. Just hope your paper is the one they mark immediately after treating themselves to a cup of coffee or a glass of wine.
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0lds0uls
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(Original post by BecauseFP)
^Seems to work.

For Brownian motion, can someone explain to me why in June 2014 they only accepted "Molecules (of the liquid) are in random / haphazard motion (AW)" and "Molecules (of liquid) are smaller than pollen grains" but earlier they required you to mention at least one of "smoke particles are continuously moving because the air molecules are continuously moving"or "small movement of smoke particles is due to the large numbers of air molecules hitting from all sides" (which would not have been accepted in June 2014)?

Is there something special about the questions that I'm not seeing? It's Q6ai of June 14 and Q5aii of Jan 11.

EDIT:
I'm tempted to ask the same about the ideal gas assumptions question, Q6bi.

In June 11, they had (5aii, emphasis theirs):
"Any 3 from:
Volume of particles negligible compared to volume of vessel OR molecules much smaller than distance between them
No intermolecular forces acting (other than during collisions) OR molecules only have kinetic energy (and no PE)
Particles travel in straight lines/at uniform velocity between collisions OR force of gravity on molecules is negligible time of collisions much smaller than time between collisions
gas consists of a large number of molecules moving randomly (both needed for the mark)"
(the elastic thing was given in the question)

But in June 14 they had (emphasis theirs, my comments in []):
"Any three from:
Collisions with the walls/container/sides are elastic
[...I would've just said "all collisions are perfectly elastic", would that get me the mark?]
Force between molecules is negligible / zero except during collisions
[for a reason I can't divine, "molecules only have kinetic energy (and no PE)" is no longer accepted]
Volume of the molecules is negligible compared to the volume of the container (AW)
[they've mysteriously switched between molecules and particles, again - the emphasis is NOT mine, they actually underlined it in both schemes]
Time within a collision is negligible compared to time between collisions"

Notice particularly that "Particles travel in straight lines/at uniform velocity between collisions OR force of gravity on molecules is negligible" and "gas consists of a large number of molecules moving randomly" are both no longer accepted.

Can anyone explain this?

EDIT 2: I'm a fool for not reading the next paragraph of the examiner's report. Here's what they had, though it doesn't fully answer my question:
"Given that the random motion of the molecules had been established in (a) it was decided not to include this in the list of critical assumptions detailed in the markscheme. The mark scheme required specific references to the 'only collisions'considered at this level and comparisons in the references to volumes and times.Candidates were not penalised for giving extra details or additional assumptions.As a result the question discriminated well with the more able candidates scoringat least 2 marks while most were able to score 1 mark"
My go to assumptions are always
collisions between molecules and container walls are elastic
no intermolecular forces between molecules except during collisions
volume of molecules negligble to volume of container
gas is a large number of molecules in rapid random motion

for me theyre the critical ones because theyre the ones that either link to one of newtons laws during the deravation of the equation or they're what differentate microscopic Kinetic Theory to Macroscopic Kinetic Theory

I guess the trick is just to look what they asked in the previous question and what is mentioned in the current question, then write what is left and then add the others if you have time at the end and the paper doesnt ask for specific number of assumptions. I'd still personally use molecules over particles because of the fact thats what used on the latest markscheme and that june 2014 is a strict scheme in my opinion so it will either be slightly more lenient this year or on par
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BecauseFP
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(Original post by 0lds0uls)
My go to assumptions are always
collisions between molecules and container walls are elastic
no intermolecular forces between molecules except during collisions
volume of molecules negligble to volume of container
gas is a large number of molecules in rapid random motion

for me theyre the critical ones because theyre the ones that either link to one of newtons laws during the deravation of the equation or they're what differentate microscopic Kinetic Theory to Macroscopic Kinetic Theory

I guess the trick is just to look what they asked in the previous question and what is mentioned in the current question, then write what is left and then add the others if you have time at the end and the paper doesnt ask for specific number of assumptions. I'd still personally use molecules over particles because of the fact thats what used on the latest markscheme and that june 2014 is a strict scheme in my opinion so it will either be slightly more lenient this year or on par
Fair enough, thoughts on the Brownian motion thing?
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0lds0uls
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(Original post by BecauseFP)
Fair enough, thoughts on the Brownian motion thing?
I think the first marking point is AW which means you can word it however you want as long as it says something similar to whats in the mark scheme, so i guess as long as you mentioned the word random with whatever explanation for that you should be alright

I think for the second point its to do with the fact it's in a liquid, there wont be a large number of molecules hitting the pollen grain because the liquid is still resticted to molecules sliding around eachother close together, so it'll be more like a few molecules hitting the pollen grain over and over because they can only slide around bumping into the thing next to them rather than zip around banging off everything and anything as they can in a gas in which it would be hit by a lot of molcules because of how much energy they have. The fact that they mention size is also to do with the size (or mass) of the molecules must be smaller than the particle or thing being bumbarded regardledss of gas or liquid so the impulse of the particle as a result of bombardment doesnt average out to 0, which it does as the size of the molecules increases

took me a while to sus out, requires more thinking than the alleged 2 marks OCR want to award for it haha, I hope that explains it
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BecauseFP
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(Original post by 0lds0uls)
I think the first marking point is AW which means you can word it however you want as long as it says something similar to whats in the mark scheme, so i guess as long as you mentioned the word random with whatever explanation for that you should be alright

I think for the second point its to do with the fact it's in a liquid, there wont be a large number of molecules hitting the pollen grain because the liquid is still resticted to molecules sliding around eachother close together, so it'll be more like a few molecules hitting the pollen grain over and over because they can only slide around bumping into the thing next to them rather than zip around banging off everything and anything as they can in a gas in which it would be hit by a lot of molcules because of how much energy they have. The fact that they mention size is also to do with the size (or mass) of the molecules must be smaller than the particle or thing being bumbarded regardledss of gas or liquid so the impulse of the particle as a result of bombardment doesnt average out to 0, which it does as the size of the molecules increases

took me a while to sus out, requires more thinking than the alleged 2 marks OCR want to award for it haha, I hope that explains it
Thanks, can anyone confirm this?
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Chung224
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guy's i've ran out of stuff to do. done every past paper and leaving june 2014 till nearer to the exam, what do you guys do?
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Mr_Cupcakes
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Probably been asked a million times but has anyone got the 2014 papers and mark schemes?
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