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# Official Thread For OCR Physics A G484 Jan 2011 Watch

1. (Original post by ViralRiver)
Okay.. I just did a past paper question, asked to work out the force using 2nd law. I got -61.1N. Would I lose the mark for having a negative force, as I took a different direction as positive?
Probably not because force is a vector, but i'd leave it as positive to be safe if in future because it isn't usually written as negative on mark schemes.
2. Would someone please explain WTF

demonstrate N2
3. Sorry about that i meant demonstrate NEWTONS 2ND LAW = N2
I have just been using N2 as i cant be bothered to write it out.
4. (Original post by Ralphus J)
Sorry about that i meant demonstrate NEWTONS 2ND LAW = N2
I have just been using N2 as i cant be bothered to write it out.
I'd say use a linear air track. Use the two light gates and the size of the card to find the velocity, and do the same for the second. From this, find the acceleration. You can do it multiple times and plot a graph, or just check using F=ma to see if the numbers work out.

Make sense?
5. Nice one Hob thats what ive done!

Experiments ive learnt:

Demonstrating Newtons 2nd
Specific heat capacity of LIQUID and SOLID
Fusion experiment (with ice)
Latent heat of vaporisation (with kettle)

Any more i should know perhaps i know i need to do Brownian which i will come to tonight .. hopefully.
6. Can someone explain to me the differences between kinetic theory and ideal gas thing? I don't know which is which >< .
7. Could someone post definitions of Temperature and heat?
8. Stuck on this simple question :

A Cannon of mass 500kg fires a shell of mass 2.5kg. The cannon recoils with a velocity of 2.4m/s, Calculate the initial velocity of the shell.
9. (Original post by ViralRiver)
Can someone explain to me the differences between kinetic theory and ideal gas thing? I don't know which is which >< .
Kinetic Theory is just a model to show how gas molecules move and that. Main things you need to know is :

* The assumptions of the kinetic theory model
* How a gas exerts a pressure e.g. on the walls of a container

Assumptions

* Gravitational Force is Negligible
* Gas molecules move in rapid, random motion
* The collisions between molecules, and walls are elastic.
* No intermolecular force exists except during collisions.
* The total volume of the molecules is negligible compared to the total volume of the container.

REMEMBER GAS HAS THE SAME VOLUME OF THE CONTAINER, AS MOLECULES ENGAGE IN CHAOTIC MOTION. IT IS A FLUID, AND IS COMPRESSIBLE, THAT'S WHY BOYLE'S LAW EXISTS, AS YOU EXERT PRESSURE, A GAS'S VOLUME DECREASES.

Ideal Gas

An Ideal Gas is a gas where the internal energy is only in the form of kinetic energy.

Internal Energy is the total sum of random distributions of kinetic and potential energies in molecules of a substance.

The minimum Internal Energy a substance has is at Absolute Zero @ -273.15 kelvin.
10. Can someone explain the following (from spec):

I understand this, but can't 'explain' it:
explain that a force perpendicular to the
velocity of an object will make the object
describe a circular path
describe how a mass creates a gravitational
field in the space around it
explain that close to the Earth’s surface the
gravitational field strength is uniform and
approximately equal to the acceleration of
free fall
describe a simple kinetic model for solids,
liquids and gases
define the term pressure and use the kinetic
model to explain the pressure exerted by
gases
state the basic assumptions of the kinetic
theory of gases
If you can help me with these, +rep coming your way <3 .
11. (Original post by bijesh12)
Stuck on this simple question :

A Cannon of mass 500kg fires a shell of mass 2.5kg. The cannon recoils with a velocity of 2.4m/s, Calculate the initial velocity of the shell.
Conservation of momentum, I presume.
(2.4 x 500) + (2.5v) = 0 (because the initial momentum was 0)

I hope that's right..
12. 1) A Force that is perpendicular the velocity of an object, will cause a the object to follow a circular path. Take a car around a bend, the centripetal force in this cause is the friction between the tyres and the ground, and it is always directed towards the centre of the circle. (Hard 1 to explain)

2) A mass creates a gravitational field in the space around it, because a mass in the gravitational field of the earth will feel a force, known as weight to us, this is directed towards the centre of the earth, and the Force (Weight) will pull the mass down to earth. Basically a force is experienced by the mass, due to the gravitational field of the earth.

3) At the earth's surface g.f.s. is uniform, if u drop a mass of 1 kg close from the earth's surface it will have a force of 9.81 N, and the g.f.s of the mass will also be 9.81 N/ kg. This is identical to the acceleration due to gravity.

4)Pressure is the force exerted per unit area. If you have the official OCR textbook read, the relevant pages.

I already stated the kinetic model for gases.
13. (Original post by WackieWatty)
Conservation of momentum, I presume.
(2.4 x 500) + (2.5v) = 0 (because the initial momentum was 0)

I hope that's right..
lol nope its 480 m/s in the opposite direction to the cannon not a clue why. I also worked it out that way, but its not right
14. (Original post by bijesh12)
Kinetic Theory is just a model to show how gas molecules move and that. Main things you need to know is :

* The assumptions of the kinetic theory model
* How a gas exerts a pressure e.g. on the walls of a container

Assumptions

* Gravitational Force is Negligible
* Gas molecules move in rapid, random motion
* The collisions between molecules, and walls are elastic.
* No intermolecular force exists except during collisions.
* The total volume of the molecules is negligible compared to the total volume of the container.

REMEMBER GAS HAS THE SAME VOLUME OF THE CONTAINER, AS MOLECULES ENGAGE IN CHAOTIC MOTION. IT IS A FLUID, AND IS COMPRESSIBLE, THAT'S WHY BOYLE'S LAW EXISTS, AS YOU EXERT PRESSURE, A GAS'S VOLUME DECREASES.

Ideal Gas

An Ideal Gas is a gas where the internal energy is only in the form of kinetic energy.

Internal Energy is the total sum of random distributions of kinetic and potential energies in molecules of a substance.

The minimum Internal Energy a substance has is at Absolute Zero @ -273.15 kelvin.
Everything there is spot on but you mean 0 Kelvin and -273.15 Celsius lol
15. There is a syllabus statement saying

" describe how there is an absolute scale of temperature that does not depend on the property of any particular substance( ie the thermodynamic scale and the concept of absolute 0)"

16. (Original post by bijesh12)
Kinetic Theory is just a model to show how gas molecules move and that. Main things you need to know is :

* The assumptions of the kinetic theory model
* How a gas exerts a pressure e.g. on the walls of a container

Assumptions

* Gravitational Force is Negligible
* Gas molecules move in rapid, random motion
* The collisions between molecules, and walls are elastic.
* No intermolecular force exists except during collisions.
* The total volume of the molecules is negligible compared to the total volume of the container.

REMEMBER GAS HAS THE SAME VOLUME OF THE CONTAINER, AS MOLECULES ENGAGE IN CHAOTIC MOTION. IT IS A FLUID, AND IS COMPRESSIBLE, THAT'S WHY BOYLE'S LAW EXISTS, AS YOU EXERT PRESSURE, A GAS'S VOLUME DECREASES.

Ideal Gas

An Ideal Gas is a gas where the internal energy is only in the form of kinetic energy.

Internal Energy is the total sum of random distributions of kinetic and potential energies in molecules of a substance.

The minimum Internal Energy a substance has is at Absolute Zero @ -273.15 kelvin.

Can you do one for Solid and Liquid i never know what to put for that, i would much appreciate it
17. (Original post by bijesh12)
lol nope its 480 m/s in the opposite direction to the cannon not a clue why. I also worked it out that way, but its not right
Momentum at start = momentum at end
Momentum at start = (500+2.5)kg x 0ms-1
Momentum at end = (500 x -2.4) + 2.5v

(500 x -2.4) + 2.5v = 0

2.5v = 1200
v = 480ms-1
18. (Original post by bijesh12)
lol nope its 480 m/s in the opposite direction to the cannon not a clue why. I also worked it out that way, but its not right
The answer I got was 480m/s in the opposite direction (-480m/s due to velocity being a vector quantity)

2.4 x 500 = 1200
1200 + (2.5v) = 0
1200 = -2.5v
v = 1200/-2.5
v = -480

Another way of looking at it, I suppose, is that momentum acting is both directions must be equal, so the initial equation would be:
(2.4 x 500) = (2.5 x -v)
19. Thanks .

What are examples of free oscillations, since the book says you can't actually have free oscillations in practice? xD
20. (Original post by Ralphus J)
There is a syllabus statement saying

" describe how there is an absolute scale of temperature that does not depend on the property of any particular substance( ie the thermodynamic scale and the concept of absolute 0)"

I'm not entirely sure what they could ask you on that.

The centigrade is found using the ice point and boiling point of water, then you stick one hundred divisions in between the two points. However, due to possible impurities in the ice/water and the fact that the boiling point is pressure dependent, this isn't an absolute scale.

Absolute zero is the temperature at which a substance has the minimum internal energy possible. This is the zero for the absolute thermodynamic scale, because it can only ever be that one temperature: -273.15 degrees Celsius.

I was talking to my teacher about the temperature scales earlier, he agrees that they're a stupid addition to the syllabus.

(Original post by ViralRiver)
Thanks .

What are examples of free oscillations, since the book says you can't actually have free oscillations in practice? xD
A cymbal rattling - but only after it's been struck! - or a child on a swing that isn't being pushed are the two I remember.

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