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# Offcial Physics A thread for the G484 Jan Exam (Newtonian World) watch

1. (Original post by LeaX)
omg thank you so much!
i owe you a rep but i've run out for today.

just wondering how do you do projectiles?
Projectiles - Think of it like Pythagoras and Trigonometry. This always makes things easy for me

So do Velocity (--> Horizontally) and solve, we assume there is no resistance always horizontally So V = S/t

And then do Velocity ( ^ Vertically Up/Down) and solve, usually a = 9.81 but you might have to resolve for Resultant Force (so use F=ma to find a). Then Use Suvat.

Then with Both Velocities solve it like its a Right angled Triangle. And you want the angle of the Vector/Line you haven't solves
2. (Original post by kanojyoxx)
In an exam, I believe it was the one with a jet-plane going in a vertical circle. It asked why the pilot felt weightless at the top of the vertical circle. That was the answer the mark scheme said.

You need to remember that unless something is pulling you or holding onto you (such as when you are on the London Eye, you have the metal bar holding onto you), there will still be a reaction force. A ball on the end of a string will feel tension from the string at the top.

If you are just moving around with nothing holding onto you, there will be no reaction force, therefore only weight provides the centripetal force.

I've updated my notes to mention the reaction force. Like I said, I might have missed some things out.

Edit: see June 2011
Finished copying these notes though I've done quite a few already but yours are much more clearly laid out! Thanks for this, hope Wednesday goes well for you!

Proper nervous :/
3. When they ask for mean force, is that (mv-mu/t) or (Ft)?
4. (Original post by Cinglis95)
When they ask for mean force, is that (mv-mu/t) or (Ft)?
I think they're the same thing as when you divide by the t on Ft you'll arrive at the same answer I'd assume as Impulse = change in momentum.
5. (Original post by Better)
Projectiles - Think of it like Pythagoras and Trigonometry. This always makes things easy for me

So do Velocity (--> Horizontally) and solve, we assume there is no resistance always horizontally So V = S/t

And then do Velocity ( ^ Vertically Up/Down) and solve, usually a = 9.81 but you might have to resolve for Resultant Force (so use F=ma to find a). Then Use Suvat.

Then with Both Velocities solve it like its a Right angled Triangle. And you want the angle of the Vector/Line you haven't solves
thank you
6. (Original post by kanojyoxx)
My notes - not sure if I have missed anything.

Spoiler:
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Newton's Laws of Motion
Definitions
Newton's First Law - a body remains in a state of constant rest or constant motion (constant velocity) unless it experiences a resultant force.

Newton's Second Law - the rate of change of momentum is proportional to the applied force and is in the same direction as the applied force.

Newton's Third Law - if body A exerts a force on body B, then body B exerts a force onto body A that is: equal, opposite, and of the same type.

Linear Momentum - product of mass and velocity

Impulse - the applied force multiplied by the time the force is applied for. This is equal to the change of momentum.

Principle of Conservation of Momentum - the total linear momentum of a system remains constant (doesn't change and is not lost) providing no external forces are acting on the system.

An Elastic Collision - total linear momentum remains constant (doesn't change and is not lost) and the total kinetic energy is not lost (remains constant and does not change into other forms.) The total energy is also constant (is not lost).

An Inelastic Collision - total linear momentum remains constant (doesn't change and is not lost) but the total kinetic energy is not conserved and some is lost into other forms (such as heat and sound) The total energy is also constant (is not lost).

1 Newton - is the force that gives a mass of one kilogram an acceleration of one metre per second per second.

Notes
In a question if you are asked to work out the force one body exerts on another, sometimes there is a question below asking what force that other body exerts on the first one - this is always equal to the force you worked out in the first question due to Newton's third law.

F=MA can be derived (using the assumption that mass is constant, and by replacing (v-u)/t with a and also using the definition of a newton to remove the constant of proportionality.

On a force/time graph if it's asking you to find the area underneath, only count the squares if you have enough time in the exam - you will not lose all of the marks by drawing a triangle, and this time could be best spent on a question with more marks.

If it asks you what is represented by the area under a force/time graph, make sure you say IMPULSE and not momentum. The area is not momentum, it is the change of momentum. One year they accepted momentum, then the next they only accepted impulse. Personally, I am going to write Impulse (change in momentum) just in case they are picky!

There is only an acceleration if there is a resultant force.

For an elastic rebound (such as an ideal gas) the change in momentum is always 2MV.

If you are in space or in a frictionless environment and you wish to change direction, throw something in the opposite direction and you will move in the direction that you want to move in.

Perfectly inelastic means the two bodies are now stuck together. So the new mass is the sum of the original two masses.

Momentum is a vector as velocity is a vector (even though mass is a scalar) - you can get negative momentum. This just means it is travelling in the opposite direction to what you take as positive.

A practical use of Newton's second law is in car safety. If you increase the time taken for the collision to come to a stop, you decrease the force applied due to the inverse proportionality (this is because the change in momentum is constant).

Momentum is always conserved due to Newton's Third Law - the force acting on both bodies is the same. Since it is the same time for both bodies in the collision, the rate of change of momentum (second law) is the same for both bodies therefore momentum is conserved. Kinetic energy might not be conserved as it might be lost (or converted into) to other forms like heat and sound. The TOTAL energy of a system is always constant due to the conservation of energy providing no external forces are acting.

If you are doing a question where it is talking about/hinting at Newton's Third Law and it is asking you why the two forces aren't the same when theoretically they aren't, remember there will be external forces acting that we haven't taken into account.

Circular Motion
Definitions
1 Radian - the angle subtended by a sector of a circle, with a radius of length R and an arc length also of length R.

Centripetal Acceleration - the rate of change of tangential velocity. Always acts towards the centre of the circle

Centripetal Force - the resultant force caused by centripetal acceleration. Always acts towards the centre of the circle.

Angular Frequency (omega, the little w) - the rate of change of angle. Measured in radians.

Notes
We get centripetal acceleration when the force acting on the body is perpendicular to the velocity.

Even if the body's speed is constant, when it is going around in a circle it's displacement is changing, therefore it's velocity is changing and it is accelerating. This acceleration is always towards the centre of the circle Make sure to use a ruler to do this - it will be an easy mark in a question if you have to draw it on a diagram.

You can use the idea behind vector diagrams from mechanics last year to work out resultant forces and therefore work out the centripetal force.

At the top of a vertical circle, the centripetal force = Reaction force + Weight.

At the bottom of a vertical circle, the centripetal force = Reaction force - Weight.

You feel weightless at the top of a vertical circle as your weight provides the required centripetal force. (There must be no reaction force)

Remember to use radians when doing circular motion questions. To work out how many radians a certain angle in degrees is: go into radians mode on your calculator; type in the angle in degrees and press equals; then press shift and you want to press "DRG>" (this is above the answer button on the Casio fx-85GT PLUS); then press number 1 (it is the circle that represents degrees) and press equals - you should now have the degree in radians. Same principle works in reverse for radians to degrees.

If you don't do maths, you should know that Sin(x)/Cos(x) = Tan(x). Might save you on calculations.

Gravitational Fields
Definitions
Gravitational field strength - force per unit mass acting on a point mass within the gravitational field.

Newton's law of gravitation - the force between two masses is directly proportional to the product of the masses and is inversely proportional to the distance between the masses squared.

Kepler's 3rd Law - the orbital time period of a body squared is proportional to the orbital radius cubed.

Density - mass per unit volume

Notes
At the Earth's surface, the gravitational field lines/strength seems uniform. So on a diagram this would be represented by straight lines. The value of this at Earth's surface is 9.81 Nkg-1

If it ever asks you why equations like mass * gravity * change in height are incorrect, it's because we aren't taking into account the fact that gravity also changes.

Geostationary satellites have the same time period as the body they are orbiting (in Earth's case this is 24 hours). They orbit the equator to ensure it always stays above the same point as the Earth. They are used in communications and in TV signals - you don't need to keep re-adjusting your TV arial because they are always above the same point.

If you are doing anything to do with gravity above a body's surface, remember to include the radius of the body if necessary.

You must be able to derive Kepler's Third Law as it is in the text book. Start from the newton's law of gravitation and incorporate centripetal force into it and the linear velocity formula you are given and just rearrange it.

Simple Harmonic Motion
Definitions
Free Oscillations - when a body which is given an initial disturbance is allowed to oscillate at it's natural frequency.

Displacement - distance from the equilibrium position in a given direction.

Amplitude - maximum displacement from the equilibrium position.

Time Period - the time taken to complete one full oscillation.

Frequency - the number of oscillations per second.

Phase Difference the difference, expressed in electrical degrees or time, between two waves having the same frequency and referenced to the same point in time.

Simple Harmonic Motion - when acceleration is directly proportional to displacement and is towards the equilibrium position. (Acceleration is in the opposite direction to displacement)

Damping - work done due to resistive forces transferring energy into other forms such as heat.

Critical Damping - damping the system so that the displacement is reduced quickly.

Resonance - occurs when the natural frequency of a body is close to/met by the frequency of another body that is driving it. This results in a maximum energy transfer between the driver and the driven, and a maximum amplitude for the driven body.

Notes
For SHM, a displacement/time graph and an acceleration/time graph are exactly the same except they are flipped in the x axis. For a velocity/time graph, remember to look at the gradient.

Remember, amplitude is independent to time period.

At the amplitude (positive and negative) there is a maximum acceleration and minimum velocity. At the equilibrium position, there is a minimum acceleration and a maximum velocity.

Good use of resonance: Cooking food in microwave ovens - the driver is the microwaves and the driven is the water molecules. Frequency of the microwaves is close to the natural frequency of the water molecules so they heat up quickly.

Problems with resonance: bridges shaking (driver is wind, driven is bridge), skyscrapers shaking (driver is vibrations in the ground, driven is skyscrapers)

When drawing a graph of amplitude/frequency showing resonance and damping has an effect, remember to move the amplitude down and to also move it slightly to the left.

Changing the mass of a body increases it's inertia and therefore increases the time period so the natural frequency occurs at a lower frequency and so the maximum amplitude at resonance decreases.

Increasing the force constant (from AS mechanics) increases the restoring force and hence the time period decreases so the natural frequency increase. Amplitude at resonance decreases. (I think of this by having a ruler on a desk - if you vibrate it and make it smaller, increasing the force constant, it moves faster with a lower amplitude).

Remember that if it does not start at the origin (displacement = 0), you need to add on the extra displacement on calculations using displacement!

Thermal Physics
Definitions
Solid - atoms are held via strong intermolecular forces and oscillate around their fixed positions with simple harmonic motion.

Liquid - atoms can slide over other planes and can only move in the bulk of the fluid. Forces of attraction between molecules are less, but the density is similar to the solid.

Gas - atoms move with a range of velocities (randomly and rapidly) in brownian motion. About 1000x less dense than solids and liquids. Movement is chaotic.

The Kinetic Model - a model used to explain pressure on a microscopic scale.

Pressure - force per unit cross sectional area.

Internal Energy - the sum of all of the random kinetic energies and the potential energies of all of the molecules/particles in a gas/system.

Thermal equilibrium - there is no net heat transfer between bodies - they are at the same temperature.

Kelvin Scale - where at absolute zero (0K) bodies have minimal energy - just PE for real gasses and zero energy for ideal gasses. All gasses have zero KE.

Specific Heat Capacity - heat energy required per unit mass to increase the temperature of a unit mass by one unit temperature (in kelvin).

Latent Heat of Fusion - energy needed per unit mass to change the state from liquid to solid (or a solid to a liquid) without a change in temperature.

Latent Heat of Vapourisation - energy needed per unit mass to change the state from liquid to vapour (or a vapour to a liquid) without a change in temperature.

Boyle's Law - for a fixed amount of gas at a constant temperature, pressure is inversely proportional to volume.

Notes
Assumptions of kinetic theory:
• All collisions are elastic.
• Particles/molecules have no intermolecular forces except during collisions (so no potential energies excist between particles/molecules).
• There are a large number of particles/molecules moving in rapid, random motion.
• The volume of the particles/molecules is neglible compared to the volume of the container.
• The time between collisions is far greater than the time a collision occurs for.
• All particles/molecules obey Newton's laws of motion
Brownian motion experiment - put some smoke into a sealed container then view it under an illuminated microscope. Results show:
• Smoke particles/molecules are moving continously - air particles/molecules must also be moving continiously.
• Smoke particles/molecules move in all directions - air particles/molecules must be hitting the smoke particles/molecules from all directions, and air particles/molecules must therefore be moving in all directions.
• Only smoke particles/molecules can be seen - air particles/molecules are very small.
• Smoke particles/molecules move randomly with different velocities - air particles/molecules have different velocities moving randomly.
How a gas exerts a pressure on a container:
• Pressure is force per unit cross-sectional area. The cross-sectional area is the face of the container.
• There are a large number of particles/molecules moving with rapid random motion. When they collide with the container, they experience a change of momentum.
• The force the particles/molecules exert on the container is equal to the force that the container exerts on the particles/molecules. - this is from Newton's third law.
• Since the time taken for the particles/molecules to collide with the wall and exert the force is the same time that the force exerted by the wall on the particles/molecules is the same, the rate of change of momentum for both is the same.
• Therefore the particles/molecules exerts a force on the wall. Since this force is acting on an area, the particles/molecules exert a pressure on the container.
Remember Q=ML where Q is the heat energy, M is the Mass and L is the value for the latent heat of fusion/vaporisation.

On a temperature/time graph, diagonal lines represent the temperature increasing - kinetic energy is increasing, potential energy is also increasing a bit. Flat lines show a phase change - no kinetic energy increase shown by the no increase in temperature, but potential energy is increasing. If you compare two diagonal lines and one is steeper, the steeper one has a lower specific heat capacity.

Experiment to find the specific heat capacity of a liquid/metal using an electric heater -
• Labeled diagram showing either a container with the liquid in, or the metal, with two holes in it. One hole is filled with an electric heater (also show the circuit - voltmeter in parallel, ammeter in series and a powerpack/battery), and also a thermometer. Also include some scales and a stopwatch.
• You want to measure the temperature increase (units don't really matter) in regular time intervals using the stop watch (in seconds), measure the mass of the liquid/solid using the scales in KG, measure the voltage (in volts) using the voltmeter, and the current (in amps) using the ammeter.
• Say that you will measure the temperature increase in regular time intervals and you will then work out {t} by doing the final temperature - the initial temperature. Also say that you will use the total time to raise to this temperature when working out the electrical energy.
• Write down the equation Q=mc{t}. Where Q is heat energy, m is mass, c is specific heat capacity, and {t} is the change in temperature. Also write down Q=ITV where Q is the electrical energy, I is the current, T is the time, and V is the voltage.
• Sub in the Q to get ITV=MC{T}. You can then rearrange to get ITV/M{T} = C. Make sure you state that you will sub in the numbers to get the value for C.
• Problems with the experiment - heat may be lost to the surroundings, and heat may be lost in heating up the container (solution is to use insulation). Temperature keeps increasing once heater is turned off beacuse heater is still higher temperature (solution wait a few seconds). Temperature reading is not accurate and is a false reading (stir liquid).
A different experiment not using electricity. Has the same general concept, except you have two containers containing water, one with a known mass. One is a much higher temperature than the other and you have a metal with a known mass.
• Dip the metal in the hot water until you reach a thermal equilibrium. Note down the temperature reading. Call this X.
• Put the thermometer in cold water with a known mass and measure the inital temperature of this (Y) and the final temperature (Z).
• The equation is MwCwTw=MmCmTm where the w means water and m means metal.
• You know Cw is 4200 and you know the mass of the water in the second cup. So you can re-write this as: (4200)(mass of water)(Z-Y)=(Mass of metal)(C of metal)(X-Z). Rerrange to get the C of the metal.
Why does pressure increase when temperature is increased?
• Temperature is directly proportional to kinetic energy. Kinetic energy is half * mass* velocity squared. So temperature increase means velocity increases.
• Velocity increases so the rate of change of momentum must also increase (there are more collisions per unit time)
• Since the rate of change of momentum is equal to the applied force from Newton's second law, the applied force must increase if the change of momentum increases.
• If the applied force increases, pressure must increase due to pressure being force per unit cross-sectional area.
Proportionalities appear quite a lot in thermal physics. Just remember to write down the initial equation and cross out what you know is constant (read the blurb of the question, this often tells you all you need to know that is constant).

General Tips
Remember A Levels are synoptic - they can ask things from AS. Commonly, they ask questions about projectiles (normally using an acceleration you worked out in a previous question). Remember if you are asked to find out the distance travelled (vertically or horizontally) and you are given an initial velocity, AND the acceleration is constant - use SUVAT. If there is no acceleration, just use speed=distance/time.

OCR are VERY VERY picky about definitions and explanations so make sure you write EVERYTHING to ensure you get the marks. Also write the formula you are using in exams!

Remember conservation of energy - it might save you some time calculating things.

Remember sig figs. Always use what is in the question, or alternatively write down your full answer and THEN round to 3 if unsure.

If you ever get stuck on a definition, as a last resort you can write out the formula and define what every single term is. This is only as a last resort! (Although I'll be doing this in addition to the definitions, just in case!)

Uses of data loggers- to record data automatically (more accurate). Can be used over long periods of time (so you could have one going for days, you couldn't have a human there for days) over short time intervals (a human can't measure things in 1 second intervals). Also you can quickly plot graphs using them.
Could you care to explain what you meant here, my peanut brain was completely baffled. If you could set me in the right direction I could give it to my teacher tomorrow as well to help me.

Increasing the force constant (from AS mechanics) increases the restoring force and hence the time period decreases so the natural frequency increase. Amplitude at resonance decreases. (I think of this by having a ruler on a desk - if you vibrate it and make it smaller, increasing the force constant, it moves faster with a lower amplitude).

Remember that if it does not start at the origin (displacement = 0), you need to add on the extra displacement on calculations using displacement!
7. (Original post by Better)
Could you care to explain what you meant here, my peanut brain was completely baffled. If you could set me in the right direction I could give it to my teacher tomorrow as well to help me.

Increasing the force constant (from AS mechanics) increases the restoring force and hence the time period decreases so the natural frequency increase. Amplitude at resonance decreases. (I think of this by having a ruler on a desk - if you vibrate it and make it smaller, increasing the force constant, it moves faster with a lower amplitude).

Remember that if it does not start at the origin (displacement = 0), you need to add on the extra displacement on calculations using displacement!
F = kx.

From what I understand, if the restoring force is bigger, the spring can be returned to equilibrium faster as this force always acts opposite to displacement and as we're keeping mass constant so a must be bigger. If this is so it makes sense that the period for an oscillation will be smaller because the amplitude is lower - it does not travel as far from equilibrium and hence returns faster.

Might be complete rubbish but that's what I can derive from it :L

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8. (Original post by Cyclohexane)
F = kx.

From what I understand, if the restoring force is bigger, the spring can be returned to equilibrium faster as this force always acts opposite to displacement and as we're keeping mass constant so a must be bigger. If this is so it makes sense that the period for an oscillation will be smaller because the amplitude is lower - it does not travel as far from equilibrium and hence returns faster.

Might be complete rubbish but that's what I can derive from it :L

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Oh I get you except when you get to the resonance is smaller. (because f=1/f)

Was there a context to this i.e. off a Past Paper Question? I could bring it to my teacher if anyone knows! Can't remember seeing anything like this...

Seems like Einstein Level Physics to me
9. (Original post by MasterYi)
Well you are supposed to count the squares for maximum accuracy. But I did 3 trapezium areas and a small triangle. And gives you an Impulse of around 2.1 Ns
Attachment 193247
Mark scheme:
area: number of squares correctly counted: 20 - 24 (500 – 600)= 2.2 Ns {allow 2.0 to 2.4}

I think it means 20 to 24 big squares. Like this:
Attachment 193248

I Hope this helps.
I see, thank you
10. i hate this exam with a passion. Good luck to howevers taking it.. just gear up for the G485 that's a nightmare
11. Does anyone know where you can find the June 2012 paper? (and markscheme)
12. (Original post by Better)
Oh I get you except when you get to the resonance is smaller. (because f=1/f)

Was there a context to this i.e. off a Past Paper Question? I could bring it to my teacher if anyone knows! Can't remember seeing anything like this...

Seems like Einstein Level Physics to me
Yeah that makes sense, like I said that's just m interpretation :P.
I remember one question about increasing mass regarding a toy plane on one of the recent past papers.

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13. (Original post by Better)
Oh I get you except when you get to the resonance is smaller. (because f=1/f)

Was there a context to this i.e. off a Past Paper Question? I could bring it to my teacher if anyone knows! Can't remember seeing anything like this...

Seems like Einstein Level Physics to me

(Original post by Better)
Could you care to explain what you meant here, my peanut brain was completely baffled. If you could set me in the right direction I could give it to my teacher tomorrow as well to help me.

Remember that if it does not start at the origin (displacement = 0), you need to add on the extra displacement on calculations using displacement!
June 2006 of the old spec, q3 for the force question.

There was a question about sea waves on Jan 11 q4, and to write an expression for the displacement, you had to include the fact that it did not start at 0 displacement. So in the x=Acos(2pift) you have to add on the equilibrium position for the wave - this was at 15.5m.
14. Does anyone know/have a list of the equations we need that aren't in the formula book? Thanks
15. (Original post by hrbrox)
Does anyone know/have a list of the equations we need that aren't in the formula book? Thanks
If I remember correctly, then there is V^2=GM/r(correct me if I'm wrong) to calculate speed of orbiting object if you can't use V=2πr/T.
It's not in the data book I don't think, but you can derive it from other equations from the formula book; if you have time.
16. By any chance does anyone have some of the older past papers that I could have? The ones that they don't give on the website?
17. Can anyone help me on this point of the specification?

(h) describe using a simple kinetic model for
matter the terms melting, boiling and
evaporation.

18. (Original post by BobFossil)
Can anyone help me on this point of the specification?

(h) describe using a simple kinetic model for
matter the terms melting, boiling and
evaporation.

Melting and boiling:
- mean separation between molecules increases
- electric potential energy increases
- internal energy increases
- mean kinetic energy of molecules remains the same

Evaporation:
- occurs at all temperatures of a liquid
- fast moving molecules escape from surface, leaving slower molecules which cools the substance
- rate increased by blowing over surface and heating

As far as I know that's all there really is to it but feel free to correct me on that
19. (Original post by BobFossil)
Can anyone help me on this point of the specification?

(h) describe using a simple kinetic model for
matter the terms melting, boiling and
evaporation.

Evaporation: At any temperature the high energy atoms in the liquid can escape from the surface of the liquid. Reducing the overall remaining internal energy of the liquid.

Boiling: Here the external pressure is equal to the internal pressure of the liquid. Particles can ecape from anywhere in the bulk of the liquid.
20. when you answer experiment questions can you answer it in bullet points under subheadings or do you have to write it out like a long essay with paragraphs describing each bit?

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