# Physics Unit 2

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• Created by: EmiLy1703
• Created on: 26-10-15 10:28

## Motion - DIstance-time graphs

• The distance-time graph for any object that is:
• stationary is a horizontal line
• moving at constant speed is a straight line that slopes upwards
• The gradient of a distance-time graph for an object represents the object's speed
• The steeper the line on a distance-time graph, the greater the speed it represents
• Speed in metres per second (m/s) = distance travelled in metres (m)/time taken in seconds (s)
• For EVERY motion graph TIME IS ALWAYS on th X axis
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## Motion - Velocity and acceleration

• Velocity is speed in a given direction
• Acceleration is change of velocity per second. The unit of acceleration is the metre per second squared (m/s2
• Acceleration = change of velocity/time taken
• Deceleration is the change of velocity per second when an object slows down
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## Motion - More about velocity-time graphs

• If a line on a velocity-time graph is horizontal, the acceleration is zero
• The gradient of the line on a velocity-time graph represents acceleration
• The area under the line on a velocity-time graph represents distance travelled
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## Motion - Using graphs

• The speed of an object is given by the gradient of the line on its distance-time graph
• The acceleration of an object is given by the gradient of the line on its velocity-time graph
• The distance travelled by an object is given by the area under the line of its velocity-time graph
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## Forces - Forces between objects

• A force can change the shape of an object or change its motion or its state of rest
• The unit of force is the newton (N)
• When two objects interact, they always exert equal and opposite forces on each other
• A force always acts outwards
• The length of the arrow = the size of the force

Vectors and Scalars

A vector is a variable which does have a direction linked to it. Eg: velocity, weight and acceleration A scalar is a variable which doesn't have a direction linked to it. Eg: speed, distance, time and colour

is the letter to represent intial speed
V is the letter to represent velocity

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## Forces - Resultant force

A RESULTANT FORCE IS THE SUM OF ALL FORCES ON AN OBJECT

• The resultant force is a single force that has the same effect as all the forces acting on an object
• If the resulant force on an object is zero, the object stays at rest or constant velocity. If the resultant force on an object is not zero, the velocity of the object will change
• If an object is accelerating there must be a resultant force acting on it
• If two forces act on an object along the same line, the resultant force is:
• their sum if the forces act in the same direction
• their difference if the forces act in opposite directions
• When the resultant force on an object is not zero, there will be an acceleration in the direction of the force This means that:
• if the object is at rest, it will accelerate in the direction of the resultant force
• if the object is moving in the same direction as the resultant force, it will accelerate in that direction
• if the object is moving in the opposite direction to the resultant force, it will decelerate
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## Forces - On the road

• Friction and air resistance oppose the driving force of a car
• The faster the speed of a vehicle, the bigger the deceleration needed to it in a particular distance, so the bigger the braking force needed
• The stopping distance of a vehicle is the distance it travels during the drivers reaction time (the thinking distance) plus the distance it travels under the brakeing force (the braking distance)
• The thinking distance is increased if the driver is tired or under the influence of alcohol or drugs
• The braking distance can be increased by:
• the condition of the car, for example worn tyres or worn brakers will increase braking distance
• The stopping distance of a car depends on the thinking distance and the braking distance

SPEED IS AN EFFECT OF BOTH BRAKING AND THINKING DISTANCE

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## Forces - Force and acceleration

• The bigger the resultant force on an object is, the greater its acceleration is
• The greater the mass of an object is, the smaller its acceleration is for a given force
• The greater the resultant force on an object, the greater the acceleration. The bigger the mass of an object, the bigger the force needed to give it a particular acceleration
• Resultant force (newtons,N) = mass (kilograms) x acceleration (metres/second2)
• F = m x a
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## Forces - Falling objects

• The weight of an object is the force of gravity on it. Its mass is the quantity of matter in it
• An object acted on only by gravity accelerates at about 10m/s2
• The terminal velocity of a falling object is the velocity it reaches when it is falling in a fluid. The weight it then equal to the drag force on the object
• When an object falls through a fluid, the fluid exerts a drag force on the object, resisting its motion. The faster the object falls, the bigger the drag force becomes, until eventually it will be equal to the weight of the object. The resultant force is now zero, so the body stops accelerating. It moves at a constant velocity called terminal velocity
• The drag force may also be called air resistance or fluid resistance
• Mass = the amount of matter in an object
• Weight = the force of gravity acting on it
• Weight in newtons (N) = mass in kilograms (kg) x the acceleration due to gravity (m/s2)
• W = m x g
• If the object is on the Earth, not falling, g is called the gravitational field strength and its units are newtons per kilogram (N/kg)
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## Forces - Force and speed issues

• Fuel economy of road vehicles can be improved by reducing the speed or fitting a wind deflector
• Average speed cameras are linked in pairs and they measure the average speed of a vehicle
• Anti-skid surfaces increase the friction between a car tyre and the road surface. This reduces skids, or even prevents skids altogether
• Fuel economy is improved by reducing the speed of a vehicle which reduces the fuel use and reducing the air resistance by making the vehicle more streamlined
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## Forces - Stretching and squashing

• The extension is the difference between the length of the spring and its original lenth
• The extension of a spring is directly proportional to the force applied to it, provided the limit of proportionality is not exceeded
• The spring constant of a spring is the force per unit extension needed to stretch it
• Directly proportional - a graph will show this if the line of best fir is a straight line throught the origin
• Limit of proportionality - the limit for Hooke's law applied to the extension of a stretched spring
• Hooke's law - the extension of a spring is directly proportional to the force applied, provided its limit of proportionality is not exceeded
• Force applied in newtons (N) = spring constant in newtons per metre (N/m) x extension in metres (m)
• F = k x x
• When an elastic object is stretched, work is done. This is stored as elasic potential energy in the object
• When the stretching is removed, this stored energy is released
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## Work, energy and momentum - Energy and work

• Work is done on an object when a force makes the object move
• Energy transferred = work done
• Work done (joules) = force (newtons) x distance moved in the direction of the force (metres)
• Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings
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## Work, energy and momentum - Gravitational potentia

• The gravitational potential energy of an object depends on its weight and how far it moves vertically
• The gravitational potential energy of an object increases when the object goes up and decreases when the object goes down
• The change of gravitational potential energy of an object is equal to its mass x the gravitational field strength x its change of height
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## Work, energy and momentum - Kinetic energy

• The kinetic energy of a moving object depends on the mass and its speed
• Kinetic energy (J) = 1/2 x mass (kg) x speed(m/s2)
• Elastic potential energy is the energy stored in an elastic object when work is done on the object
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## Work, energy and momentum - Momentum

• Momentum = mass x velocity
• The unit of mementum is kg m/s
• Momentum is conserved whenever objects interact, provided the objects are in a closed system so that no external forces act on them
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## Work, energy and momentum - Explosions

• Momentum is mass x velocity
• Velocity is speed in a certain direction
• When two objects push each other apart, they move apart:
• with different speeds if they have unequal masses
• with equal and opposite momentum so their total momentum is zero
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## Work, energy and momentum - Impact forces

• When vehicles collide, the force of the impact depends on mass, change of velocity, and the duration of the impact
• The longer the impact time is, the more the impact force is reduced
• When two vehicles collide,
• they exert equal and opposite forces on each other
• their total momentum is unchanged
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## Work, energy and momentum - Car safety

• Seat belts and air bags spread the force across the chest and they also increase the impact time
• Side impact bars and crumple zones 'give way' in an impact so increasing the impact time
• We can use the conservation of momentum to find the speed of a car before an impact
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## Current electricity - Electrical charges

• Certain insulating materials become charged when rubbed together
• Electrons are transferred when objects become charged:
• insulating materials that become positively charged when rubbed lose electrons
• insulating materials that become negatively charged when rubbed gain electrons
• Like charges repel; unlike charge attract
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## Current electricity - Electric circuits

• Every component has its own agreed symbol. A circuit diagram shows how components are connected together
• A battery consists of two or mor cells connected together
• The size of an electric current is the rate of flow of charge
• Electric current = charge flow/time taken
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## Current electricity - Resistance

• The potential difference across a component (in volts) = work done or energy transferred (in joules)/charge (in coulombs)
• Resistance (in ohms) = potential difference (volts)/current (amps)
• Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor
• Reversing the current through a component reverses the pd across it
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## Current electricity - More current-potential diffe

• Filament bulb: resistance increase with increase of the filament teperature
• Diode: 'forward' resistance low; 'reverse' resistance high
• Thermistor: resistance decreases if its temperature increases
• LDR: resistance decreases if the light intensity on it increases
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## Current electricity - Series circuits

• For components in series:
• the current is the same in each component
• adding the potential differences gives the total potential differene
• Adding the resistances gives the total resistance of resistors in series
• For cells in series, acting in the same direction, the total potential difference is the sum of their individual potential differences
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## Current electricity - Parallel circuits

• For components in parallel:
• the total current is the sum of the currents through the separate components
• the bigger the resistance of a component, the smaller its current is
• In a parallel circuit the potential difference is the same across each component
• To calculate the current through a resistor in a parallel circuit, use this equation:
• current (amps) = potential difference (volts)/resistance (ohms)
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## Mains electricity - Alternating current

• Direct current is in one direction only. Alternating current repeatedly reverses its direction
• The peak voltage of an alternating potential difference is the maximum voltage measured from zero volts
• A mains circuit has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts
• To measure the frequency of an a.c. supply, we measure the time period of the waves then used the formula:
• frequency = 1/time taken for 1 cycle
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## Mains electricity - Cables and plugs

• Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical insulator
• Mains cable consists of two or three insulated copper wires surrounded by an outer layer of flexible plastic material
• In a three pin plug or a three core cable, the live wire is brown, the neutral wire is blue and the earth wire is green and yello
• The earth wire is connected to the longest pin and is used to earth the metal case of a mains appliance
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## Mains electricity - Fuses

• A fuse contains a thin wire that heats up, mlets, and cuts off the current if the current is too large
• A fuse is always fitted in series with the live wire. This cuts the appliance off from the live wire if the fuse blows
• A circuit breaker is an electromagnetic switch that opens (i.e. 'trips') and cuts off the current if too much current passes through the circuit breaker
• A mains appliance with a plastic case does not need to be earther because plastic is an insulator and cannot become live
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## Mains electricity - Electrical power and potential

• The power supplied to a device is the energy tramsferred to it each second
• Electrical power supllied (watts) = current (amps) x potential difference (volts)
• Correct rating (in amps) for a fuse: = electrical power (watts)/potential difference (volts)
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## Mains electricity - Electrical energy and charge

• An electric current is the rate of flow of charge
• Charge (coulombs) = current (amps) x time (seconds)
• When an electrical charge flows through a resistor, energy transferred to the resistor makes it hot
• Energy transferred (joules) = potential difference (volts) x charge flow (coulombs)
• When charge flows round a circuit for a certain time, the electrical energy supplied by the battery is equal to the electrical energy transferred to all the components in the circuit
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## Mains electricity - Electrical issues

• Electrical faults are dangerous because they can cause electric shocks and fires
• Never touch a mains appliance (or plug or socket) with wet hands. Never touch a bare wire or a terminal at a potential of more than 30V
• Check cables, plugs and sockets for damage regularly. Check smoke alarms and infrared sensors regularly
• When choosing an electrical appliance, the power and effiency rating of the appliance need to be considered
• Filament bulbs and halogen bulbs are much less efficient than low energy bulbs
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• A radioactive substance contains unstable nuclei that become stable by emitting radiation
• There are three main types of radiation from radioactive substances - alpha, beta and gamma radiation
• Radioactive decay is a random event - we cannont predict or influence when it will happen
• Background radiation is from radioactive substances in the environment or from space or from devices such as X-ray machines
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## Radioactivity - The discovery of the nucleus

• Rutherford used the measurements from alpha-scattering experiments to prove that an atom has a small positively charge central nucleus where most of the mass of the atom is located
• The plum pudding model could not explain why some alpha particles were scattered through large angles
• The nuclear model of the atom correctly explained why the alpha particles are scattered and why some are scattered through large angles
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• Isotopes of an element are atoms with the same number of protons but different numbers of neutrons. Therefore they have the same atomic number but different mass numbers
• Alpha decay:
• Change in nucleus = loses 2 protons and 2 neutrons
• Particle emitted = 2 protons and 2 neutrons emitted as an alpha particle
• Equation =
• Beta decay:
• Change in nucleus = a neutron in the nucleus changes into a proton
• Particle emitted = an electron is created in the nucleus and instantly emitted
• Equation =
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• Alpha radiation is stopped by paper, has a range of a few centimetres in air and consists of particles, each composed of two protons and two neutrons
• Beta radiation is stopped by thin metal, has a range of about a metre in air and consists of fast-moving electrons emitted from the nucleus
• Gamma radiation is stopped by thick lead, has an unlimited range in air and consists of electromagnetic radiation
• A magnetic or an electric field can be used to separate a beam of alpha, beta and gamma radiation
• Alpha, beta and gamma radiation ionised substances they pass through. Ionisation in a living cell can damage or kill the cell
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• The half like of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve
• The activity of a radioactive source is the number of nuclei that decay per second
• The number of atoms of a radioactive isotope and the activity both decrease by half every half life
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• The use we can make of a radioactive isotope depends on:
• its half life
• the type of radiation it gives out
• For monitoring, the isotope should have a long half life
• Radioactive tracers should be beta or gamma emitters that last long enough to monitor but not to long
• For radioactive dating of a sample, we need a radioactive isotope that is present in the sample which has a half life about the same as the age of the sample
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## Energy from the nucleus - Nuclear fission

• Nuclear fission is the splitting of a nucleus into two approxiamately equal fragments and the release of two or three neutrons
• Nuclear fission occurs when a neutron hits a uranium-235 nucleus or a plutonium-239 nucleus and the nucleus splits
• A chain reaction occurs in a nuclear reactor when each fission event causes further fission events
• In a nuclear reactor, control rods absorb fission neutrons to ensure that, on average, only one neutron per fission goes on to produce further fission
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## Energy from the nucleus - Nuclear fusion

• Nuclear fusion is the process of forcing two nuclei close enough together so they form a single larger nucleus
• Nuclear fusion can be brought about by making two light nuclei collide at very high speed
• Energy is released when two light nuclei are fused together. Nuclear fusion in the Sun's core releases energy
• A fusion reactor needs to be at a very high temperature before nuclear fusion can take place. The nuclei to be fused are different to contain
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## Energy from the nucleus - Nuclear issues

• Radon gas is an alpha emitting isotope that seeps into house in certain areas through the ground
• There are thousands of fission reactors safely in use in the world. None of them are of the same type as the Chermobyl reactors that exploded
• Nuclear waste is stored in safe and secure conditions for many years afer unused uranium and plutonium (to be used in the future) is removed from it
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## Energy from the nucleus - The early universe

• A galaxy is a collection of billions of stars held together by their own gravity
• Before galaxies and stars formed, the universe was a dark patchy cloud of hydrogen and helium
• The force of gravity pulled matter into galaxies and stars
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## Energy from the nucleus - The life history of a st

• A protostar is a gas and dust cloud in space that can go on to form a star
• Low mass star:
• Protostar
• Main sequence star
• Red giant
• White dwarf
• Black dwarf
• High mass star:
• Protostar
• Main sequence star
• Red supergiant
• Supernova
• Neutron star
• Black hole (if sufficient mass)
• The Sun will eventually become a black dwarf
• A supernova is the explision of a supergiant after it collapses
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## Energy from the nucleus - How chemical elements fo

• Elements as heavy asiron are formed inside stars as a result of nuclear fusion
• Elements heavier than iron are formed in supernovas as well as light elements
• The Sun and the rest of the Solar System were formed from the debris of a supernova
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## Forces - Newton's laws

Newton's first law

• An object is stationary or moves at a steady speed unless a resultant force acts upon it

Newton's second law

• The resultant force exerted on an object is equal to the product of its mass and resultant acceleration
• F= ma
• F - force measured in newtons
• m - mass measured in kg
• a - acceleration measured in m/s2

Newton's third law

• Any exerted force on an object causes an equal and opposite force to be produced
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