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• Created by: lilyexall
• Created on: 26-03-16 08:15

## Motion

Distance-time graph:

• Distance-time graph tells you how an object's distance changes over time
• The gradient represents the speed
• The steeper the gradient the greater the speed of the object
• Flat-line = no moment
• Downwards line = returning to starting position
• Upwards curve = deceleration

Speed (m/s)=Distance(m)/Time taken(s)

Velocity and acceleration:

• Velocity is the speed in a given direction
• Acceleration is the change of velocity per second

a = v-u / t

• a = acceleration, m/s2
• v = final velocity, m/s
• u = initial velocity, m/s
• t = time taken, s
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## Motion

Velocity-time graphs;

• Gradient represents acceleration
• Steeper gradient = bigger acceleration
• Flat (horizontal) line = no acceleration
• The area under the line in the graph = distance travelled (Total distance = total area) [H]

Using graphs;

• If you calculate the gradient of the line on the distance-time graph for an object, the answer will be the speed of an object
• If you calculate the gradient of the line on a velocity-time graph for an object, the answer will be the acceleration of the object
• Calculating the area under the line between two times on a velocity-time graph gives the distance travelled between those times
• Always use the numbers from the graph scales in any calculations
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## Forces

Forces between objects;

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

Resultant forces;

• The resultant forces = a single force that has the same effect as all the forces acting on an object
• If an object is accelerating there must be a resultant forces acting on it
• If the resultant force is:
• Zero (balanced) - the object will continue to move at the same speed and same direction
• Not Zero (unbalanced) - the object will accelerate in the direction of the resultant (single) force

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## Forces

• Friction and air resistance oppose the driving force of a car
• The stopping distance depends on the thinking distance and the braking distance
• The stopping distance of a vehicle is the distance it travels during the drivers reaction time (thinking distance) plus the distance it travels under the braking force (the braking distance)
• The thinking distance is increased if the driver is tired or under the influence of drugs or alcohol
• The braking distance can be increased by: 1) - Poorly maintained roads or bad weather conditions. 2) - The condition of the car (e.g. Worn tyres or worn brakes)

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## Forces

Stretching and squashing

• The extension is the difference between the length of the spring and its original length
• Hooke's law - 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
• F = k x e

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
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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
• W = F x d
• Work done to overcome friction is transferred as energy that heats the objects that rub together and the surroundings

Gravitational potential energy;

• The gravitational potential energy of an object depends on its weight and how far it moves vertically
• Ep = m x g x h

Kinetic energy;

• The kenetic energy of a moving object depends in its mass and its speed
• Ek = 1/2 x m x V2
• 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;

• p = m x v
• The unit of momentum is kg m/s
• Momentum is conserved whenever objects interact, provided no external forces act on them

Explosion;

• Momentum = mass x velocity
• When two objects push each other apart, they move apart: 1) With different speeds if they have unequal masses, 2) With equal and opposite momentum so their total momentum is zero.

Impact force;

• When vehicles collide the force of the impact depends on; mass, change of velocity and the duration of the impact
• When two vehicles collide: 1) They exert equal and opposite forces on each other, 2) 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 away' 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
• Like charges repel; unlike charges attract

Electric circuit;

• I = Q / t
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## Current electricity

Resistance;

• V = W/Q = E/Q
• R = V/I
• Ohm's law statues that the current through a resistpr at contant temperature is directly proportional to the protential difference across the resistor
• Reversing the current through a component reverses the protential difference across it

More current - protential difference graphs;

• Filament bulb: resistance increases with increase of the filament temperature
• Diode: 'forward' resistance low; 'reverse' resistance high
• Thermistor: resistance decreases if it temperature increases
• LDR: resistance decreases if the light intensitiy on it increases

Series circuit;

• For components in series; 1) The current is the same in each component, 2) adding the potential differences gives the total protential difference, 3) adding the resistances gives the total resistance
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## Current electricity

Parallel circuits;

• For components in parallel; 1) The total current in the sum of the currents through the separate component, 2) The bigger the resistance of a component the smaller the 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 I = V/R
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## Mains electricity

Alternating current;

• Direct current is in one direction only - Alternative current repeatedly reverses its direction
• The peak voltage of an alternative potential difference is the maximum voltage measured from zero volts
• A mains circuit has a live wire that is alternatively positive and negative every cycle and a neutral wire at zero volts. f = 1/T

Fuses;

• A fuse contains a thin wire that heats up and melts if too much current passes through it. This cuts off the current
• A circuit breaker is an electromagnetic switch that opens (i.e. 'trips') and cuts the current pff if too much current passes through it
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## Mains electricity

Cables and plugs;

• Sockets and plugs are made of stiff plastic materials, which enclose the electrical connections
• Cables consist 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: 1) The live wire is brown, 2) The neutral wire is blue, 3) The eath wire is green and yellow
• The earth wire is used to earth the metal case of a mains appliance

Electrical power and protential difference;

• The power supplied to a device is the energy transferred to it each second
• P = I x V
• Correct rating (in amperes) for a fuse = electrical power (watts) / protential difference (volts)
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## Mains electricity

Electrical energy and charge;

• An electric current is the rate of flow of charge
• Q = I x t
• When charge flows through a resistor, energy transferred to the resistor makes it hot
• E = V x Q

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 30 V
• Check cables, plugs and sockets for damage regularly
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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 cannot predict or influence when it happens
• Background radiation is from radioactive substances in the enviroment, or from space, or from devices such as X-ray machines

The discovery of the nucleus;

• Rutherford used the measurements from alpha particle scattering experiments as evidence that an atom has a small, positively charged, central necleus where most of the mass of the atom is located
• Most alpha particles passed straight through = most of the atom is atom
• Some alpha particles defected through small angles = nucleus has a positive charge
• Few reflected back = nucleus has a large mass, very positivley charged
• The nuclear model of the atom correctly explained why the alpha particles are scattered and why some are scattered through a large angles
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Nuclear reactions;

• Isotopes of an element are atoms with the same number of protons but different number of neutrons. Therefore, they have the same atomic numbers but different mass numbers
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More about alpha, beta and gamma radiation;

• a-radiation is stopped by paper or a few centimetres of air
• B-radiation is stopped by a thin metal or about a metre of air
• y-radiation is stopped by a thick lead and has an unlimited range in air
• A magnetic or an electric field can be used to separate a beam of alpha, beta and gamma radiation
• Alpha, beta and gamma radiation ionise substances they pass through

Half-life

• The half-life 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, and the type of radiation it gives out
• 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 approximately 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 when neutrons from the fission go on to cause other 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

Nuclear fusion;

• Nuclear fusion in the process of forcing two nuclei close enough together so they form a single larger nucleus
• Energy is released when two light nuclei are fused together
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## Energy from the nucleus

Nuclear issues;

• Radon gas is an a-emitting isotope that seeps into houses in certain areas through the ground
• There are hundreds of fission reactors safely in use throughout the world. None of them are the same type as the Chernobyl reactors that exploded
• Nuclear waste is stored in safe and secure conditions for many years after unused uranium and plutonium (to be used in the future) is removed from it

The early universe;

• A galaxy is a collection of billions of stars held together by their own gravity
• Before galaxies are stars formed, the universe was formed 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 star;

• A protostar is a gas and dust cloud in space that can go on to form a star
• Low mass star (sun):
• Protostar > main sequence star > red giant > white dwarf > black dwarf
• High mass star:
• Protostar > main sequence star > red supergiant > supernova > black hole (if sufficient mass)
• The son will eventually become a black dwarf
• A supernova is the explosion of a supergiant after its collapses

How the chemical elements formed;

• Elements as heavy as iron are formed inside stars as a result of nuclear fusion
• Elements heavier than iron are formed in supernovas, along with lighter elements
• The sun and the rest of the solar system were formed from the debris of a supernova
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## Force

Force and acceleration;

• The bigger the resultant force on an object, the greater its acceleration
• The greater the mass of an object, the smaller its acceleration for a given force
• F = m x a

Falling object;

• The weight of an object = force of gravity on it
• The mass of an object = 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 = the velocity it reaches when falling through a fluid. The weight is then equal to the drag force on the object
• The drag force may also be called air resistance or fluid resistance
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