Resultant Force = The combined effect of forces acting on an object
Whenever two objects interact, the forces they exert on each other are equal and opposite.
A resultant force acting on an object may cause a change in its state of rest or motion.
* If the resultant force on a stationary object is zero, then the object will not move
* If the resultant force on a stationary object is not zero, then the object will accelerate
* If the resultant force on a moving object is zero, then it will be moving at a steady speed
* If the resultant force on a moving object is not zero, then the object will accelerate in the direction of the resultant force
Forces and Motion
* The gradient of a distance-time graph represents speed.
* The steeper the gradient, the faster the object is moving.
* A horizontal line means the object is stationary.
Units = a - m/s2 , v - m/s , t - s
Velocity is speed in a given direction.
* The gradient of a velocity-time graph represents acceleration.
* The steeper the gradient, the greater the acceleration.
* A horizontal line represents steady acceleration.
Forces and Braking
* When a vehicle travels at a steady speed, the resistive force balances the driving force.
* Most of the resistive forces are caused by air resistance.
* The greater the speed of a vehicle the greater the breaking force needed to stop it in a certain distance.
* When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.
* The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver's reaction time (thinking distance) and the distance it travels under the braking force (braking distance).
* The faster a car travels, the longer the braking distance is.
* A driver's reaction time can be affected by tiredness, drugs or alcohol.
* A vehicle's braking distance can be affected by adverse road conditions (wet/ icy conditions) and poor condition of the vehicle (brakes/tyres).
Forces and Terminal Velocity
* The faster an object moves through a fluid (liquid/air), the greater the frictional force acting on it.
* An object falling through a fluid will initially accelerate due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity (steady speed).
>The initial two forces acting on a parachutist are air resistance (upwards) and weight (downwards).
2) The person starts moving faster so the air resistance increases. They accelerate slower
3) The two forces balance each other out so the person falls at a steady speed (terminal velocity)
4) The parachute opens and there is a bigger air resistance force so the person slows down
5) The two forces become balances so the person reaches a new steady speed, but it is slower so it is a slower terminal velocity.
Forces and Elasticity
Work = To transfer energy
* A force acting on an object may cause a change in shape of the object.
* When a force is applied to an elastic object, such as a spring, energy is transferred to the spring and is stored as elastic potential energy. The object can be stretched or compressed.
* When the object is unstretched/leg go of, the energy is released. An elastic object regains its shape once the forces are removed.
"The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded"
* F (N) = K (N/m) x e (m)
* The Extension - Difference between the length of the spring and its original length
* The Spring Constant - Force per unit extension needed to stretch it (Stiffer the sring = Greater the spring constant)
* Limit of Proportionality = After this point, when you add more force to the spring, it doesn't go up in a proportianl way.
Forces and Energy
* When a force causes an object to move through a distance work is done.
* Work means energy transferred and is measured in Joules (J).
* Work done, force and distance are related by the equation; W (J) = F (N) x d (m) d is the distance moved in the direction of the force.
* Power is the work done or energy transferred in a given time; P (W) = E (J) / t (s)
* Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field; Ep (J) = m (kg) x g (N/kg) x h (m)
* When an object is raised vertically work is done against gravitational force and the object gains gravitational potential energy.
* The kinetic energy of an object depends on its mass and speed; Ek (J) = 1/2 x m (kg) x v2 (m/s)
* Every moving object has kinetic energy. A fast moving object turns K.E. into heat enrgy.
Example: A rocket has lots of kinetic energy. When it hits the atmosphere, you get friction against the Earth's atmosphere. Energy is transferred from movement into heat energy.
* Weight is a force, which is measured in Newtons (N).
* The weight always acts downwards towards the centre of the Earth.
* The weight of an object depends on two things; The mass of an object and the amount of force pulling it towards the centre of the Earth.
* To calculate weight we need to know the mass of an object, and the force that is pulling it towards the centre of the Earth. This force is known as Gravitational Field Strength (N/kg2)
* For all objects on Planet Earth, the gravitational field strength is 10 N/kg2.
* For all objects on the moon, the gravitational field strength is 1.6 N/kg2.
* The mass of an object stays the same, regardless of where it is.
* Weight (N) = Mass (g) x Gravitational Field Strenght (N/kg2)
* Momentum is a property of moving objects.
* To calculate momentum, we need to know two things; the Mass of the object and its Velocity.
* p (kg m/s) = m (kg) x v (m/s)
* Conservation of Momentum = The total momentum before an event is equal to the total momentum after the event.
- By an event,we mean collisions and explosions.
* To calculate the total momentum, we calculate the momentum of both objects before the evnt, then add them together.
* The more mass and velocity you have, the more momentum you have.
* If you lose momentum quickly, you feel a large force.
* When two vehicles collide, they exert equal and opposite forces on eachother.
* The longer the impact time, the more the impact force is reduced.
Impact Time = The time taken for a collision to take place.
Crumple zones in cars are designed to fold in a collision. They increase the impact time and so reduce the fore on the car.
* Seat belts are designed to increase the impact time. A seat belt stops the wearer being flung forward if the car stops suddenly.
* Air Bags are designed to do the same as seat belts. They too increase the impact time.
* Seat belts and air bags spread the force across the body and increase the impact time.
* Side impact bars increase the amount of time for a collision, to increase the impact time.
* When certain insulating materials are rubbed against each other they become electrically charged.
* Insulating materials may include a polyethene/perspex rod and a cloth.
* Negatively charged electrons are rubbed off one material and onto the other.
* The material that gains electrons becomes negativly charged. The material that loses electrons is left with an equal positive charge.
* When two electrically charged objects are brought together they exert a force on each other.
* Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract.
* Electrical charges can move easily through some substances, e.g.metals.
* Examples of static electricity include; Van der Graaf, Baloons, Jumper (wool), Trampolines, Carpets.
- A voltmeter measures Voltage / Potential Difference
- An ammeter measures Current, in amps
- A resistor lowers Current
- A thermistor allows more current to flow through when there are high temperatures (Thermostat)
- A light-dependant resistor changes its resistance depending on light (Automatic lights)
- A light emmiting diode can be made to make efficient lighting
- A diode allows current to flow only one way in a circuit.
* Electric current is the flow of electric charge / flow of elctrons. This is measured by an Ammeter.
* Electric charge is the rate of the flow of the electric charge / flow of electrons.
* The size of the current is given by the equation; I (A) = Q (C) / t (s)
* The potential difference (voltage) between two points in an electrical circuit is the work done (energy transferred) per coulomb of charge that passes between the points; V (V) = W (J) / Q (C)
- The potential difference is the work done (energy transferred) per coulomb of charge that passes through the component.
* A current-potential difference graph is used to show how the current through a component varies with the potential difference across it.
* The resistance of a component can be found by measuring the current through, and the potential difference across, the component.
* The current through a resistor (at a constant temperature) is directly proportional to the potential difference across it.
* The current through a component depends on its resistance. The greater the resistance the smaller the current for a given potential difference across the component.
* The P.D. provided by cells connected in series is the sum of the P.D. of each cell (depending on the direction in which they are connected)
Series and Parallel Circuits
Series Circuit - There is only one pathway in which the current can flow.
* The total resistance is the sum of the resistance of each component.
* The total potential difference of the cells is the sum of the individual cells.
* The potential difference of the supply is shared between the components.
* The current is the same through each component.
Parallel Circuit - The current can flow through different paths of the circuit.
* The potential difference across each component is the same.
* The current is different in different branches of the circuit.
* The total current through the whole circuit is the sum of the currents through the seperate components
> The HIGHER the the resistance, the LOWER the current.
LDRs, LEDs and Thermistors
* The resistance of a light-dependant resistor (LDR) decreases as light intensity increases.
l----> High Resistance = Low Current.
> Uses: Atomatic lights.
* The resistance of a thermister decreases as the temperature increases.
l----> The higher the temperature, the lower the resistance
> Uses: Automatic heating systems
* A light emmiting diode (LED) emits light when a current flows through it in the forward direction
* It only works when the current flows in one direction
> Uses:Can be used for lighting systems. They are much more efficient (70-80%) than other forms of lighting (e.g. filament lamps). They use much smaller current than other lighting forms.
Current through Lamps and Diodes
* The resistance of a filament bulb increases as the temperature of the filament increases.
* The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.
* There are two types of electrical current; A.C. and D.C.
* Cells and batteries supply current that always passes in the same direction. This is called direct current (D.C.)
* An alternating current (A.C.) is one that is constantly changing direction.
* Mains electricity is is an A.C. supply. In the UK it has a frequency of about 50 cycles per second (50 hertz) and is about 230V.
* Most electrical appliences are connected to the mains using cable and a three-pin plug.
* Inside a cable (we call it a three-core cable) we have three different wires. The wires are made from strands of copper twisted together as it is a good conductor of electricity. A two core cable has two wires.
* A plug is surrounded by a tough rubber/plastic case as they are good insulators.
* A cable is made from tough plastic, with a cable grip in place at the top of the cable.this prevents the wires from being pulled away from the pins.
* The three pins are made from brass because it is hard and strong.
* A fuse is fitted in series with the live wire. It melts if the current gets too high, disconnecting the circuit.
Saftey, Fuses and RCCBs
* An applience has a two core cable if it does not have a metal casing. Examples of these appliences include hairdryers. We say that these appliences are double insulated.
* Three core cables are connected to an applience with a meatl casing (e.g. kettle).
* Appliences with metal cases are usually earthed. They need to be earthed as, if the live wire comes lose and touches the metal, the whole metal case becomes live. If a person touched the case, instead of the current flowing through the circuit, it could flow through you and you will be electricuted.
l---> If there is a live wire in contact with the outer metal casing, the current would not flow through the person, it would go through the earth wire which would direct it back to the plug and into the ground.
* The earth wire has a very low resitance and this creates a surge of current that flows up to the plug. The fuse in the plug melts (brakes) and the applience is disconnected.
* Some circuits are protected by Residual Current Circuit Breakers (RCCBs).
* A RCCB detects a difference in current between a live wire and a neutral wire. It disconnects the circuit if one is detected, no matter how small.
* An RCCB is much quicker than a fuse, and can be reused (unlike a fuse).
* They can be used in lawnmowers.
Current, Charge and Power
* When an electrical charge flows through a resistor, the resistor gets hot.
* When a current flows through a wire, the wire gets warmer.
* The lower the current, the thinner the wire.
* The amount of current flowing determines the size of the fuse needed (e.g. 3A, 5A, 13A)
* A lot of energy is wasted in a filament lamp as heat. Less energy is wasted in power saving lamps such as Compact Fluorescent Lamps (CFLs).
* A high efficiency rating means the applience wastes less energy as heat to the environment.
* The rate at which energy is transferred by an applience is called the power; P (W) = E (J) / t (s)
* Power, potential difference and current are related by the equation; P (W) = I (A) x V (V)
l---> Potential Difference is the energy transferred per coulomb.
* We can find out what type of fuse we need for an applience by using the rearranged equation;
- I (A) = P (W) / V (V)
* Once we've work out what the current is, we can choose an appropriate fuse for it.
* The known fuses are; 3A, 5A and 13A
* The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons.
* Relative Masses: Protons +1, Neutrons +1, Electrons = Very small
* Relative Charges: Protons +1, Neutrons 0, Electrons -1
* In an atom the number of electrons is equal to the number of protons in the nucleus. The atom has no overall electric charge.
* Atoms may lose or gain electrons to form charged particles called ions.
* The atoms of an element always have the same number of protons, but have a different number of neutrons for each isotope.
* The total number of protons in an atom is called its atomic number. The total number of protons and neutrons in an atom is called its mass number.
Atoms and Radiation
* Some substances give out radiation from the nuclei of their atoms all the time. These substances are said to be radioactive.
* A Guiger-Muller tube is used to measure radioation.
* Background Radiation = Radiation that is present naturally that comes from the environment.
- Food and Drink
- Cosmic Rays (outer space)
- Air --> Radon gas (50%)
- Man-Made; Medical, Nuclear reactors, Air travel, Nuclear accidents, Weapons.
* The 'Plum Pudding Model' was the work of two scientists called Rutherford and Marsden.
* The Plum Pudding Model = The scientific idea that an atom is a sphere of positive charge, with negatively charged electrons inside it.
* An Alph particle scattering experiment was devised, in which they fired alpha particles at thin gold foil. Alpha particles have a charge of 2+
* At the time, scientists were expecting the particles to pass straight through the foil.
* During the experiment, most of the particles passed straiht through, however some were deflected at various angles. Some of the particles even came straight back.
* Most Alpha particles passed straight through the gold foil - Most of the atom is empty space
* Some alpha particles passed through but were deflected by varying amounts - There is a positive centre to the atoms
* Very few came straight back - The positive centre is very dense
Alpha, Beta and Gamma Radiation
* Alpha - 2 Protons, 2 Neutrons (2+ charge) - Mass = 4
* Beta - An electron (ejected from the nucleus) (-1 charge) - Mass = Very tiny
* Gamma - Electromagnetic wave (no charge) - No mass
* Alpha - Stopped by a few cm of air ---> Absorbed by thin paper/skin ---> Least Penetrating
* Beta - Stopped by 1m of air ---> Absorbed by a few mm of aluminium
* Gamma - Unlimmited through air ---> Absorbed by a few cm (5+) of lead ---> Least Ionising
* Beta is deflected most in a magnetic field
* Alpha is deflected less than Beta, and in the opposite direction
* Gamma is not deflected
Half Lives & Uses of Radioactivity
* 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, or the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.
* Alpha particles are ionising. This means that an alpha particle can collide with another atom and knock off an electron. This can cause the atom to become unstable. If atoms in the DNA become ionised, it can cause the DNA to not work properly, and can cause cells to become cancerous.
* Beta particle are less likely to knock off electrons, however as they can travel further than Alpha radiation, they are more likely to damage the atom.
Gamma - Sterilise, tracers (needs a very short half-life), Killing cancer cells (radiotherepy)
Beta - Controlling foil thickness (very long half-life)
Alpha - Controlling paper thickness, smoke alarms (very long half-life)
Other Radiation - Radioactive dating
* Nuclear fission is the splitting of an atomic nucleus.
* The nucleus undergoing fission splits into two smaller nuclei and two or three neutrons and energy is released.
* The neutrons may go on to start a chain reaction.
* The process does produce radioactive waste.
* It is a process that generates heat, and can be used to generate heat in a power station.
* There are two fissionable substances in common use in nuclear reactors; Uranium-235 and Plutonium-239. The majority of nuclear reactors use Uranium-235.
* For the fission to occur, the Uranium-235 or Plutonium-239 nucleus must first absorb a neutron.
* Nuclear fusion is the joining of two atomic nuclei to form a larger one.
* Nuclear fusion is the process by which energy is released in stars.
* Nuclear fussion releases energy and there are no radioactive waste products, however it is not acheived at low enough temperatures.
* Stars form when enough dust and gas from space is pulled together by gravitational attracktion. Smaller masses may also form and be attracted by a larger mass to become planets.
* During the 'main sequence' period of its life cycle a star is stable because the force within it are balanced.
* Fusion processes in stars produce all of the naturally occuring elements. These elements may be distributed throughout the Universe by the explosion of a massive star (supernova) at the end of its life.
Life Cycle of Stars
* A star goes through a life cycle. This life cycle is determined by the size of the star.
Low Mass Star (stars about the same size as the sun):
Nebula ---> Protostar ---> Main Sequence Star ---> Red Giant ---> White Dwarf ---> Black Dwarf
High Mass Star (stars much bigger than the sun):
Nebula --> Protostar --> Main Sequence Star --> Red Super Giant --> Supernova --> Black Hole / Neutron Star