Physics 2a Flashcards
What do speed and velocity both show?
How fast you’re going
What is the difference between speed and velocity?
Speed is just how fast you’re going whereas velocity must also have the direction e.g. 30 mph north
How do you work out speed?
Distance divided by time
What does the gradient of a distance-time graph tell you?
How fast your object is travelling because the gradient is the change in the distance divided by the change in time
What do flat sections in the graph tell you?
That the object/person is stationary
What do straight uphill/downhill sections mean?
Its travelling at a steady speed
The steeper the graph……?
The faster its going
What do downhill sections mean?
That its going back to its starting point
What do curves represent?
Acceleration or deceleration
What does an increasing gradient show?
Its speeding up
What does a decreasing gradient show?
Its slowing down
How do you work out the speed from a distance-time graph?
Speed=vertical divided by horizontal
What is acceleration?
How quickly the velocity is changing
What can cause he velocity to change?
Change in speed or change in direction
What is the formula to work out acceleration?
Change in velocity divided by time taken
What does the gradient mean in a velocity-time graphs?
Acceleration
What do flat sections represent?
Steady speed
The steeper the graph the….?
The greater the acceleration/deceleration
What do uphill sections show?
Acceleration
What do downhill sections show?
Deceleration
What does the area under any section/all of it, equal?
It is equal to the distance travelled in the time interval
What does a curve mean?
Changing acceleration
How do you work out acceleration from velocity-time graphs?
Gradient- vertical change divided by horizontal change
What is gravitational force?
The force of attraction between all masses
What does gravity attract?
It attracts all masses, but you only notice it when one of the masses is really bug e.g. a planet, anything near a planet to star is attracted to it very strongly
What are the two important effects of gravity?
On the surface of a planet, all things accelerate towards the ground, and it gives anything a weight
Are weight and mass the same?
No
What is mass?
Mass is the amount of ‘stuff’ in an object, for any given object this will have the same value anywhere in the universe
What is weight?
It is caused by the pull of the gravitational force
What is the difference between weight and mass?
An object has the same mass whether its on Earth or on the moon-but its weight will be different. And weight is a force measured in newtons whereas mass isn’t a force and is measured in kilograms with a mass balance
What is the formula relating mass, weight and gravity?
Weight=mass x gravitational field strength
What is resultant force?
The overall force on a point or object
What will the overall effect of these forces decide?
The motion of the object: accelerate/decelerate/steady speed
What can you do if you have a number of forces acting at a single point?
You can replace them with a single force, as long as the single force has the same effect on the motion as the original forces acting all together
How is the overall effect found if the forces all act along the same line (parallel and acting in the same or opposite direction) ?
By adding or subtracting them - The overall effect is the resultant force
What is an example of something where all the forces are balanced?
A stationary teapot. Gravity acting downwards, causes a reaction force from the surface pushing up on the object of equal force
What would happen if there wasn’t a reaction force?
The object would accelerate downwards
What would the resultant force be on a stationary teapot that weighed 10N?
10N - 10N=0N Resultant force = 0
What happens if there is a resultant force acting on an object?
The object will change its star of rest or motion-it changes its velocity
What is the statement about stationary objects?
If the resultant force on a stationary object is zero, the object will remain stationary
What is the statement about resultant force and velocity?
If there is no resultant force on a moving object it’ll just carry on moving at the same velocity
How does something travel at a constant speed?
If all the forces are balanced, there must be zero resultant force
What is the statement about resultant force and speed?
If there is a non-zero resultant force, then the object will accelerate in the direction of the force
What wil a non-zero resultant force always cause?
Acceleration or deceleration
What are the five different thing caused by an acceleration in the direction of the force?
Starting, stopping, speeding up, slowing down and changing direction
What does a (non-zero) resultant force produce?
Acceleration
What does a (non-zero) resultant force produce?
Acceleration
What is the formula for this?
F=ma OR a=F/m
What do each of the letters stand for in that formula?
M= mass in kg. a=acceleration in m/s2. F= resultant force in newtons
What are reaction forces (in relation to each other)?
Equal and opposite
What is the statement for this?
When two objects interact, the forces they exert on each other are equal and opposite
What is an example of this using a trolley?
When you push it, it pushes back with equal force and so when you stop, so does it
How does anything move if there are equal forces?
Eg ice skaters pushing equal force onto each other both accelerate backwards, one more than the other as they would be different masses. Also with swimming, you push back on the water with your arms and legs and the water pushed you forwards with equal sized force in opposite direction
Why does an object always slow down and stop if it has no force against it?
Because of friction (except in space) as friction always acts in the opposite direction of movement
How do you travel at a steady speed?
Driving force has to balance the frictional force which happens between two surfaces in contact or when an object passes through a fluid (drag)
What are most resistive forces caused by?
Air resistance
What is the most important factor in reducing drag in fluids?
Keeping the shale if the object streamlined because the opposite extreme is a parachute which is about as high drag as possible and isn’t streamlined
When does drag increase?
As speed increases. Frictional forces from fluids always increase with speed
What do objects falling through fluids reach?
Terminal velocity
What is the process of an object reaching terminal velocity?
When something first starts falling, the force of gravity is much more than the friction force so they accelerate. As the speed increases, the friction builds up. This gradually reduces the acceleration until the frictional force balance the accelerating force and then it won’t accelerate anymore-steady speed/terminal velocity
What does the terminal velocity of falling objects depend on?
Drag in comparison to its weight. The frictional force depends on shape and area
What is the accelerating force acting on all falling objects?
Gravity
What is an example of how terminal velocity works?
A skydiver. Without parachute the person is small and reaches terminal velocity at about 130mph but with parachute open they are bigger so more air resistance and terminal velocity is down to 15mph
What is the total stripping distance of a vehicle?
The distance covered in the time between the driver first spotting a hazard and the vehicle coming to a complete stop (the sum of the thinking distance and the braking distance)
What is the reaction time?
The time between the driver spotting a hazard and taking action
What can thinking distance be affected by?
How fast your going and how aware you are which is affected by tiredness, drugs, alcohol and careless attitudes
What other factors cause accidents?
Distractions and weather
What factors affect braking distance?
How fast your going, how good your brakes are, how good the tires are (minimum tread depth of 1.6mm to get rid of water in wet conditions) and how good the grip is which depends on road surface, weather conditions and tyres
What is the definition of work?
When a force moves an object through a distance, energy is transferred and work is done
What does the statement mean?
When something makes, something else provides it with effort to move it, that thing needs a supply of energy and then does work by moving the object so it transfers the energy it receives into other forms
Can you say work is done if the energy is transferred usefully or if it is wasted
Both
What is work done and energy transferred measured in?
Joules
What is the formula for this?
Work done=force X distance
What is gravitational potential energy?
Energy due to height
What is the formula for gravitational potential energy?
Gravitational potential energy=mass X g X height
What is the symbol formula?
Ep = m X g X h
What is the proper definition of gravitational potential energy?
The energy that an object has because of its vertical position in a gravitational field.
What happens when an object is raised vertically?
Work is done against the force of gravity and the object gains gravitational potential energy
What is the gravitation field strength on earth?
About 10n/kg
What is kinetic energy?
Energy of movement
What is the formula for kinetic energy?
Kinetic energy = 1/2 X mass X speed2
What is the symbol formula?
Ek=1/2 X m X v2
What does the kinetic energy of something depend on?
Mass and speed. The more it weighs and the faster its going, the bigger the kinetic energy will be
How is work done when a car is slowing down?
The kinetic energy needs to be converted into other tules of energy using the law of conservation energy. Then to stop, it must be converted into heads energy as friction between the wheels and the brake pads cause the temperature of the breaks to increase
What is the formula for this?
Kinetic energy transferred = work done by brakes
What is the symbol formula?
1/2mv2=f X d
What do the symbols stand for?
M=mass of car and passengers. V = speed of car. F = maximum braking force. D = braking distance
What do falling objects convert?
Ep into ek. Kinetic energy gained = potential energy lost
What is some of the ek then transferred into?
Heat and sound eg when meteors or space shuttles enter the atmosphere they have very high kinetic energy, fiction due to collisions with particles in atmosphere transfers some of the kinetic into heat energy and work is done. Temperature is so hot most meteors burn up without hitting earth so only the biggest ones called meteorites hit Earth
How do space shuttles get to earth without burning away?
Heat shields made from special materials which lose hear quickly allowing the shuttle to re enter the atmosphere without burning up
What is work done to an elastic object stored as?
Elastic potential energy
What sort of objects are elastic objects?
Ones that scan stretch and change shape and then return to its original shape once the force is removed
What happens when an elastic object changes shape?
Work is done, the energy isn’t lost but is stored as elastic potential energy
What then happens to the elastic potential energy?
It’s converted to kinetic energy when the force is removed and the object returns to its original shape eg when a spring or an elastic band bounces back
What is the extension of a stretched spring directly proportional to?
The load or forces applied.
What is the extension measured in?
Meters
What is the force measure in?
Newtons
What is the equation?
F=k X e
What is k
The spring constant. It’s value depends on the material that you are stretching and its measured in newtons per meter
Why does the direct proportional part stop working eventually?
There’s a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally
What would a graph show that is showing force against extension for an elastic object?
For small forces, force and extension are proportional so the first part of the graph shows a straight line relationship between force and extension. There is a maximum force and the elastic object can take and still extend proportionally. This is known as the limit of proportionality and is shown on a graph by a point marked “P”
What is power in terms of work?
The rate of doing work ie how much per second
What is a powerful machine?
One that transfers a lot of energy in a short space of time
What is the formula for power?
Power=work done (or energy transferred) divided by time taken
What is the symbol formula for this?
P=e divided by t
What is power measured in?
Watts or j/s
What is one watt in joules?
1 joules of energy transferred per second
What does power mean?
How much energy per didn’t, so watts is the same as joules per second but don’t say watts per second
What is one way to measure the power output of a person?
The timed run upstairs. The energy transferred is the potential energy you gain. ( energy transferred divided by time)
What is another way to measure the power output of a person?
The timed acceleration. The energy transferred is the kinetic energy you gain ( energy transferred divided by time)
What is the formula for momentum?
Momentum = mass x velocity
What is momentum?
A property of moving objects
What does the mass and velocity of an object show about its momentum?
The greater the mass of an object and the greater its velocity, the more momentum the object has
Momentum is a vector quantity, meaning what?
It has size and direction
What is the conservation of momentum?
In a closed system, the total momentum before an event is the same as after the event
What is the collision example?
Two skaters approach each other, collide and move off together, what is their velocity? Total momentum before = momentum of first skater + momentum of second skater. This then equals the momentum after
What is the explosion example?
A gun fires a bullet, at what speed does the gun move backwards? Momentum before = 0, moment after = momentum of bullet + momentum of gun. The momentum of a system before an explosion is zero so due to conservation of momentum, after is also zero
What causes changes in momentum?
When a force acts on an object
What does a larger force mean?
A faster change of momentum (greater acceleration)
What happens if someones momentum changes very quickly e.g. in a car crush?
The forces on the body will be very large and more likely to cause injury which is why cars are designed with safety features to slow people down over a longer time when they have a crash
Why do safety features in cars reduce the risk of injury?
The longer it takes for a change in momentum, the smaller the force
What do brakes do work against?
Against the kinetic energy of the car
What happens when the brakes are applied?
Work is done, they reduce the kinetic energy by transferring it into heat and sound energy. In traditional systems that would be it but new regenerative braking systems used in some electric or hybrid cars make use of the energy but convert it all into heat during braking
How do regenerative systems works?
Use the system that drives the vehicle to do the majority of the braking. Rather than converting kinetic to heat energy, the brakes put the vehicles motor into reverse so the wheels are slowed.
What else does the motor do during this?
It acts as an electric generator, converting kinetic energy into electrical energy that is stored as chemical energy in the vehicles battery, this takes advantage of regenerative brakes-they store the energy of braking rather than wasting it
If a car crashes, what happens if it slow down very quickly?
So a lot of kinetic energy is converted into other forms of energy in a short amount of time which can be dangerous for the people in the car
What happens if a car crashes and theres a big change in momentum in a short amount of time?
The people inside experience huge forces that could be fatal
How are cars designed to make crashes safer?
Increasing the time over which momentum changes happen, lessoning the force on the passengers
What are car safety features?
Crumple zones, side impact bars, seat belts and air bags
What do crumple zones do?
They’re at the front and back of car to crumple up on impact, the cars kinetic energy s converted into other forms of energy by the car body as it changes shape. Crumple zones increase the impact time, decreasing the force produced by the change in momentum
What do side impact bars do?
They are string metal tubes fitted into car door panels. They help direct the kinetic energy of the crash away from the passengers to other areas of the car, such as the crumple zones
What do seat belts do?
They stretch slightly, increasing the time taken for the wearer to sop. This reduces the forces acting in the chest. Some of the kinetic energy of the wearer is absorbed by the seat belt stretching
What do air bags do?
They also slow you down more gradually and prevent you from hitting are surfaces inside the car
What determines how powerful a car is?
The size and design of car engines
What happens when an engine is more powerful?
The more energy it transfers from its fuel every second, and so the faster its top speed can be
What is the typical power output for a small car and sports car?
Small car would be 50KW and a sports car would be 100KW
What does aerodynamic mean?
The cars are shaped in such a way that air flows very easily and smoothly past them, minimising their air resistance
When do cars reach their top speed?
When the resistive force equals the driving force provided by the engine. So with less air resistance to overcome, the car can reach a higher speed before this happens, aerodynamic cars can, therefore, have higher top speeds
What is a build up of static caused by?
Friction
What happens when certain insulating materials are rubbed together?
Negatively charge elections will be scraped off one and dumped on the other, leaving a positive static charge on one and a negative static charge on the other
What does the way the electrons are transferred depend on?
The two materials involved
What do electrically charged objects attract?
Small objects placed near them
What is an example of this?
Polythene and acetate rods being rubbed with a cloth duster, with the polythene ride electrons move from the duster to the rod, with the acetate rod, electrons move fro the rod to the duster
Do positive charges move?
NO, A positive charge is always caused by electrons moving away elsewhere. The material that loses the electrons loses some negative charge and is left with an equal positive charge
Opposites attract and like charges…?
Repel
What are currents?
The flow of electric charge round the circuit. Current will only flow through a component if there is a potential difference across that component. Unit = ampere, A
What is potential difference?
The driving force that pushes the current round. Unit = Volt, V
What is resistance?
Anything in the circuit which slows the flow down. Unit = ohm, Ω
What is the statement that includes these three things?
The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component).
What does the total charge through a circuit depend on?
Current and time
What happens when current flows past a aunt in a circuit for a length of time?
The charge that has passed is given by current=charge divided by time
What is charge measured in?
Coulombs C
How do you make more charge pass around a circuit?
When a bigger current flows
What is potential difference (work done)?
Its the work done per coulomb of charge that passes between two points in an electrical circuit
What is the formula for this?
P.D.=work done divided by charge
What means the same thing as potential difference?
Voltage
LEARN CIRCUIT SYMBOLS
THERE ON PAGE 59
What circuit do you use if you want to know the resistance of a component?
The standard test circuit-you measure the current through and the potential difference across the component
What does a standard circuit consist of?
Variable resistor, ammeter, voltmeter and component
What does the ammeter do?
Measures the current in amps, flowing through the component. It must be placed in series but can be anywhere in a series in the main circuit but never in parallel
What does the voltmeter do?
Measures the potential difference in volts across the component. It must be placed in parallel around the component under test, NOT around the variable resistor or battery
What is this circuit for?
Testing components and getting V-I graphs from them
What does it mean by the component, ammeter and variable resistor are all in series?
They can be put in any order in the main circuit however the bolster can only be placed in parallel around the component under test
What happens as you vary the variable resistor?
The current flowing through the circuit alters, allowing you to take several pairs of readings from the ammeter and the voltmeter which can be plotted ion a V-I graph
What do V-I graphs show?
How the current varies as you change the potential difference
What are the three different potential difference graphs?
Different resistors, filament lamp and diode
What does the graph for different resistors show?
The current through a resistor (at a constant temperature) is directly proportional to P.D. Different resistors have different resistances, hence the different slopes (different straight lines crossing through 0,0)
What does the graph for a filament lamp show?
As the temperature of the filament increase as, the resistance increases, hence the curve (stretched s shaped line)
What does the graph for a diode show?
Current will only flow through a diode in one direction, the diode has very high resistance in the opposite direction (straight line across x axis on 0, then goes up at 0,0)
What happens when an electrical charge flows through a resistor?
Some of the electrical energy is transferred to heat energy
What does the heat energy cause?
The ions in the conduct to vibrate more. With the ions moving around more its more difficult for the charge-carrying electrons to get through the resistor (current cant flow as easily and the resistance increases)
What does resistance increase with?
Temperature
What does more current mean?
An increase in temperature, meaning an increases in resistance so the current decreases again which is why the filament lamp levels off at high currents
What is the formula to find potential difference?
Potential difference=current x resistance
The steeper the graph the…….?
Lower the resistance
What does it mean if the graph curves?
The resistance is changing
How does a current flow through a diode?
Only in one direction
What is a diode?
A special device made from semiconductor material such as silicon
What are diodes used for?
To regulate the potential difference in circuits by letting the current flow freely in one direction but not in the other (very high resistance in the reverse direction)
What is a light emitting diode (LED)?
Emits light when a current flows through it in the forward direction, being used more as lighting as they use a much smaller current than other forms of lighting
What do LEDs do?
Indicate the presence of current in a circuit, often used in appliances e.g. TVs to show they’re switched on, and also for the numbers on digital clocks, traffic lights and in remote controls
What does LDR stand for?
A light dependent resistor
What is an LDR?
A resistor that is dependant on the intensity of light, in bright light the resistance falls and in darkness the resistance is highest
Where are LDR’s used?
Lots of applications including automatic night lights, outdoor lighting and burglar detectors
What does a graph for an LDR look like?
Slopes downwards from left to right
What is the symbol for an LDR?
A circle with a rectangle in it and then a line going straight horizontally out of the circle from both shorter sides of the rectangle
What is a thermistor?
A temperature dependent resistor, in hot conditions, the resistance drops, in cool conditions, the resistance goes up
When are thermistors used?
Temperature detectors e.g. car engine temperature sensors and electronic thermostats
What does a graph for a thermistor look like?
The same as an LDR, like a slide
What is the symbol for a thermistor?
A rectangle with horizontal straight lines coming off both shorter ends of the rectangle. Then there is a small horizontal straight line that then goes upwards diagonally through the rectangle up to the right
How does a series circuit work?
The different components are connected in a line, end to end, between the +ve and -ve of the power supply except for voltmeters which are always connected in parallel but they don’t count as part of the circuit
What happens if you remove or disconnect one component in a series circuit?
The circuit is broken and they all stop, this is generally not very useful and in practice very few things are connected in series
What happens to the potential difference (P.D) in a series circuit?
The total P.D is shared between the various components. The voltages round a series circuit always add up to equal the source voltage
What is the symbol formula for this?
V=V1 +V2 + …
What is the current like in a series circuit?
The same current flows through all parts of the circuit (A1=A2), the size of the current is determined by the total potential difference of the cells and the total resistance of the circuit (I=V/R)
What is the resistance like in a series circuit?
The total resistance is just the sum of all the resistances (R=R1+R2+R3), the bigger the resistance of a component, the bigger its share of the total P.D.
What is the cell voltage like in series circuits?
There is a bigger P.D. when more cells are in series, provided the cells are all connected the same way. For example when two batteries of voltage 1.5V are connected in series they supply 3 V between them
How does a parallel circuit work?
Each component is separately connected to the +ve and -ve of the supply
What happens if you remove or disconnect one component in a parallel circuit?
It will hardly effect the others at all, this is how most things are connected for example in cars and in household electrics as you have to be able to switch everything on and off separately
What is the potential difference like in parallel circuits?
All components get the full source of P.D. so the voltage is the same across all components (V1=V2=V3), this means that identical bulbs connected in parallel will be at the same brightness
What is the current like in parallel circuits?
The total current flowing around the circuit is equal to the total of all the currents through the separate components (A=A1+A2+…)
What happens in parallel circuits where the current either splits or rejoins?
There are junctions. The total current going into a junction has to equal the total current leaving
In parallel circuits, what happens if two identical components are connected in parallel?
The same current will flow through each component
What are exceptions to the rules for circuits?
Voltmeters and ammeters
Why are ammeters exceptions?
Exceptions to series and parallel rules, they are always connected in series even in a parallel circuit
Why are voltmeters exceptions?
They’re always connected in parallel with a component even in a series circuit
