P2 Part A Flashcards
What is the difference between speed and velocity?
Speed is how fast an object is travelling, and velocity is the same but with direction specified too
What does the gradient of a distance time graph tell you?
How fast the object is travelling
How do you calculate speed from a distance/time graph?
Speed = Gradient = vertical ÷ horizontal
What is acceleration?
Acceleration is how quickly the velocity is changing. This change in velocity can either be a change in speed or a change in direction.
What is the formula for acceleration?
acceleration = change in velocity(final velocity - initial velocity) ÷ time taken
What is the unit used for acceleration?
m/s2
What is the formula for calculating acceleration in a certain time fram from a V-T (Velocity-Time) graph?
Acceleration = gradient = Vertical (velocity) change ÷ Horizontal (time) change
What force is the force of attraction between all masses?
Gravity
On the …… of a planet, …… makes all things ……. towards the ground (all with the same acceleration, which is about …. m/s2 on Earth)
On the surface of a planet, gravity makes all things accelerate towards the ground (all with the same acceleration, which is about 10m/s2 on Earth)
What is ‘mass’?
Mass is the amount of substance in an object. For any given object this will have the same value anywhere in the universe (measuref in g/kg)
What is ‘weight’? How can it be measured?
The weight of an object is the amount of gravity acting on an object (measured in Newtons). This will be different on different planets due to varying gravitational forces on each.
Weight can be measured using a spring balance or newton meter.
What is the formula for working out the weight of an object?
weight = mass x gravitational field strength
What is the resultant force of an object?
A resultant force is the total force on an object, taking into account forces in both directions
eg. on a car the total of the driving force - air resistance
What happens if the resultant force on a stationary object is 0?
It will stay stationary
What happens if there is no resultant force on a moving object? How does this effect velocity?
If there is no resultant force on a moving object, it will carry on moving at a steady speed. There will be no change in velocity.
What does it mean if an object has a resultant force? How is this shown on a force diagram?
It means that the object will accelerate in the direction of that resultant force. On a force diagram, this will be shown by arrows that are unequal.
What is the equation for resultant force? How can this be rearranged to work out acceleration?
Resultant force = mass x acceleration
(F = ma)
Acceleration = resultant force ÷ mass
(a = F/m)
Fill in the gaps:
When two objects interact, the …… they exert on each other are …… and ………..
When two objects interact, the forces they exert on each other are equal and opposite.
If two objects interact with equal and opposite forces, how does anything ever move anywhere?
The different objects will have different masses, meaning they accelerate at different speeds (a = F/m)
Why does friction always slow objects down? When does friction occur?
Friction always acts in the opposite direction to movement. Friction happens between two surfaces when they come in to contact with one another, or when an object passes through a fluid (drag).
Why are cars streamlined? And why do parachutes have a large surface area?
Cars are streamlined to reduce the area available for resistive forces to work on (eg. air resistance), as they slow moving objects down (and would make the engine work harder).
A parachute, on the other hand, has a large surface area to so that the area resistive forces (drag) can work on is greater, making for a slow and gentle landing.
Drag increases with speed - true or false?
True. car has much more friction to work against when travelling at 70mph compared to 30mph. This means the engine will have to work much harder to maintain a steady speed.
Outline the three stages of a falling object
- At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.
- As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.
- Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.
What is terminal velocity?
The terminal velocity is the fastest velocity that an object can have as it is falling.
How does a parachute change someone’s terminal velocity?
Without a parachute, a person would have a small surface area and a force of mass x gravity pulling them down. They would reach terminal veolcity at about 120mph.
With a parachute open, there’s much more air resistance (and still the same force mass x gravity) pulling them down, meaning their terminal velocity would decrease to about 15mph (a safe speed to hit the ground)
What is ‘thinking distance’ and what factors impact on it?
Thinking distance is the distance a vehicle travels during the driver’s reaction time to needing to stop. Affected by:
- How fast the vehicle is travelling (faster you go, further the thinking distance)
- Mental state - tiredness/drugs/alcohol/careless attitude
- Bad visibility
What is braking distance and what factors affect it?
Braking distance is the distance a vehicle travels under the breaking force.
Affected by:
- How fast the vehicle is going
- How good the brakes are
- How good the tyres are - minimum tread depth of 1.6mm
- How good the grip is - road surface, weather conditions, tyres
Fill in the gaps:
When a …… moves an object through a distance, ……. is transferred and …….. is ……….
When a force moves an object through a distance, energy is transferred and work is done
What is the equation for work done?
work done = force x distance
If an od tractor tyre is dragged 5m across the ground and with a force of 340 N, what is the energy transferred?
Energy transferred means work done
W = f x d
W = 340 x 5
W= 1700 joules
What is gravitational potential energy?
Gravitational potential energy (measured in joules) is the energy that an object has due to its vertical position in a gravitational field.
What is the approximate gravitational field strength on Earth?
10 N/kg
What is the formula for working out gravitational potential energy?
Gravitational potential energy = mass x gravitational field strength x height above the ground
Kinetic energy is the energy of movement. Give the equaton for calculating it.
kinetic energy = 0.5 x mass x speed2
In symbol form: kinetic energy = 0.5mv2
A car of mass 2450kg is travelling at 38m/s. Calculate its kinetic energy.
kinetic energy = 0.5 x 2450 x 382
= 1,768,900 joules
The more an object …….. and the ……. its going, the …….. its kinetic energy will be.
The more an object weighs and the faster its going, the greater its kinetic energy will be.
What are the symbols for gravitational potential energy and kinetic energy?
Gravitational potential = Ep
Kinetic = Ek
To slow a car down, its kinetic energy, must be converted into other types of energy (using the law of conservation of energy). Fill in the gaps:
To stop a car, the ……. energy (0.5 x …… x speed2) has to be converted into …… energy as ……. between the wheels and the brake pads, causing the temperature of the brakes to increase.
To stop a car, the kinetic energy (0.5 x mass x speed2) has to be converted into heat energy as friction between the wheels and the brake pads, causing the temperature of the brakes to increase.
When a car is stopping:
Kinetic energy transferred = Work done by brakes
Convert this in to a formula, give it in symbol form.
Kinetic energy transferred = Work done by braks
0.5 x m x v2 = F x d
m = mass of car AND passengers (kg)
v = speed of car (m/s)
F = maximum braking force (in N)
d = braking distance (in m)
Fill in the gaps”
When something falls, its …… energy is converted into ……… energy so the further it falls, the …… it goes.
Kinetic energy ……. = Potential energy lost
When something falls, its potential energy is converted into kinetic energy so the further it falls, the faster it goes.
Kinetic energy gained = Potential energy lost
Falling objects convert Ep into Ek, and then the Ek is converted in to what? How does this impact the design of space shuttles?
Ek is converted into heat and sound energy as friction due to collisions with particles in the atmosphere treansfer some of their kinetic energy and work is done.(This means only the biggest meteors make it through to the Earth’s surface as most will burn up completely.)
Space shuttles have heat shields made from special materials, which lose heat quickly and allow the shuttle to re-enter the atmosphere without burning up.
What is elastic potential energy and how does it occur?
When you apply a force to an object you may cause it to stretch and change shape. Any object that can return to its original shape after the force has been removed is elastic. Work is done to an elastic object to change its shape. This energy is not lost but it is stored by the object as elastic potential energy. The elastic potential energy is then converted to kinetic energy when the force is removed and the object returns to its original shape.
What is the eqution for calculating force applied to an object that extends?
Force = spring constant x extension
F = k x e
Why is there a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally?
There is a maximum force that an elastic object can take and still extend proportionally to the force being applied. This is known as the limit of proportionality. If you increase the force past this point, the material will be permanently stretched. When the force is removed, the material will be longer that at the start.
What is power and what unit is it measured in?
Power is the rate of work done. It is measured in either watts or joules per second.
1 watt = 1 joule of energy transferred per second.
What is the equation for power?
Power = Work done (or energy transferred, joules) ÷ Time taken (seconds)
A motor transfers 4.8 kJ of useful energy in 2 minutes. Find it’s power output.
Power = energy ÷ time
Power = 4.8J ÷ 2 minutes
Power = 4800J ÷ 120 seconds
= 40watts
How might a person’s power output be calculated? Give the equations used for each method
- A timed run upstairs. In this case energy transferred is the potential energy you gain so: Power = (mass x gravitational field strength x height) ÷ time
- A timed acceleration. The energy transferred is the energy you gain so: Power = (0.5 x mass x speed2) ÷ time
What is momentum?
Momentum is a property of moving objects. The greater the mass of an object, and the greater its velocity, the more momentum it will have.
What is the equation for momentum?
Momentum (kg m/s) = mass (kg) x velocity (m/s)
When two objects collide, how do you calculate the momentum before and after the collision?
Momentum before = Momentum after
Choose which direction is positive velocity.
Calculate the total momentum before collision by adding the momentum of the two objects together
Work out the total momentum after the collision by adding the mass of the two objects together.
If a 1kg gun fires a 0.01 kg bullet at 150 m/s, at what speed does the gun move backwards? Designate the positive direction as the way the bullet is being fired.
The total momentum before firing = 0 kgm/s
Total momentum after firing = momentum of bullet + momentum of gun
= (0.01 x 150) + (1 x v)
= 1.5 + v
The momentum of a system before an explosion is zero, so, due to conservation of momentum, the total momentum after an explosion is 0 too.
So 1.5 + v = 0, meaning v = -1.5 m/s
The gun moves backwards at 1.5m/s.
What causes a change in momentum?
Forces. A larger force means a faster chance of momentum, and thus a greater acceleration.
How are cars designed to convert kinetic energy safely in a crash?
- Crumple zones - at the front and back of the car, crumple up on impact. This converts the car’s kinetic energy into other forms by the car body as it changes shape. increases impact time, reduces force produced.
- Seat belts - reduces the forces acting in the chest.
- Side impact bars - strong metal tubes fitted to the door panels, direct kinetic energy away from the passengers and to other areas of the car (eg. crumple zones).
- Air bags - slow passengers slow passengers down more gradually and prevent them from hitting hard surfaces.
How do car brakes work?
They reduce the kinetic energy of the car by converting it into heat and sound energy
How do regenerative braking systems work?
Regenenerative brakes put the car’s motor into reverse. With the motor running backwards, the wheels are slowed. At the same time, the motor acts as an electrical generatior and converts the car’s kinetic energy into electrical, which is stored as chemical energy in the car’s battery.
What is static caused by?
Static is caused by friction. When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other. This will leave a positive static charge on one material, and negative on the other.
Give a classic example of static charge being generated
- When a polythene rod is rubbed with a cloth duster, electrons move from the duster to the rod (giving it a negative charge)
- When an acetate rod is rubbed with a cloth, duster, electrons move from the rod to the duster (giving the rod a positive charge).
Both positive and negative ……… charges are only ever produced by the movement of …………
A ………… charge is caused by electrons moving elsewhere.
Both positive and negative electrostatic charges are only ever produced by the movement of electrons
A positive charge is caused by electrons moving elsewhere.
Two things with opposite electric charges will repel each other, true or false?
False. Two things with opposite electric charges are attracted to each other. Two things with the same charge will repel each other.
Define and give the units used for
a) Current
b) Potential difference
c) Resistance
a) Current is the flow of electrical charge around a circuit. It will only flow through a component if there is potential difference. Unit: ampere/amps, A
b) Potential difference is the driving force that pushes the current round. Unit: volts, V
c) Resistance is anything in a circuit, which slows the current down. Unit: ohm, Ω
What is the equation for working out current? What is the unit for charge and how can you rearrange the first equation to find it?
Current = charge ÷ time (seconds)
I = Q ÷ t
The unit for charge is coulombs (C).
Charge = current x time
Q = I x t
A battery charger passes a current of 2.5 A through a cell over a period of 4 hours. How much charge does the charger transfer to the cell altogether?
Q = I x t
= 2.5 x (4 x 60 x 60)
= 36,000 C (or 36 kC)
How is the potential difference/voltage of an electrical compenent calculated?
PD = Work done (the energy transferred across the component, joules) ÷ Charge (coulombs)
Draw as many electrical circuit symbols as you can
How must a voltmeter be placed in a circuit to measure the potential difference across a component?
It must be placed in parallel around the component
Sketch a potential difference-current (V-I) graph for the current through a resistor
The current through a resistor (at a constant temperature) is directly proportional to potential difference.
Sketch a potential difference- current (V-I) graph for a filament lamp
As the temperature of the filament lamp increases, the resistance will too - hence the curve.
Sketch a potential difference-current graph for a diode
Current will only flow through a diode in one direction, the diode has a very high resistance in the opposite direction.
Why does resistance increase with temperature?
Heat energy causes the ions in the conductor to vibrate more, this makes it more difficult for charge-carrying electrons to go through the resistor (the current can’t flow as easily).
Why does the V-I graph for the filament lamp level off at high currents?
More current means an increase in temperature, which means an increase in resistance, which means the current decreases again.
What is the equation for calculating potential difference/voltage in a circuit?
Potential difference = current x resistance
What is a diode and what is it used for?
A diode is a special device made from semiconductor material such as silicon. It is used to regulate the potential difference in circuits - it lets the current flow freely in one direction, but not in the other.
Why are light-emitting diodes so useful?
LEDs emit light when a current flows through in the forward direction. They are being used more as lighting because they use a much smaller current. They indicate the presence of current in a circuit so are often used in appliances (eg. TVs) to show they’re switched on.
What is a light-dependent resistor and what are they useful for?
An LDR is a resistor that is dependent on the intensity of light; in bright light the resistance falls, in darkness the resistance is highest. They can be used in automatic night lights, outdoor lighting and burglar detectors.
What is a thermistor and what are they useful for?
A thermistor is a temperature dependent resistor. In hot conditions, the resistance drops and in cool conditions the resistance increases. They make useful temperature detectors eg. in car engine temperature sensors and electronic thermostats.
What are the three rules that must be considered when working out the potential difference, current and resistance in a series circuit?
- Potential difference is shared between the various components.
- Current is the same everywhere.
- Resistance adds up across all components to equal the total resistance in a circuit.
What is the only real-life example of things connected in series and why are they inconvenient?
Christmas fairy lights. This is inconvenient because when one of the bulbs break, the whole lot stop working. The only advantage is the bulbs can be small because the total 230v can be shared out between them.
What are the two rules that must be considered when calculating the potential difference and current in series circuits?
- Potential difference is the same across all components.
- Current is shared between brances.
Why are voltmeters and ammeters exceptions to the series and parallel circuit rules?
Ammeters are always connected in series, even in a parallel circuit.
Voltmeters are always connected in parallel with a component, even in a series circuit.
Why is everything electrical in a car connected in parallel?
- To ensure everything can be turned on and off separately
- So everything gets the full voltage from the battery.
