Y10 Physics End of Year

what is a scalar quantity

has a magnitude but no direction

what is a vector quantity

has both magnitude and direction

the 5 scalar quantities

• speed

• distance

• mass

• time

• energy

the 6 vector quantities

• velocity

• displacement

• weight

• force

• acceleration

• momentum

velocity definition

speed in a stated direction

equation for speed

speed (m/s) = distance (m)
time (s)

what does a straight line mean on a distance/time graph

constant speed

what does the slope of the line represent on a distance/time graph

the magnitude of the speed:

• steep slope = large speed

• shallow slope = small speed

• flat slope = stationary (not moving)

what do curves mean on distance/time graphs

changing speed

• if the slope is increasing the speed is increasing

• if the slope is decreasing the speed is decreasing

how do you work out speed from a distance/time graph

using the gradient of the line:

speed = gradient = rise / run

equation of acceleration

acceleration (m/s²) = change in velocity (m/s) / time taken (s)

a = v-u
t

equation for change in velocity using acceleration and distance

final velocity² - initial velocity² = 2 x acceleration x distance

v² - u² = 2ax

what does a straight line mean on a velocity/time graph

constant acceleration - the object is speeding up at a constant rate

what does the slope of the line represent on a velocity/time graph

the magnitude of acceleration

• steep slope = large acceleration (speed changes quickly)

• gentle slope = small acceleration (speed changes gradually)

• flat line = zero acceleration (object is at a constant velocity)

how do you calculate acceleration from a velocity/time graph

acceleration = gradient = rise / run

what does the area under a velocity/time graph represent

the displacement - the distance travelled by the object

split the area up into triangles and rectangles to work it out

what are light gates

pieces of digital equipment that accurately measure times. using it you can accurately work out speed

using 2 light gates to measure time

the moving object will block the beam of light on the light gate when it passes through, triggering the equipment

1. a timer will start when the object passes through the light gate

2. the timer is stopped when the object passes through the second light gate

the time can then be used to work out speed

using a single light gate to measure speed

1. there is a flag/object on top of the moving object

2. as the object passes through the light gate it blocks the beam of light for a set amount of time as it passes through

3. the timer measures how long the light is blocked for

4. the distance is the length of the flag

5. then you can work out speed

typical speeds table

acceleration due to gravity

10 m/s²

typical accelerations table

newton’s first law

when the resultant force on a body is zero (still or at a constant speed), it will stay at zero unless an external force acts upon it

when the resultant force on a body is not zero, the object will accelerate, deaccelerate, or change direction

no force = no change in motion // force = change in motion

newton’s second law

force (N) = mass (kg) x acceleration (m/s²)

f = m x a

---

force = change in momentum (kg m/s) / time (s)

F = (mv - mu)
t

weight definition

the force acting on an object due to gravitational attraction

weight equation

weight (N) = mass (kg) x gravitational field strength (N/kg)

W = m x g

how to measure weight

using a newton-meter, which measures force in newtons. spring fixed at one end with a hook to attach an object on the other

you can indirectly measure weight using a top pan balance (scales) to find mass and then use w=mg

gravitational field strength (g) on earth

10 N/kg

relationship between weight & gravitational field strength

mass always stays the same

weight changes depending on gravity:

• if gravity increases, weight increases

• if gravity decreases, weight decreases

equipment for investigating acceleration practical

• metre ruler: to measure distance

• car / trolley: the object for which acceleration is measured

• pencil / chalk: to mark intervals of distance

• bench pulley & string: to connect masses to the trolley

• weights & weight hanger: to provide force on the trolley

• stopwatch: to time the trolley between distance intervals

• blu tac: to attack extra weight to trolley if needed

independent variable

the variable you change, that is unaffected by other variables

dependent variable

the variable you measure, depends on other variables

control variable

something you keep the same

what happens when an object is moving in a circular orbit at a constant speed

even though speed is constant, the velocity is constantly changing because it depends on direction

centripetal force

The resultant perpendicular force towards the centre of the circle required to keep a body in uniform circular motion

inertial mass

how hard it is to change an object’s velocity (speed or direction), something with a large inertial mass resists acceleration more than one with a small inertial mass

mass = force / acceleration (comes from f = ma)

the more mass an object has, the more it resists changes in velocity

newton’s third law

if object A exerts a force on object B, then object B will exert a force on object A that’s exactly the same size, at exactly the same time, but in the opposite direction

action reaction forces

newton’s third law in equilibrium situation

if an object is at rest or moving at a constant velocity, the forces acting on it are balanced

e.g. for a book on a table, the book pushes down on the table (its weight) and the table pushes up on the book with an equal force (reaction force) meaning nothing moves.

newton’s third law in collisions with momentum conservation

if e.g. two ice skaters push apart, A pushes on B, and B pushes back on A, their combined total momentum before and after is the same

this is because they each have the same momentum but going in opposite directions so together it equals zero

law of conservation of momentum in collisions

total momentum before = total momentum after

momentum definition

how much motion an object has

equation for momentum

momentum (kg m/s) = mass (kg) x velocity (m/s)

p = m x v

elastic collsision

when objects collide and move in opposite directions

each object will have a different velocity depending on its mass and the initial momentum of the system

inelastic collisions

when objects collide and move in the same direction together

objects have a combined velocity

reaction time

how much time passes between seeing something and reacting to it

a reaction time for someone who is alert (waiting to react to something) is between 0.2 and 0.9 seconds

ruler method for measuring reaction time

1. person a holds a ruler vertically at the top end so that the bottom end hovers over the top of person b’s hand

2. person a releases the ruler unexpectedly

3. as soon as person b sees the ruler move, they close their hand to catch it

4. the ruler is marked at the point at which it was caught by person b

5. the greater distance, the longer reaction time

formula of stopping distance of a vehicle

thinking distance + braking distance

thinking distance = distance travelled in the time it takes the driver to react (reaction time) in meters
braking distance = distance travelled under the braking force in meters

the greater the speed of the vehicle, the greater the stopping distance

factors affecting stopping distance of a vehicle

• vehicle mass

• vehicle speed

• driver’s reaction time

• state of the brakes

• state of the road

• friction between the tire and the road (e.g. icy road = less friction)

factors affecting a driver’s reaction time

• tiredness

• distractions e.g. using a phone

• intoxication: alcohol / drugs

dangers of large decelerations

overheating of brakes: kinetic energy of vehicle is converted into thermal energy of the brakes, and if they get too hot they can fail, meaning they won’t work effectively

injury: when a vehicle decelerates the driver & passengers also decelerate and this can cause injuries e.g. whiplash, a neck injury when your head moves suddenly relative to your body

loss of control: it’s more difficult to control a vehicle that’s decelerating, and loss of control can cause a collision

equation for work done

work done (J) = force (N) x distance moved in direction of force (m)

E = F x d

braking distance with kinetic energy & work done

when a vehicle brakes, its kinetic energy is being transferred to thermal energy.

so work is being done on the vehicle by the braking force. the work done = the total kinetic energy transferred to heat

you can calculate braking distance and/or work done and/or braking force using E = F x d:
E = work done = KE
F = braking force
d = braking distance

because work done is the same as kinetic energy, we can say that ½ x m x v² = F x d

equation for change in GPE

change in GPE (J) = mass (kg) x gravitational field strength (n/kg) x change in vertical height (m)

ΔGPE = m x g x Δh

kinetic energy equation

kinetic energy (J) = ½ x mass (kg) x speed² (m/s)

KE = ½ x m x v²

sankey diagrams

sankey diagrams show the amount of energy transferred and where to

different thicknesses of arrows represent how much energy is transferred in that way

percentages are put in brackets

other energy transfer diagrams

principle of conservation of energy

energy can’t be created or destroyed, it can only be transferred from one store to another

so the total amount of energy in a closed system remains constant

the total energy transferred into a system must be equal to the total energy transferred out

energy transferred when a ball being thrown up

• before ball is thrown the person holding the ball has energy in their chemical store

• when the ball is thrown some of that energy is transferred to the kinetic store of the ball as it begins to move upwards

• as the height of the ball increases, energy from the kinetic store of the ball is transferred to its gravitational potential store

energy transfers with a moving object hitting an obstacle

• when a car is moving energy in the chemical store of fuel is transferred to the kinetic store

• when it hits a wall the speed and therefore the energy in the kinetic store decreases quickly

• most of this energy is dissipated

• but some is transferred mechanically to the thermal store of the wall (the force of the car on the wall)

• and some is transferred by heating to the thermal store of the air as the sound waves transfer energy away from the system

energy transfers with a vehicle being accelerated by a constant force

• when the vehicle is stationary it has energy in the chemical store of its fuel

• when it accelerates, the energy is transferred to the kinetic store of the car

energy transfers with a vehicle slowing down

• the moving vehicle has energy in its kinetic store

• as it slows energy is transferred to the thermal store of the surroundings (dissipated)

• energy is also transferred by heating due to friction between the tyres and the ground and the the brakes and the brake pads

• energy is transferred by heating as sound waves transfer energy away from the system

energy transfers with boiling water in an electric kettle

• energy is transferred electrically from the mains supply to the thermal store of the heating element in the kettle

• as the heating element gets hotter energy is transferred by heating to the thermal store of the water

energy transfers with a trampoline

elastic potential energy → kinetic energy → gravitational potential energy

• while jumping the person has energy in their kinetic store

• when the person lands on the trampoline most of that energy is transferred to the elastic potential store of the trampoline

• that energy is transferred back to the kinetic store of the person as they bounce up

• as the person gains height energy is transferred from kinetic → gravitational potential

• some of the energy is dissipated by heating to the thermal store of the surroundings (person, trampoline, air)

how do mechanical processes become wasteful (energy)

when they cause a rise in temperature (usually through friction) because energy dissipates through heating into the surroundings

useful energy definition

an energy transfer that serves an intended purpose

wasted energy definition

an energy transfer that is not useful for the intended purpose and is dissipated to the surroundings

dissipated energy

energy that spreads out into the surroundings is hard to ‘gather’ so that it can be used usefully again. this means that the energy becomes less useful

whenever a process produces unwanted heating, light, or sound, the energy is dissipated and wasted

however not all dissipated energy is wasted e.g. a tv generates light & sound energy that is useful, or a heater generates useful heat energy

reducing energy loss

producing lots of waste energy can take up lots of energy and be very expensive, so we need to find ways to reduce waste

lubrication and insulation

Insulation

reduces energy lost from conduction

effectiveness depends on:

• thermal conductivity (must be low)

• density of material (must be low)

• thickness of material (must be high)

insulation is made of fibreglass / glass fibre, a reinforced plastic composed of woven matieral with glass fibres laid across and held together. the air trapped between the fibres makes it a good insulator

cavity wall insulation: gaps (cavities) between external walls are filled with insulation (a special type of foam) to lower conduction from inside to outside

thermal conductivity

how well a material allows heat to move through it

depends on thickness and temperature across the material

a good insulator needs low thermal conductivity

three ways in which energy is transferred through heating

conduction: vibrations passed between particles in a solid

convection: part of a fluid is warmer (less dense) and rises up, on the other side the colder (more dense) fluid sinks, creating a convection current going around in a circle

radiation: energy transferred through waves. infrared radiation can pass through some solid objects. works in a vacuum

lubrication

friction is a major cause of wasted energy transfers, and the amount of wasted energy can be reduced if the amount of friction is reduced. this is done by lubricating the parts that rub together

efficiency equation

useful energy transferred by device / total energy supplied to device

how to improve efficiency of a device

by reducing wasted energy transfers due to friction, air resistance, electrical resistance, or noise

reducing friction

• adding bearings to prevent components from directly rubbing together

• lubricating parts

reducing electrical resistance

in electrical circuits there is resistance as current flows through the wires and components, resulting in unwanted energy transfers by heating to the wires, components & surroundings

this can be reduced by using components with lower resistance or reducing the current

reducing air resistance

air resistance causes frictional force between the moving object and the air that opposes its motion, resulting in unwanted energy transfers by heating to the object and the surroundings

this can be reduced by streamlining the shapes of moving objects

e.g. cyclists adopt more streamlined posture to reduce the effects of air resistance. their bicycle, clothing, and helmet are streamlined too

reducing noise

sound is created by moving parts of machinery, resulting in unwanted energy transfers by heating to the surroundings as sound waves cause the particles in the air & nearby objects to vibrate

this can be reduced by tightening loose parts to reduce vibration or lubricating parts

energy resources: solar energy

heating & lighting from the sun are either used with solar cells to generate electricity, or radiation from the sun is used to heat water for homes using solar panels

energy resources: wind energy

using wind turbines we can use the kinetic energy of the wind to turn a turbine and generate electricity

energy resources: hydroelectric energy

water flowing in a river turns a turbine to generate electricity. we usually need to build a dam and let water flow through it gradually

energy resources: wave energy

the sea’s waves have kinetic energy and using machines we can turn that into electricity

energy resources: nuclear energy

the energy in uranium nuclei is transferred into heating, which is used to create steam that turns a turbine and generates energy

nuclear store of fuel → thermal store of water → kinetic store of turbine → kinetic store of generator

energy resources: fossil fuels

chemical energy in coal, oil, and gas is transferred into heating, which is used to create steam which turns a turbine and generates electricity

energy in chemical store of fuel → thermal store of water → kinetic store of turbine → kinetic store of generator → transferred electrically to national grid

energy resources: biomass energy

chemical energy in things that were once alive e.g. trees is transferred to heating when they are burned

energy resources: geothermal energy

rocks deep underground are very hot, and we can use this heat to generate electricity by producing steam to turn a turbine

energy resources: tidal energy

at high tide, water is trapped behind a dam, and at low tide it is released, turning turbine which is used to generate electricity

energy resources

energy resources: large stores of energy that can be used to generate electricity and heat homes & businesses

can be renewable or non-renewable

in most cases a turbine is turned, which turns a generator, which generates energy. what changes is how the turbine is made to turn

renewable energy resources

an energy resource that is replenished at a faster rate than the rate at which it is being used. as a result of this it won’t run out

• solar energy

• wind

• biofuel/biomass

• hydroelectricity

• geothermal

• tidal

• wave

non-renewable energy resources

fossil fuels (coal, oil, natural gas) and nuclear fuel

reliable energy resource

one that can product energy at any time

non-reliable ones can only produce energy some of the time e.g. when its windy / when its sunny

energy resources advantages & disadvantages

three main uses of energy resources

• transport

• electricity generation

• heating

use of energy resources: transport

most vehicles are powered by petroleum products (e.g. petrol, diesel, kerosene) which originate from crude oil, a fossil fuel

but more vehicles now are being powered by electricity, which produces no carbon emissions but uses both renewable and non-renewable energy sources

vehicles can also be powered by biofuel, which is a renewable resource but some believe that the process isn’t carbon-neutral

use of energy resources: electricity

increasing population + increasing use of electricity = high electricity demand

this means that all available energy resources are needed for electricity generation, however the majority of this is non-renewable & carbon-emitting. this has a negative effect on the environment

use of energy resources: heating

central heating systems utilise natural gas to heat up water which can be pumped around radiators, but gas isn’t renewable

some geologically active countries (e.g. Iceland) can heat their homes using geothermal energy

waves

an oscillation (vibration) that transfers energy, not matter, from one place to another

wavelength

the distance from one point on the wave to the same point on the next wave aka the length of one cycle e.g. from crest to crest or trough to trough

symbol λ (lambda)
measured in meters

amplitude

the size of the wave, ½ of the crest to trough in meters, symbol A

frequency

the number of complete waves to pass a point per second in hertz (Hz)