PHYSICS UNIT 2

Distance-Time Graphs

• The line is straight - this means that the object is moving at a constant speed (40 seconds to go every 1000m)

• If the car had gone faster then the line would have been steeper. In other words the gradient would have been greater (would have gone further than a 1000m every 40 seconds)

• The gradient of a line on a Distance-Time graph represents speed

• Y and Z went at a constant speed because the lines are straight

• X went the fastest because its gradient is greatest• Part of Z’s line is straight – this means the object is stationary

(the car stopped)

Calculating Speed• SPEED = Distance / Time• Distance in metres• Time in seconds• Speed is m/s (metres per second)• Distance – 1000m• Time – 40 seconds• Speed = 1000/40 = 25m/s

Calculating Speed

• Speed of Y = 30,000 / 1250 = 24m/s• For Z you can only work out the average speed• Speed of Z = 20,000 / 1500 = 13.33 m/s

What is Velocity?• An object moving at a constant speed in a straight line

has a constant velocity• An object moving in a circle at a constant speed does

not have a constant velocity

Velocity is speed in a given direction

• Cars travelling at the same speed in different directions have different velocities

What is Acceleration?• Acceleration is change of velocity per second• Unit is metres per second squared

Velocity-Time Graphs

The greater the gradient – the greater the acceleration

Calculating change in velocityChange of velocity = Final velocity (v) – initial velocity

(u)Change of velocity = v - u

Calculating VelocityAcceleration (A) = Change in velocity

Time taken for the change (T)

Acceleration (a) = v-u t

The area under the line on a velocity-time graph represents distance travelled

What is a force?A force can change the shape of an object. It can change its state of rest. It can change its motion

The unit is Newton (N)

Resultant Force

We can work out the effect of the forces acting on an object by replacing them with a single force, the

resultant force

This is a single force that has the same effect as all the forces acting on an object

Zero Resultant Force• If an object remains stationary then the resultant

force is zero• If the object continues to move at the same velocity

then the resultant force is zero• In this case: If only two forces act on an object, they

must be equal to each other and act in opposite directions

Calculating Resultant Force• Resultant force is their difference if the forces act in

opposite directions• Resultant force is the sum of the two forces if they act

in the same direction

Calculating Resultant Force

Resultant Force (F) = Mass (M) x Acceleration (A)

Resultant Force in NewtonsMass in Kilograms

Acceleration in metres/second squared

The greater the mass of an object, the smaller its acceleration is for a given force

Breaking ForceBreaking force to stop a vehicle = Mass * Acceleration

Acceleration when the breaks are first applied

Stopping DistancesShortest distance a vehicle can safely stop

Stopping distance = Thinking Distance + Braking distance

What increases thinking distance?

• Poor reaction time (Tired / Alcohol / Drugs)• High speed

What increases braking distance?

• High speed• Poor weather conditions• Poor maintenance

We can solve this using speed cameras

• Fall in speed by 17%• 55% reduction in deaths and serious injury

How to prevent skidding when breaking

• Anti skid surfaces• Rougher• This increases friction• Reduces skidding when breaking hard• Used at junctions

What is Weight?• Weight of an object is the force of gravity on it• Mass of an object is the quantity of matter in it

Calculating Weight• Weight = Mass * Gravitational Field Strength• Weight in Newtons (N)• Mass in Kg• Gravitational Field Strength in N/Kg

Where will you weigh less?• The gravitational field strength is weaker at the

equator• If you want to reduce your weight, go to the equator

Releasing an object• When you release an object it falls because of its

weight• Weight is the resultant force

Releasing an object into liquid• Drag force acts in the opposite direction to the weight

of the object• Resultant Force = Weight – Drag Force• As it falls the acceleration will decrease• It will reach a constant velocity when the Drag Force =

Weight. The resultant force will be zero• This velocity is its TERMINAL VELOCITY

What is an Elastic Object?• It is an object that regains its original shape• For example rubber

Extension• Increase in length of an elastic object is known as

extension• Extension = New length – Original length

Hooke’s Law• Extension of a spring is directly proportional to the

force applied• As long as the limit of proportionality is not exceeded• It obeys Hooke’s law

Energy and Work

• Work is done on an object when a force makes the object move• Energy transferred = work done• Work done (joules) = force (newtons) x distance moved in the direction of

the force (metres)

Gravitational Potential Energy

• This increases when the object goes up and decreases when it goes down• Change of gravitational potential energy = mass * gravitational field

strength* change in height

Kinetic Energy and Momentum

• Kinetic energy (J) = ½ * Mass (Kg)* Speed (m/s)²• Momentum (Kg m/s) = Mass * Velocity• In a closed system, the total momentum before an event is equal to the

total momentum after an event

Inside an atom

• In the nucleus there are protons and neutrons• Electrons move around the nucleus

• PROTONS – Positive charge• ELECTRONS – Negative charge• NEUTRONS – Uncharged

Equal number of electrons and protons means that the object is UNCHARGED

Objects become charged if electrons or added or lost

STATIC ELECTRICITY

• Adding electrons makes it negative – more electrons than protons

• Removing electrons makes it positive – more protons than electrons

Electric Current

• This is a flow of electrons around a circuit (flow of charge)• Move from negative end of battery (repelled from it) to positive end

of battery• They transfer energy from the battery to the bulb• Battery is two or more cells connected together• In a circuit every component has an agreed symbol• A circuit diagram shows how they are linked together

Calculating Electric Current

• Electric current is the flow of charge (flow of electrons) per second• More electrons = More current

• Current = Charge flow / Time• I = Q/T• I is current in amperes (A)• Q is charge in coulombs (c) • T is time in seconds (s)

Ammeter

• Ammeter measures current• In this case it measures the current going through the bulb (flow of

electrons which carry the energy)

Voltmeter

• Voltmeter measures the potential difference across the torch bulb• This is the amount of energy transferred to the bulb by each coulomb

of charge• It is called voltage• The unit is volts (V)

Calculating Voltage

• Voltage = Work done /charge • V = W/Q• V is Voltage in volts (V)• W is work done in Joules (J)• Q is charge in Coulombs (C)

What is Resistance?

• Electrical components RESIST the passage of electrons• Resistance = Voltage /Current• R = V/I• R is Resistance in Ohms• V is Voltage in Volts• I is current in amperes

OHMS Law

• The current through a resistor at constant temperature is directly proportional to the potential difference across the resistor• Double the voltage – double the current• This means that the resistance is constant• Changing the current does not change the resistance

Series: Parallel:

Current Same current passes through components in series with each other

Current splits. Thus total current is sum of the currents

Potential Difference

This is shared between the components. Thus you add the potential differences to

get the total potential difference

This is the same for each component in parallel

Cells Add the potential difference from each of the cells together to get the total

potential difference

Resistance Add resistances to get total resistance The bigger the resistance the smaller the current

• The battery in a torch makes the current go round the circuit in one direction only. This is called DIRECT CURRENT (dc)• At home you use ALTERNATING CURRENT (ac) which means the current

repeatedly reverses direction. It flows one way and then the opposite way.• Its FREQUENCY is the number of cycles it passes through each second• An OSCILLOSCOPE measures this

• Frequency = 1 / Time taken for one cycle• Frequency = 1/0.02 = 50Hz

Cables

• Every mains circuit has a LIVE wire and a NEUTRAL wire• The live wire alternates between + and – volts• The neutral wire is always at 0V

Earth Wire

• Live wire touches the metal case• Big current will flow from the live into the metal case• This will give you an electric shock• The metal case is attached to the Earth wire • The earth wire takes the current away and blows the fuse.• The fuse in the plug must be in series with the appliance. The fuse

melts.• Alternatively there is a circuit breaker which pulls a switch open

Plugs

• Remember the colours• Cable grip and case and

made of plastic because they are good insulators• Wall sockets are

connected in parallel

1. POWER = Energy transferred / Time – UNIT IS WATTS (W)

2. POWER SUPPLIED = Current * Potential Difference – UNIT IS VOLTS (V)

3. CORRECT RATING FOR A FUSE = Electrical power / Potential Difference

4. CHARGE = Current * Time – UNIT IS COULOMBS (C)

5. ENERGY TRANSFERRED = Potential difference * Charge – UNIT IS JOULES

• Atoms have a nucleus that are unstable. • They become stable by emitting particles

Alpha 2 protons and 2 neutrons stuck

together

Positively charged

Beta Fast moving electron Negatively charged

Gamma Electromagnetic radiation

Uncharged

Nuclear Reactions

• Atomic number: Number of protons in a nucleus• Mass number: Number of protons and neutrons• Number of neutrons = Mass number – Atomic number• Alpha emission reduces the atomic number by 2 and the mass number by 4• Beta emission adds 1 to the atomic number and does not change the mass

number

Half life is the time it takes for the number of nuclei of an isotope (mass of atoms to half)

What are the uses of radioactivity?1. Measure thickness2. Trace the flow of a substance through a system3. Find the age of an ancient material

• Nucleus releases 2 or 3 neutrons and energy (radiation)• This causes other nuclei to split – CHAIN REACTION• This happens in a nuclear reactor• The energy is used to generate electricity• Uses Urannium-235 or Plutonium-239• https://www.youtube.com/watch?v=EsuRgxsd6bk

Nuclear Fission

1. Small nuclei are fused into large nuclei (for example hydrogen)2. This releases energy3. It happens in stars and the sun4. Could do this with water – lots of hydrogen atoms5. The nuclei need to move fast or they will repel

Nuclear Fusion

1. 13 billion years ago- THE BIG BANG2. A hot glowing ball of radiation and matter were created3. The university expanded and the temperature fell – it became cold and dark

– THE DARK AGES4. Uncharged matter was attracted to one another and formed gigantic

clumps5. This became galaxies and stars

The birth of the universe

1. 13 billion years ago- THE BIG BANG2. A hot glowing ball of radiation and matter were created3. The university expanded and the temperature fell – it became cold and dark

– THE DARK AGES4. Uncharged matter was attracted to one another and formed gigantic

clumps5. This became galaxies and stars

The Birth of the Universe

1. Gravity pulls dust and gas together. This forms a PROTOSTAR

2. As it gets denser –it gets hotter

3. Nuclear fusion changes hydrogen to helium. This is the MAIN SEQUENCE. The star is stable because the force of gravity is balanced by the outward force of radiation.

4. Star runs out of hydrogen

5. Those the same size of the sun swell out, cool down and turn red. This is called a RED GIANT

6. Gravity then causes it to collapse in on itself. It heats up and turns white. This is called a WHITE DWARF

7. It then goes cold and becomes a BLACK DWARF

5. Those bigger than the sun swell out and become a RED SUPERGIANT

6. It starts to collapse but then explodes in a massive explosion called a SUPERNOVA

7. Most then become NEUTRON STARS. Very heavy stars create a BLACK HOLE

The Life Cycle of a Star