Paper 2 GCSE Physics AQA

What is a force?
How are they measured
Quantity

A push or pull that acts on an object due to its interaction with another object (Newtons)

Vector Quantity

What is the difference between a vector quantity and a scalar quantity?

Vector quantities have magnitude and an associated direction whereas Scalar quantities have magnitude only

Examples of Vector and Scalar quantities

Scalar - Distance, mass, temperature, time
Vectors - Velocity, Displacement, Acceleration, Force, Momentum

What are the 2 categories of forces?

Contact Forces - The objects are physically touching
Non-contact forces - The objects are physically separated

Examples of Contact and Non-contact forces

Contact forces: Friction, Air resistance, tension and normal contact force
Non-contact forces - gravitational force, electrostatic force and magnetic force

What is a normal contact force?
Example

A rock on a table has mass therefore weight is being exerted downwards.
The table exerts an equal and opposite force. This is the normal contact force

How are vector quantities represented?

Arrow - The length represents the magnitude and the direction of arrow represents direction

What is a resultant force?

The overall force of the object
Some force cancel each other out

What is a balanced force/equilibrium of an object?

When there is no resultant force acting upon it

What is weight and what does it depend on?

Weight is the force acting on an object due to gravity
The weight of an object depends on the gravitational field strength at the point where the object is

What is deformation?
2 Types

When an object changes shape it is deformed
Elastic deformation and Inelastic deformation

Difference between Elastic and Inelastic deformation

Elastic deformation is when an object returns to its original shape after forces are removed whereas inelastic deformation is when an object stays deformed after the forces are removed

What is extension?
Spring with mass added onto it

Increase in length of a spring when stretched
As force is increased on the spring (in the form of extra mass added), the extension increases proportionally
F∝e

What does an object’s extension depend on?

Spring constant (k) - How many newtons it would take to stretch the object by 1 Metre

Equation linking force, extension and spring constant

F = k e
(N) (N/m) (m)

Describe a force-extension Graph

As the force increase, so does the extension
Straight line that passes through the origin so force and extension are directly proportional
All elastic deformation

What happens when force-extension line starts to curve?

Object has reached its elastic limit/limit of proportionality
Hooke’s law no longer applies
It will have inelastically deformed

Force-extension graph information before it curves

Gradient of line = Spring constant (k)

Area under curve/line = Energy transferred to spring (EPE)

What is elastic potential energy?

Energy transferred to an object as it is stretched

Elastic potential energy equation

EPE = ½ k e²

(J) (N/m) (m)

What is a moment?
Example

The rotational or turning effect of a force
Applying a force down at the end of a spanner - it will turn around the central point (pivot) - turning effect is the moment

Moment Equation
2 moments on one object

M - F d
Moment (Nm) = Force (N) x Perpendicular distance between pivot and place where force is being applied (m)
Clockwise or anticlockwise
If two moments are on one object, the leftover moment is the overall moment

What is the perpendicular distance?

Perpendicular distance from the line of action of the force to the axis of rotation

What do levers do?
Input and output forces and where they are in relation to the pivot

Transmit the turning effect of a force

If the input and output forces are on different sides of the pivot, they act in different directions

If the forces are on the same side of the pivot, they act in the same direction

Output force is closer to the pivot, the force will be larger

What do gears do?

Example

Transmit turning effects

Example:

Gear A (engine) and Gear B (wheels)
Turning effect of engine - rotates gear a - rotates gear b - wheels rotate
Gears turn in opposite directions
Gear B radius is 2x larger than Gear A so turning effect is 2x bigger
Turning effect has been doubles
Gear A has to rotate 2x as much as Gear B
Work done remains the same

What is pressure?
Formula

Force per unit of Area
P = F/A

Pressure (Pa) = Force (N) / Area (m)

Fluid pressure

Particles bang into surroundings and generate pressure
Collisions of particles with wall creates pressure (applying force to inside area of wall)

What is perpendicular force?

Component of the force which is at right angles to the surface its colliding with

Example:

particle at perfect right angle will exert entire force in comparison to particle at an angle

What is pressure acting on an object in water due to? 4 things

Surrounding Water molecules constantly colliding with object
Weight of the water above the object - the downward force of it
Density of the Liquid - denser, the larger the mass per volume therefore the larger the weight
Gravitational field strength - determines weight for a given mass

Pressure in a liquid equation

P = hpg
Pressure in a liquid (Pa) = Depth (m) x density of liquid (kg/m3) x gravitational field strength (N/kg)

What is upthrust?

Water exerts force on an object in water
The bottom of an object is deeper therefore will experience a larger upwards force that downwards force.
Therefore resultant force upwards

What causes an object to not float?

An object’s weight may be larger than upward force therefore the object will sink
If an object is more dense, it will sink

Where is pressure highest in the earth’s atmosphere?

The density of the gas particles is highest near the Earth’s surface therefore pressure is highest

Why is the pressure highest at sea level?

There are more particles therefore more collisions and a greater force overall therefore there is more pressure
There is also a larger weight of all the particles above pressing down on an object at sea level

Velocity equation

V = s/t

Average velocity = total displacement/total time

Displacement vs Distance

Distance is how far an object moves and doesn’t involve direction - scalar

Displacement includes both the distance an object moves and the direction - vector

Speed

Speed doesn’t involve direction - scalar quantity
Speed of a moving object is rarely constant as it is always changing

Speed of different things

Walking - 1.5m/s
Running - 3 m/s
Cycling - 6 m/s
Car - 13 - 30 m/s
Train - 50 m/s
Aeroplane - 250m/s
Speed of sound in air - 330 m/s

Why is the velocity not constant if an object moves at a steady speed in a circle?

Velocity is a vector quantity therefore is measures both direction and magnitude. Since the direction is constantly changing as it moves around the circle, the velocity is also constantly changing

Why is an object moving in a circle accelerating?

Acceleration is the change in velocity per second
Since direction is changing, velocity is changing therefore the object must be accelerating

Why would a satellite orbiting the earth accelerate?

The satellite is in orbit due to a gravitational centripetal force
This causes the satellite to accelerate
This causes the change in the direction to keep it moving in a circular path therefore changing the velocity (scalar quantity)

How can you interpret speed of an object at any particular time on a distance time graph?

Drawing a tangent on the line

How do you find acceleration from a velocity-time graph?

Calculate the gradient of the line

How do you find distance travelled from a velocity- time graph?

Calculate the area under the line

What are the three stages of falling?

Initial Acceleration - the only force is gravity therefore there is a large resultant force downwards so the object accelerates

Increasing Resistance - As the speed increases, air resistance increases which reduces the resultant force so the acceleration decreases

Terminal velocity - Air resistance will equal to the weight so the resultant force will be zero and the object will stop accelerating and will move at a constant speed

Why and how does an object reach terminal velocity? (sky diver 4 marks)

• As their velocity increases, the air resistance acting on them increases

• Eventually the air resistance will equal the weight of the sky diver

• At this point the resultant force acting on the skydiver is zero 

• This means their acceleration will be zero and so their velocity will be constant - terminal velocity

What is the formula for acceleration?

a = change in v x t
Acceleration (m/s²) = change in velocity (m/s) x time (s)

What is the equation used for motion with a constant acceleration?

v² - u² = 2as
Final velocity² (m/s) - initial velocity² m/s = 2 acceleration (m/s²) x distance (m)

What is Newton’s first law?

An object’s motion will not change unless a resultant force acts on it

Newtons first law stationary and moving objects

If a resultant force is zero and the object is stationary, the object stays stationary
If a resistance force is zero and the object is moving, the object continues to move at a constant velocity

What happens when an object travels at a steady speed (car)?

The driving force from the engine is exactly equal to the resistive forces (air resistance and friction)
Therefore, the resultant force is zero
Because the forces are balanced, the car maintains a uniform velocity

What happens when there is a Change in Speed in an object? Acceleration and Deceleration

A resultant force in the direction of motion causes acceleration
A resultant force against the direction of motion causes deceleration

What happens when there is a Change in direction in an object?

Even if the speed stays the same, a change in direction is a change in velocity which requires a resultant force

What is Inertia?

Inertia is the tendency of objects to continue in their state of rest or of uniform motion

What is inertial mass?

Measure of how difficult it is to change the velocity of an object
Ratio of force over acceleration: m=F/a

Key concept of Inertial mass

An object with a large inertial mass requires a much larger resultant force to speed it up or slow it down compared to an object with a small inertial mass

What is Newton’s second law?

If a non-zero resultant force acts on an object, it will cause the object to accelerate
The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass

What is the equation for Newton’s second law?

F = m x a
Resultant force (N) = Mass (kg) x Acceleration (m/s²)

If two objects are pushed together with the same force… (inertial mass)

the one that accelerates less has the higher inertial mass

Car on a main road and a motor way Typical velocity and acceleration

Main road velocity = 13 m/s
Motorway velocity = 30 m/s
Main road acceleration = 2 m/s²
Motorway acceleration = 1 m/s² at high speed

What is Newton’s third law?

Whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction

For a pair of forces to be a true Newton’s Third law pair, they must:

Be the same type of force
Always acts on 2 different objects
Be equal in size
Act in opposite directions

Newton’s third law vs. Equilibrium (Newton’s first law)

Newton’s third law - Object A exerts a force on Object B and Object B exerts equal and opposite force on Object A
Equilibrium - Two or more forces act on Object A, if they are equal and opposite, Object A doesn’t move

What is Stopping distance formula?

Stopping Distance = Thinking Distance + Braking Distance

What is thinking distance? + Relationship to speed

The distance travelled during the driver’s reaction time
Relationship to speed: Directly proportional to speed

What is braking distance? + Relationship to speed

The distance travelled once the brakes are applied
Relationship to speed: Increases with the square of speed

Estimating Stopping distance (20mph, 40mph and 70mph)

20 mph TD: 6m BD: 6m SD: 12m
40 mph TD: 12m BD: 24m SD: 36
70 mph TD: 21m BD: 75m SD 96m

What will the lines of each distance look like on a speed-distance graph?

Thinking Distance line - straight line through the origin
Braking Distance line - Curve that gets steeper as speed increases
Stopping Distance Line - Sum of both and also a steep curve

What is a method that can be used to measure reaction time?

Ruler drop Test
One person holds a ruler while the person being tested puts their hand at the 0cm mark. The ruler is dropped and the person catches as fast as possible
Measure the distance the ruler fell
Calculate the time using s = ut + 1/2at²

Typical reaction time range

0.2 and 0.9s

Factors effecting Reaction times (therefore thinking distance)

Tiredness - increases
Alcohol/Drugs - Increases
Distractions - Increases
Caffeine - Decreases (stimulant)

Factors affecting braking Distance 4 Reasons + Why

Icy/Wet roads - reduces friction between tyre and road surface
Worn tyres
Worn Brakes - worn pads or discs cannot apply much force to the wheels
Mass of Vehicle - Heavier vehicle has more kinetic energy therefore requires more work done to be stopped

What happens when a force is applied to the brakes of a vehicle?

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 greater the speed of a vehicle, the greater the braking force needed to stop the vehicle in a certain distance

What are the dangers of a large deceleration? 3 reasons

Brakes overheating - the brakes can get so hot that they lose their grip or even catch fire
Loss of control - large forces can cause tires to lose traction with road leading to skidding
Injury to Passengers - exerts huge forces on human body

What is momentum?

A measure of how difficult it is to stop a moving object. It is a vector quantity

Momentum equation

p = m x v
Momentum (kg m/s) = mass (kg) x velocity (m/s)

What is the law of conservation of momentum?

In a closed system, the total momentum before an event is equal to the total momentum after the event

Changes in momentum + Equation

When a force acts on an object, it causes a change in velocity, therefore a change in momentum
Force (N) = Change in momentum (kg m/s) / Time taken for the change (s)
F = mv - mu (final momentum - initial momentum) / t

How do safety features work? 4 marker

The safety feature increases the time taken for the change in momentum to occur
If you increase the time, you decrease the rate of change of momentum (use equation)
this results in a smaller force acting on the person
A smaller force reduces the risk of a severe injury

What do waves do?

Transfer energy and information from one place to another without transferring matter

What is a Transverse Wave? + Examples

The oscillations are perpendicular to the direction of energy transfer
Peaks and Troughs
Examples: Ripples on water, EM Waves, S-waves

What is a Longitudinal Wave? + Examples

The oscillations are parallel to the direction of energy transfer
Compressions (particles are bunched together) and rarefactions (particles are spread out)
Examples: Sounds waves, ultrasound, P-Waves

Give one piece of evidence that shows that it is the wave that moves, not the medium

A floating object only bobs up and down as a wave passes and it does not travel across the water with the wave

What is Amplitude?

Middle to the top of the wave
The maximum displacement from the rest position

What is a wavelength?

The distance from one point on a wave to the equivalent point on the next

What is Frequency?

The number of waves passing a point each second (Hz)

What is a period? + the equation

Time it takes for one complete wave to pass a point (seconds)
T = 1/f
Period = 1/Frequency

What is the wave equation?

v = fλ
Wave speed (m/s) = frequency (Hz) x Wavelength (m)

What happens when a wave travels from one medium to another?

Frequency stays the same
Speed changes (usually faster is solids than in gases)
Wavelength changes

Describe a method to measure the speed of sound waves in the air

Two people stand a measured distance apart
Person A clashes two wooden blocks together
Person B starts stopwatch when they see the blocks hit and stops it when they hear the sound
Calculate the speed (d/t)

What are the three things that can happen when a wave hits a boundary

Reflection - Wave bounces off the surface (Incoming energy sent back)
Absorption - Energy of the wave is taken up by the material (Internal energy increases)
Transmission - The wave passes through the material (may refract)

5 key components in a ray diagram

Incident ray - Incoming wave
Reflected ray - Wave bouncing off
Normal - dashed line drawn perpendicular to the surface where wave hits
Angle of incidence - the angle between the incident ray and the normal
Angle of Reflection - the angle between the reflected ray and the normal

What is the law of reflection?

Angle of incidence is equal to the angle of reflection

What are the types of Reflection?

Specular Reflection
Diffuse Reflection

What is Specular Reflection?

Occurs on a smooth surface. All parallel rays are reflected in the same direction, producing a clear image

What is Diffuse reflection?

Occurs on a rough surface
They rays are reflected in many different directions as the normal is different for every bump on the surface

How does sound travel through solids?

Sound is a longitudinal wave and when the waves hit a solid object, the compressions and rarefactions create a varying pressure on the surface of the solid.
This force causes the particles in the solid to vibrate back and fourth at the same frequency as the sound wave

How does sound travel in the ear?

Sound waves enter the ear canal and hit the diaphragm of the ear drum, causing it to vibrate at the same frequency as the sound wave

What is the Human auditory range? + Infrasound and Ultrasound

20 Hz - 20,000 Hz
Infrasound - Sound lower than 20 Hz
Ultrasound - Sound higher than 20000 Hz

Why is the Human auditory range limited?

The physical parts of the ear (ear drum and ossicles) have a limited frequency range - cannot vibrate fast enough to transmit frequencies higher than 20,000 Hz
If the frequency is too low, the vibrations don’t have enough energy to pass through the ear effectively