AQA GCSE Physics - Paper 2

What are vectors?

Quantities that have a magnitude and a direction

What are scalars?

Quantities that only have a magnitude

Examples of vectors

Force

Velocity

Displacement

Acceleration

Momentum

Examples of scalars

Speed

Distance

Mass

Temperature

Time

What are contact forces?

Forces that act when two objects are touching

What are non-contact forces?

Forces that act without the need for two objects to be touching

Examples of contact forces

Friction

Air resistance

Tension in ropes

Normal contact force

Examples of non-contact forces

Magnetic force

Gravitational force

Electrostatic force

What is mass?

The amount of material an object is made of

It is the same value everywhere

Measured using a mass balance

What is weight?

The force acting on an object due to gravity

It depends on the strength of the gravitational field at the location of the object

Measured using a calibrated spring balance - newtonmeter

Equation for Weight

Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)

W=mg

What is weight directly proportional to?

Mass

What are free body diagrams?

Diagrams that show all the forces acting on an object

What is the minimum number of forces acting on an object in real situations?

At least 2 forces

What is the resultant force?

The single force that replaces multiple forces acting at a single point

How is work done?

When a force moves an object through a distance, energy is transferred and work is done on the object

Equation for 'Work Done'

Work done (J) = Force (N) x Distance (m)

W=Fs

What is 1J equal to?

1Nm

What could happen when you apply a force to an object?

It may stretch, compress or bend

What happens when an object is inelastically deformed?

The object won't return to its original shape and length after the force has been removed

Equation for Force 1

Force (N) = Spring Constant (N/m) x Extension (m)

F=ke

What is extension directly proportional to?

The force applied

F∝e

The stiffer the spring...

...The greater the spring constant

What is the limit of proportionality?

The point at which extension is no longer directly proportional to force

Equation for 'Moments'

Moment of a force (Nm) = Force (N) x Distance (m)

M=Fd

What is a moment?

The turning effect of a force

When will an object not turn?

When the object is balanced - the total anticlockwise moment equals the total clockwise moment about a pivot

What affects the size of a moment?

1) The size of the force applied - a larger force will produce a larger moment

2) The perpendicular distance from the pivot to the line of action of the force - any angle smaller than 90° will result in a smaller distance, thus a smaller moment

What do levers do?

They increase the distance from the pivot at which the force is applied

Do levers make it easier or harder to do work?

Easier to do work

What are gears and what do they do?

Circular discs with 'teeth' around their edges

Their teeth interlock so that turning one causes another to turn in the opposite direction

They are used to transmit the rotational effect of a force from one place to another

What is pressure?

The force per unit area

Equation for Pressure - Surface of a fluid

Pressure (Pa) = Force normal to a surface (N) / Area of that surface (m²)

p=F/A

What is the pressure of a fluid?

A force is exerted normal (at right angles) to any surface in contact with the fluid

What is density?

A measure of the 'compactness' of a substance

Equation for Pressure - Liquid

Pressure (Pa) = Height of the column of liquid (the depth)(m) x Density of the liquid (kg/m³) x Gravitational field strength (N/kg)

p=hρg

What is upthrust?

The force exerted on the bottom of the object is larger than the force acting on the top of the object - the resultant force upwards on an object when it is submerged in water

What is upthrust equal to?

The weight of fluid that has been displaced by the object

Why do objects float?

The upthrust of the object is equal to the object's weight, thus the force balances

It has a low density

Why do objects sink?

The object's weight is more than the upthrust

It has a high density

Low density

If an object is less dense than the fluid it is placed in, it weighs less than the equivalent volume of fluid

It will displace a volume of fluid that is equal to its weight before it is completely submerged

High density

If an object is denser than the fluid it is placed in, it will be unable to displace enough fluid to equal its weight, therefore its weight will be larger

How do submarines make use of upthrust?

To sink, large tanks are filled with water to increase the weight of the submarine so that it is more than the upthrust

To float, the tanks are filled with compressed air to reduce the weight so that it is less than the upthrust

What is atmospheric pressure?

A layer of air that surrounds Earth

It's created on a surface when air molecules collide with the surface

What is atmospheric pressure inversely proportional to?

Altitude (height above Earth)

If the altitude increases, atmospheric pressure decreases and becomes less dense

What else happens when altitude increases?

There are fewer air molecules above a surface as the height increases, therefore the weight of the air above it (which contributes to atmospheric pressure) decreases

What is distance?

How far an object has moved

What is displacement?

It measures the distance and direction in a straight line from an object's starting point to its finishing point

What is speed?

How fast you're going

What is velocity?

Speed (how fast you're going) in a given direction

Equation for Speed

Distance Travelled (m) = Speed (m/s) x Time (s)

s=vt

What is the typical speed of a person walking?

1.5m/s

What is the typical speed of a person running?

3m/s

What is the typical speed of a person cycling?

6m/s

What is the typical speed of a car?

25m/s

What is the typical speed of a train?

55m/s

What is the typical speed of a plane?

250m/s

What factors affect speed?

Fitness of the person

Age of the person

Distance travelled

Terrain

Climate

Gender of the person

What factors affect wind speed?

Temperature

Atmospheric pressure

Any large buildings or structures nearby e.g. forests reduce wind speed travelling through them

What is acceleration?

The change in velocity in a certain amount of time

Equation for Acceleration

Acceleration (m/s²) = Change in Velocity (m/s) / Time (s)

a=Δv/t

What is deceleration?

Negative acceleration - when something slows down, the change in velocity is negative

What is constant acceleration?

Uniform acceleration - acceleration due to gravity is uniform for objects in free fall

9.8m/s² near the Earth's surface

Equation for Uniform Acceleration

Final velocity² (m/s) - Initial velocity² (m/s) = 2 x Acceleration (m/s²) x Distance (m)

v²-u²=2as

Distance-Time Graphs - Features

1) Gradient = speed

2) Flat sections = object is stationary

3) Straight uphill sections = object is travelling at a steady speed

4) Curves = object is accelerating or decelerating

5) Steepening curve = object is speeding up

6) Levelling off curve = object is slowing down

Velocity-Time Graphs - Features

1) Gradient = acceleration

2) Flat sections = object is travelling at a steady speed

3) Uphill sections = object is accelerating

4) Downhill sections = object is decelerating

5) Curves = object is changing acceleration

The steeper the graph, the greater the acceleration or deceleration

What does friction do?

It causes objects to slow down when they rub against another surface

It always acts in the opposite direction to movement

It's always smaller than the driving force

Where do you get friction?

When two surfaces are in contact or when an object passes through a fluid (drag)

What is drag?

The resistance you get in a fluid

Air resistance is a type of drag

How do you reduce drag?

Keep the shape of an object streamlined

Process of a falling object

1) When a falling object first sets off, the force of gravity is much more than the frictional force slowing it down, therefore the object accelerates

2) As the speed increases, the friction builds up

3) The acceleration is gradually reduced until eventually, the friction force is equal to the accelerating force - the resultant force is 0

4) At this point, it will have reached maximum speed or terminal velocity and will fall at a steady speed

What does air resistance do?

It causes things to fall at different speeds

What determines the terminal velocity?

The terminal velocity of any object is determined by its drag in comparison to its weight

What affects the frictional force?

The shape and area of an object

Newton's First Law - Law of Inertia

If the resultant force on a stationary object is zero, the object will remain stationary

If the resultant force on a moving object is zero, it will just carry on moving at the same velocity

What does a non-zero resultant force always produce?

Acceleration or deceleration in the direction of the force

Which forms does acceleration take on?

1) Starting

2) Stopping

3) Speeding up

4) Slowing down

5) Changing direction

Newton's Second Law

Force ∝ Acceleration

Acceleration is inversely proportional to the mass of an object

F=ma

What is inertia?

When objects continue in the same state of motion before they are acted upon by a resultant force

What is the inertial mass?

It measures how difficult it is to change the velocity of an object

It's the ratio of force over acceleration

Newton's Third Law

When two different objects interact, the forces they exert on each other are equal and opposite

An action always has an equal and opposite reaction

What is equilibrium?

When two different forces are equal when acting on the same object

E.g. a book resting on the ground is in equilibrium - the weight of the book is equal to the normal contact force

Equation for Stopping Distance

Stopping Distance = Thinking Distance + Braking Distance

What is the thinking distance?

How far the car travels during the driver's reaction time

What is the braking distance?

The distance taken to stop under the braking force

Typical car braking distances

14m at 30mph

55m at 60mph

75m at 70mph

What is thinking distance affected by?

1) Speed - the faster you're going, the further you'll travel during your reaction time

2) Your reaction time - the longer it is, the longer your thinking distance

3) Alcohol

4) Drugs

5) Sleep deprivation

6) Distractions

What is braking distance affected by?

1) Speed - the faster a vehicle travels, the longer it takes to stop

2) Weather/Road surface - if it's wet or icy, there is less grip (and less friction) between a vehicle's tyres and the road, which can cause tyres to skid

3) Condition of tyres - if the tyres are bald, then they cannot get rid of water in wet conditions, thus leading to skidding on top of the water

4) Quality of brakes - if brakes are worn or faulty, they won't be able to apply as much force as well-maintained brakes, which could be dangerous when wanting to brake hard

What happens when the brake pedal is pushed?

The brake pads are pressed onto the wheels, thus causing friction, which causes work to be done. The work done between the brakes and the wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes, which as a result, increase in temperature

What happens when a vehicle is going really fast?

It has more energy in its kinetic energy stores, so the more work needs to be done to stop it - a greater braking force will be needed to make the vehicle stop within a certain distance, therefore the deceleration will be larger

The larger the deceleration, the more dangerous it will be as the brakes could overheat or cause the vehicle to skid

What is the typical reaction time?

Between 0.2s and 0.9s

Typical thinking distances

30mph - 9m

50mph - 15m

70mph - 21m

Typical space needed to be left for stopping distances

30mph - 6 car lengths

50mph - 13 car lengths

70mph - 24 car lengths

What is momentum?

How much 'oomph' an object has

All moving objects have it

The momentum of one thing is always equal to the momentum of another thing e.g. a skateboarder has the same momentum as the skateboard

Equation for Momentum

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

p=mv

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 a closed system?

When no external forces act

Equation for Force 2

Force (N) = Mass (kg) x Acceleration (m/s²)

F=ma

Equation for Force 3

Force (N) = Change in momentum (kg m/s) / Change in time (s)

F=mΔv/Δt

Safety Features of Cars

1) Crumple zones crumple on impact, increasing the time taken for the car to stop

2) Seat belts stretch slightly, increasing the time taken for the wearer to stop

3) Air bags inflate before you hit the dashboard of a car. The compressing air inside it slows you down more gradually than if you had just hit the hard dashboard