Chapter 9: Application of Forces and Transfer of Energy (Summary Sheet)

Part A · Forces

1 Contact and Non-Contact Forces

A force is a push or a pull that one object exerts on another.
Every force belongs to one of two families, and the test is whether the two objects have to touch.

Family	What it needs	Examples
Contact force	The objects must be physically touching	Friction, the normal force from a surface, the elastic force of a stretched spring, air resistance, a hand pushing a door
Non-contact force	The force reaches across a gap — no touching needed	Magnetic force, electrostatic force, gravitational force

The quick classification test: imagine lifting the objects apart. If the force is still there, it is non-contact — the Earth keeps pulling a book downwards even after you take it off the table, but its friction and the table's push disappear at once.

2 What an Interaction Does: the Four Changes

When two or more objects interact, there is a transfer of energy, and applying a force can change four things:

What the force changes	Everyday example
State of rest or motion of an object	A push starts a stationary trolley, another stops or speeds it up; brakes slow a bicycle
Turning effects in objects	A spanner turned on a nut, a lever prying open a tin — the force makes the object rotate instead of slide
Size and/or shape of an object	Squeezing a sponge, stretching a rubber band, denting a can — the object is deformed
Pressure on objects	Pressing a drawing pin into a board, a knife edge against a chopping board — the force is concentrated on a small area

Notice that the same push can do more than one of these at once, so always name the change the question is asking about.

3 Measuring Force: the Newton

Force is measured in newtons (N) — that is the SI unit named by the syllabus.
The instrument is a spring balance (also called a Newton meter): the pull stretches a spring, and the pointer shows the force on a scale.
Using one correctly: check the zero before you start, hold it so the scale is vertical, read the pointer at eye level to avoid parallax, and keep the force inside the range marked on the balance.
Forces only add simply when they act along the same line, so a force arrow always needs a size and a direction.

Part B · Mass and Weight

4 Weight and Mass Compared

	Mass	Weight
What it is	The amount of matter in an object	The force of gravity pulling on that matter
SI unit	Kilogram (kg)	Newton (N)
How it is measured	Beam balance or electronic balance	Spring balance
Does it change with place?	No — the same everywhere in the universe	Yes — weaker gravity means less weight
On the Moon	Still 60 kg of matter	About one-sixth of the Earth weight

The two are linked by weight = mass × gravitational field strength, with g ≈ 10 N/kg on Earth: a 60 kg student weighs about 600 N here.
Because weight is a force it has a direction (downwards, towards the centre of the Earth); mass is a quantity of matter and has none.

Part C · Pressure

5 Pressure = Force ÷ Area

Pressure is the force acting perpendicular to a surface, divided by the area it presses on: P = F / A.
Units: pressure is measured in pascals (Pa), where 1 Pa = 1 N/m². F is in newtons and A is in square metres.
The area in the formula is the contact area — only the part actually being pressed on, which is why the same crate on four small feet makes far more pressure than one lying flat.
Two rules to quote: spread a force over a larger area and the pressure falls; squeeze it into a smaller area and the pressure rises.
Worked comparison: a 600 N person on boots (contact area 0.03 m²) makes 600 ÷ 0.03 = 20 000 Pa, but on snowshoes (0.30 m²) the same 600 N makes 600 ÷ 0.30 = 2 000 Pa — ten times less, so the snow holds.

6 Pressure in Everyday Life

Object	Contact area	Pressure	Why it is designed that way
Sharp knife edge	Very small — ground to a thin line	Very high	So a modest push parts the food instead of skidding
Blunt knife	Large — rounded edge	Low	The same push is spread out, so it does not cut
High-heeled shoes	Small under the heel	Very high	Sink into soft ground and mark floors; flat soles spread the same weight
Snowshoes	Large — wide flat boards	Very low	Keeps the walker on top of soft snow
Tractor tyres	Wide and broad	Low	Stops the wheels sinking into wet soil

In any “why” question, say which of the two has changed — the force or the area — and therefore whether the pressure rises or falls. The weight usually has not changed at all.

7 Atmospheric Pressure

The air above us has weight, so the atmosphere presses on everything it touches: about 100 000 Pa (100 kPa) at sea level.
It acts in every direction, not only downwards — which is why you do not feel it crushing you from above.
Suction cups: pressing the cup squeezes the air out, so the atmosphere pushes it hard against the surface and holds it there.
Drinking from a straw: you lower the pressure inside the straw, and the atmosphere pushes the drink up into it. Nothing is really “sucked” — the push comes from outside.

8 Pressure Due to Liquids

A liquid pushes on anything inside it, and it pushes in all directions — sideways and upwards as well as down.
The pressure grows with depth: a deeper point has more liquid above it pressing down, so each extra metre adds more pressure.
A denser liquid makes more pressure at the same depth — seawater presses harder than fresh water.
Submarines have depth limits for exactly this reason: past a certain depth the water pressure would crush the hull.
That steady rise with depth is also why a dam wall is thickest at the bottom, where the water pushes hardest.

Part D · Work and Energy

9 Work Done and the Joule

The SI unit of both work and energy is the joule (J).
Work is done when a force moves an object in the direction of the force — it is an example of energy transfer, which is why work and energy share a unit.
Work is done: lifting a bag upwards, pushing a trolley along, pedalling a bicycle up a slope.
No work is done against gravity when you carry a bag along level ground: the force you apply is upwards, but the motion is horizontal, so the force and the movement do not point the same way.
Watch the units in an answer: force in newtons, weight in newtons, energy and work in joules — a mass in kilograms is not a force.

10 Energy Converted From One Form to Another

Form of energy	What it is	Where you meet it
Kinetic	Energy of movement — a moving object has it, a stationary one has none	A cyclist freewheeling downhill; wind turning a turbine
Gravitational potential	Energy a body has because it is raised above a level we have chosen	Water held behind a dam; a book on a high shelf
Elastic potential	Energy stored while a material is deformed, given back when it springs into shape	A drawn bow; a stretched rubber band; a trampoline at the bottom of a bounce
Chemical potential	Energy held in the bonds between particles, released when substances react	Petrol, a dry cell, the food you ate for lunch
Electrical	Energy carried by charge on the move through a circuit	Anything plugged into the mains; a lightning strike
Thermal	The total kinetic energy of all the particles in a body — their collective jostling	A hot drink; a car exhaust; a bath that has gone cold
Light and sound	Energy carried by waves — light can cross empty space, sound needs a medium	Sunlight and a glowing filament; a speaker cone or a drum
Nuclear	Energy stored inside a nucleus, released when nuclei are rearranged	A nuclear power station; the Sun

To infer a conversion, follow the energy and name each form in order: a torch is chemical → electrical → light (and thermal); water falling over a dam is gravitational potential → kinetic → electrical.
Usually more than one form comes out — say so, and name the form you actually wanted as the useful one.
Energy is not used up in these changes; it is transferred and changed in form, which is why we can chase it through a chain.

Part E · Forces and Energy in the World

11 The Destructive Power of Forces in Nature

Event	The forces it produces
Tropical cyclone	Moving air pushes hard on walls, roofs and trees, and the pressure difference across a roof can lift it off the walls
Earthquake	The ground lurches sideways and vertically, loading buildings with forces far greater than their own weight, while weakened soil can no longer push up hard enough to support a foundation
Volcanic eruption	Gas at very high pressure blasts rock and ash out of the vent, and falling debris strikes buildings with destructive contact forces
Tsunami	A moving wall of water pushes along a whole sea wall; because water is dense and fast, the force is enormous, and the pressure beneath can scour sand away from a foundation

Engineers answer these forces with the same pressure idea as the knife and the snowshoe: spread a very large force over more area, and reinforce where the load is greatest.

12 Energy Sources and Their Impact

Renewable: natural processes replace it about as fast as we use it, so the stock does not shrink — sunlight, wind, flowing water, underground heat, plant matter.
Non-renewable: a fixed stock that took millions of years to form, so what we use is gone — coal, crude oil, natural gas, uranium.

Source	Type	How we get energy from it, and its impact on the environment
Fossil fuels (coal, oil, gas)	Non-renewable	Burnt to heat water into steam that spins turbines; releases carbon dioxide and pollutants, and the impacts are felt globally, not only locally
Nuclear	Non-renewable	Splitting uranium releases heat to make steam — almost no carbon dioxide while running, but radioactive waste stays dangerous for thousands of years
Solar	Renewable	Sunlight strikes a cell and becomes electrical energy directly; nothing burns, but output stops at night and a wide area is needed
Wind	Renewable	Moving air turns a turbine and generator; no fuel and no emissions, but output follows the weather and blades can harm flying wildlife
Hydroelectric	Renewable	Water held behind a dam falls and spins turbines; steady and clean in use, but the reservoir floods the valley and cuts the river in two
Geothermal	Renewable	Heat from the Earth's interior turns water to steam; steady output, but only where that heat lies near the surface
Biofuels	Renewable	Plant matter or waste is burnt for heat; still releases carbon dioxide and soot, though the plants regrow

Compare the two sides when you answer: renewables add little or no carbon dioxide while running but their output is not controllable, while fossil fuels are dense, cheap to store and available on demand but release carbon dioxide every day they are used.
An answer on this outcome needs both halves — what the source gives us, and the impact that comes with it.

Part F · Exam Tips

13 Exam Tips: What Each Syllabus Pointer Asks For

The pointer	What earns the mark
Show an understanding that a force can be a contact force (e.g. friction) or non-contact force (e.g. magnetic force, gravitational force)	Name both families and give the touching test: friction is contact, magnetic and gravitational are not
Recognise that interactions between two or more objects result in a transfer of energy which may cause changes to the state of rest or motion, turning effects, size and/or shape, and pressure	Say transfer of energy, then name the change: motion, turning, deformation or pressure — with one example each
Show an appreciation of daily-life pressure phenomena — pressure (high-heeled shoes, cutting edge of a knife), atmospheric pressure (suction cups, straws), pressure due to liquid (submarine depth limits)	Use the example and say whether the area shrank or grew (knife, heels), or that the atmosphere pushed (straw, suction cup), or that pressure rises with depth (submarine)
State the SI unit of work and energy as the joule	Write joule (J) — not newton, not watt — and note that work and energy share the unit
Identify that work is done is an example of energy transfer that occurs when an object moves in the direction of a force	Check the two directions match: force and movement along the same line. Carrying a bag level does no work against gravity
Measure force, using newton as the SI unit	Use a spring balance, zero it, read at eye level, and quote the answer in newtons
Compare weight and mass	Give the pairings: mass in kg, constant everywhere; weight in newtons, changes with gravity
Investigate pressure using the formula, pressure = force/area	Write P = F / A, keep F in newtons and A in m², and answer in pascals (1 Pa = 1 N/m²)
Infer that energy can be converted from one form to another	Chain the forms in order and name where the energy ends up, e.g. chemical → electrical → light + thermal
Show curiosity about the destructive power of forces in nature (earthquakes, tsunamis, volcanic eruptions, tropical cyclones)	Name the event and say which force does the damage, then how design reduces it (spreading the force, reinforcing)
Show an appreciation of the uses of various sources of energy and their impact on the environment	Give one benefit and one impact per source, and say whether it is renewable

Three errors to avoid: quoting a weight in kilograms (that is mass — weight is in newtons); writing the pressure unit as newtons instead of pascals; and saying the atmosphere “sucks” a drink up a straw when it is the higher pressure outside that pushes it in.