Forces and Energy

In Grade 6 you saw that forces can start, stop and reshape objects. Now meet another effect: a force can make something turn.

Think of pushing a door. You always push the handle, far from the hinge — never near the hinge. The hinge is the pivot, the fixed point the door turns around. The turning effect of a force is called a moment.

A moment depends on two things, not one:

• the size of the force (a bigger push turns it more), and
• the distance of the force from the pivot.

That is why a door handle is placed far from the hinge — the extra distance makes the same push much more effective. We can write this as:

$\text{moment} = \text{force} \times \text{distance from pivot}$

Moment is measured in newton metres ($N\,m$): newtons for the force, metres for the distance. So a force of $20\ N$ pushing $0.8\ m$ from a pivot gives a moment of $16\ N\,m$.

Spotting the pivot is the first move with any turning question — find it, then measure your distances from there.

A lever is a simple machine that uses moments. It has a pivot, an effort (the force you apply) and a load (the force you are working against).

The clever trick is distance. If your effort acts far from the pivot and the load is close to the pivot, a small effort can move a large load. A see-saw balances when the two moments are equal:

$\text{effort} \times \text{its distance} = \text{load} \times \text{its distance}$

You use levers every day without noticing: scissors, a wheelbarrow, a bottle opener, a spanner and even your own forearm are all levers. A crowbar lets you lift something far too heavy to lift by hand — the long bar gives your effort a huge moment.

Why does a drawing pin slide easily into a board, while your blunt thumb does not? The force can be the same — what differs is the area.

Pressure tells you how concentrated a force is. It is the force pressing on each unit of area:

$\text{pressure} = \dfrac{\text{force}}{\text{area}}$

Pressure is measured in pascals ($Pa$), where $1\ Pa$ is one newton on each square metre.

• A small area concentrates the force — high pressure. A pin point, a knife edge, a stiletto heel.
• A large area spreads the force out — low pressure. Snowshoes, tractor tyres, a camel's wide feet on sand.

This is why a sharp knife cuts well — its thin edge has a tiny area, so even a gentle force gives huge pressure. And it is why lying flat on thin ice is safer than standing: more area, less pressure.

Pressure is not only about solid objects pushing down. Liquids and gases also exert pressure — and they do it in all directions at once, not just downwards.

Two key facts about pressure in a liquid:

• Pressure pushes outwards and upwards as well as down, which is why water squirts sideways out of a hole in a bottle.
• Pressure increases with depth. Deeper down, there is more liquid above pushing down, so the pressure is greater.

Gases behave the same way. The air around you is a gas, and it presses on you from every side — this is atmospheric pressure. You do not feel it because it is balanced by the pressure inside your body.

These ideas explain real things: a dam is built thicker at the bottom to withstand the greater pressure deep down, and deep-sea divers must protect themselves from crushing pressure.

In Grade 6 you followed energy moving between stores. But where does all that energy come from in the first place? From energy resources — and these split into two groups.

Non-renewable resources will run out one day, because we use them far faster than nature can replace them:

• fossil fuels — coal, oil and natural gas, formed over millions of years,
• nuclear fuel — such as uranium.

Renewable resources will not run out, because they are constantly being replaced:

• solar (energy from the Sun),
• wind, hydroelectric (flowing water), wave and tidal,
• geothermal (heat from inside the Earth),
• biomass (wood and plant material).

Each resource has trade-offs. Fossil fuels are reliable and give a lot of energy, but burning them releases gases that warm the planet. Renewables are cleaner, but some depend on the weather — solar needs sunshine, wind needs wind.

You learned that energy is always conserved — never created or destroyed. So why do we talk about 'saving' energy? Because in every transfer, some energy becomes wasted energy — usually spread out as heat that we cannot easily use again.

When energy is transferred:

• the part that does the job we wanted is useful energy,
• the rest is wasted energy, often heat and sometimes sound.

An old filament bulb is a clear example: only a small fraction of the electrical energy becomes light — most leaves as heat. A modern LED bulb wastes far less, so much more of the energy becomes useful light.

We can reduce waste in sensible ways:

• insulation — loft insulation and double glazing trap heat inside a home so less is wasted,
• efficient appliances — LED bulbs and modern motors waste less energy,
• reducing friction — oiling moving parts means less energy is lost as heat,
• switching things off when they are not needed.

The total energy is still conserved — but reducing waste means we need fewer resources to get the same useful result. Less waste means lower bills and fewer fossil fuels burned.

The Stage 8 ideas in this topic open up a lot of later science:

• Machines and engineering — moments and levers explain cranes, gears and tools.
• Floating, sinking and flight — pressure differences explain how ships and aeroplanes work.
• Weather and the atmosphere — air pressure drives winds and weather systems.
• Climate and the environment — choosing energy resources wisely is one of the biggest challenges of your lifetime.
• Electricity generation — power stations turn an energy resource into electricity, useful and wasted energy together.

Whenever a turning question appears later, find the pivot first. Whenever pressure puzzles you, ask 'how big is the area?' Take your time here — these ideas reach into almost every branch of physics ahead.