Forces and Energy

Think about the last time you opened a door. You either pushed it or pulled it. That push or pull is a force.

You can never actually see a force — but you can always see what it does. A force can:

• start something moving (kicking a still football),
• stop something moving (catching that football),
• make something speed up or slow down,
• change the direction of something moving (a tennis racket hitting a ball),
• change the shape of something (squashing a sponge).

Forces are drawn as arrows. The arrow points the way the force pushes or pulls, and a longer arrow means a bigger force.

Whenever you see something change, ask yourself: what force made that happen?

There are several forces you will meet again and again.

• Gravity — a pulling force that pulls everything towards the centre of the Earth. It is why a dropped pencil falls down and never up.
• Friction — a force that acts when two surfaces rub together. It always pushes against movement, slowing things down. Friction is bigger on rough surfaces than on smooth ones.
• Air resistance — a kind of friction from the air. It pushes against anything moving through air, like a cyclist or a falling leaf.
• Water resistance — the same idea but in water. It pushes against a swimmer or a boat.
• Upthrust — an upward push from a liquid or gas on an object in it. Upthrust is why a beach ball pops back up when you push it under water.

Some of these forces need contact (friction, resistance, upthrust) — the objects must touch. Gravity is different: it works without touching, even across empty space.

When you swim, you can feel several of these at once: gravity pulling you down, upthrust pushing you up, and water resistance pushing back as you move forward.

Just as length is measured in metres, force is measured in newtons. We write newtons with a capital $N$.

To give you a feel for it, holding a small apple takes a downward pull of roughly $1\ N$ from gravity. A bag of school books might pull with around $30\ N$.

We measure force with a force meter (also called a newton meter). Inside it is a spring. When you pull on it, the spring stretches — a bigger force stretches it further. A pointer moves along a scale marked in newtons so you can read the value.

To read a force meter well, look straight at the scale so you do not read the wrong line by mistake.

Most objects have more than one force acting on them at once. What happens next depends on whether those forces are balanced or unbalanced.

Picture a tug-of-war. If both teams pull equally hard, the rope does not move — the forces are balanced. If one team pulls harder, the rope shoots their way — the forces are unbalanced.

Here is the rule to remember:

• Balanced forces — the forces are equal in size but opposite in direction. The object stays still, or keeps moving the same way at the same speed. Nothing changes.
• Unbalanced forces — one force is bigger than the other. The object speeds up, slows down, or changes direction — in the direction of the bigger force.

A parked car has balanced forces, so it stays still. Press the accelerator and the forces become unbalanced, so the car speeds up.

Energy is what makes things happen. Nothing can move, heat up, light up or make sound without energy.

Energy sits in stores, ready to be used. Some everyday stores are:

• chemical store — in food, fuel and batteries,
• kinetic store — in anything that is moving,
• gravitational store — in anything lifted up high,
• thermal store — in anything hot,
• elastic store — in anything stretched or squashed, like a drawn catapult.

Energy does not stay still — it moves between stores. We call this an energy transfer. Following the energy from store to store gives an energy transfer chain.

The most important rule in this whole topic: energy cannot be made or destroyed — only moved from one store to another. This rule is called the conservation of energy. The total amount of energy always stays the same; it just changes its form.

Understanding forces and energy now makes the rest of physics much easier:

• Speed and motion — you will use forces to explain why things speed up and slow down.
• Electricity — circuits are all about transferring energy from a cell to other components.
• Light and sound — these are forms of energy that travel and transfer.
• Floating and sinking — you will use gravity and upthrust together to explain why ships float.
• Machines and engineering — every machine is a story about forces acting and energy moving.

Whenever a physics topic feels tricky later, go back to two questions: what forces are acting? and where is the energy going? Take your time here — a strong start with forces and energy is worth far more than rushing ahead.