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# How big, which way?

Year 7 Physics · Forces and motion

<!-- Speaker note: Students represent forces as arrows with the right size and direction, and name them (AC9S7U04, node 1, a hinge node). About 25 minutes (Engage and Explore 6 min · Explain 7 min · Practise 6 min · Evaluate 3 min · Typical adds 3 min, Wider about 3 min). -->

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## Today

**We are learning to** a force has a size and a direction, so we draw it as an arrow. In a force diagram every force on an object gets an arrow of the right length, pointing the right way, with its name.

- I can draw a force as an arrow with the right direction and relative size.
- I can name the forces acting on an object.
- I can draw a force diagram that fits a situation.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace force meters and real pushes. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge risk is imprecise diagrams: wrong direction, missing forces (especially support), sizes that don't match. Every later node uses diagrams to reason, so the Practise step matters. Board notes: arrows start at the object's centre, the usual convention for a force diagram. The boards use g = 9.8 N/kg, as the weight lesson does; masses are chosen so the weights are whole numbers (5 kg weighs 49 N). Teacher: In your own words: collect the trolley diagram. -->

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## Why it matters

> Push a shopping trolley gently, then hard. Same direction, different force. Push it from the other side: same size, different force. Two things to get right every time.

<!-- Speaker note: Forces are invisible, so scientists draw them. This lesson is about drawing them precisely. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**In a force diagram, each force is drawn as an arrow. What do you predict the arrow shows?**

- Both how big it is (its length) and which way it acts (its direction)
- Only how big the force is
- Only which way the force acts
- Which way the object will move

<!-- Speaker note: What happens: Both. The arrow's length shows the size of the force and it points the way the force acts. (It does not always point the way the object moves.) Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: ask what else an arrow could show; 'which way it moves' is the misconception the trolley exposes (friction points backwards). -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-7%2Farrows%2Fstep-2&code=widget%3A+forces%0Aobject%3A+Trolley%0Amass%3A+5%0Ag%3A+9.8%0Aforce+W%3A+down%2C+weight%2C+Weight%0Aforce+N%3A+up%2C+support%2C+Support%0Aforce+P%3A+right%2C+10%2C+Push%2C+adjust+0-60+by+5%0Afriction%3A+10%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-7/arrows/?full=step-2#step-2)

<!-- Speaker note: Use the **Push** slider. Watch the push arrow, and watch friction too. -->

<!-- Speaker note: Teacher: ask students to make the push smaller than 10 N and notice friction shrinking with it. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* Both. The arrow's length shows the size of the force and it points the way the force acts. (It does not always point the way the object moves.)

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Length is size

**When you made the push bigger, what happened to its arrow?**

- It got longer, pointing the same way
- It stayed the same; only the number changed
- It changed direction

* Answer: It got longer, pointing the same way

<!-- Speaker note: Length shows size. A force twice as big gets an arrow twice as long. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Naming forces

**Match each force to what causes it.**

| | |
|---|---|
| `Weight` | A. Air pushing against a moving object |
| `Support (normal force)` | B. Surfaces rubbing, acting against sliding |
| `Friction` | C. Earth's gravity pulling the object down |
| `Air resistance` | D. Something in contact pushing or pulling |
| `Push or pull` | E. A surface pushing back up on the object |

* Answer: Weight → C. Earth's gravity pulling the object down; Support (normal force) → E. A surface pushing back up on the object; Friction → B. Surfaces rubbing, acting against sliding; Air resistance → A. Air pushing against a moving object; Push or pull → D. Something in contact pushing or pulling

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Two equal forces

**The trolley's weight is 49 N and the support is 49 N. How should their arrows look?**

- Both pointing down
- Weight longer, because it is heavier
- The same length, pointing in opposite directions

* Answer: The same length, pointing in opposite directions

<!-- Speaker note: Equal sizes, equal lengths. Weight points down; the support from the ground points up. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## draw it right

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-7%2Farrows%2Fstep-4&code=widget%3A+forces%0Aobject%3A+Books%0Amass%3A+5%0Ag%3A+9.8%0Aforce+W%3A+down%2C+weight%2C+Weight%0Aforce+N%3A+up%2C+0%2C+Table+pushing+up%2C+adjust+0-80%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-7/arrows/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Repeated diagrams with immediate feedback build precision: correct direction, relative size and a name for every force. -->

<!-- Speaker note: Teacher: the book task is the commonest omission (the table's push). Stop and discuss it. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: four situations

**Which diagram fits each situation? (Forward is the way the object moves.)**

Groups: Forward arrow longer than backward arrow · Forward and backward arrows equal · Backward arrow longer than forward arrow

A cyclist speeding up · A car driving at a steady 60 km/h · A sled slowing down on flat snow · A box being pushed but not moving · A runner speeding up out of the blocks

* Answer: A cyclist speeding up → Forward arrow longer than backward arrow; A car driving at a steady 60 km/h → Forward and backward arrows equal; A sled slowing down on flat snow → Backward arrow longer than forward arrow; A box being pushed but not moving → Forward and backward arrows equal; A runner speeding up out of the blocks → Forward arrow longer than backward arrow

<!-- Speaker note: What this step is for: Matching diagrams to situations (at rest, speeding up, slowing down, falling) uses arrows as reasoning, not decoration. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: a boomerang

**First Peoples of Australia shaped boomerangs with a curved upper surface, like an aircraft wing. In flight, as air moves over it, which force does that shape mainly create?**

- Thrust: the shape keeps pushing it forward, like an engine
- More weight
- Lift: air pushing the boomerang up (and sideways, which curves its path)

* Answer: Lift: air pushing the boomerang up (and sideways, which curves its path)

<!-- Speaker note: The aerofoil shape makes the air push on it: lift. Spinning, the lift tilts the boomerang so it curves back. Its force diagram: weight down, lift, and air resistance against its motion. What this step is for: A shaped object in flight shows forces that aren't pushes in contact, extending the list of forces students can name. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- A force has a **size** (newtons, N) and a **direction**, so it is drawn as an **arrow**.
- The arrow's **length** shows the size; it **points** the way the force acts.
- A **force diagram** shows every force on one object, each one named.
- Common forces: weight (down), support (up from a surface), friction (against sliding), air resistance (against motion through air), pushes and pulls.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**A box sits on the floor. Name the two forces on it and describe their arrows.**

* Weight, pointing down, and support from the floor, pointing up; the same length because they are equal.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: node 2, balanced and unbalanced forces: what the diagram tells you about the motion. In class: force meters pulling a block along the bench; tug of war with two force meters. -->