---
marp: true
theme: default
html: true
---

# What forces act on it?

Year 10 Physics · Newton's laws

<!-- Speaker note: Students draw free body diagrams for objects with several forces, find the net force in each direction, and use Newton's first and second laws to say what the motion does (AC9S10U05, node 6). About 25 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

---

## Today

**We are learning to** a free body diagram shows one object and every force acting on it, as arrows from the object: size by length, direction by the way it points. Add the forces along each direction to find the net force, then use Newton's laws to say what the object's motion will do.

- I can draw a free body diagram with every force on an object.
- I can find the net force in each direction.
- I can say what the object's motion will do from its free body diagram.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It follows Push the wall, and it pushes you? and pulls together the first and second laws. Integrating all three laws in problems (nodes 7 and 8) is a class task. Not a hinge node, but students carry two errors into it: a moving object needs a net force forward, and forces the object exerts belong on its diagram. The plane prediction targets the first; the "what goes on it" question the second. Board notes: the board draws forces from the object's centre (friction at the ground) with arrow length by size; large forces are capped in length, so read the labels. The plane's weight is given directly in newtons. Teacher: In your own words: the cyclist's balanced diagram is the evidence. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 27px; }</style>

---

## Why it matters

> A plane cruising at 900 km/h has four big forces on it, each hundreds of thousands of newtons. Yet the passengers can pour a drink without spilling it. How?

<!-- Speaker note: You can find a net force along one line. This lesson draws all the forces on an object at once. -->

---

<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

**A plane cruises at a steady speed and height. What can you say about the forces on it?**

- Thrust is bigger than drag, or it would slow down
- There are no forces on it
- They balance both ways: lift equals weight, thrust equals drag
- Lift is bigger than weight, or it would fall

<!-- Speaker note: What happens: They balance in both directions. Steady speed in a straight line means no change in motion, so no net force (Newton's first law): lift balances weight, and thrust balances drag. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Thrust is bigger' is the Aristotelian idea. -->

---

<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>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-10%2Ffree-body%2Fstep-2&code=widget%3A+forces%0Aobject%3A+Sled%0Amass%3A+20%0Ag%3A+9.8%0Aforce+W%3A+down%2C+weight%2C+Weight%0Aforce+N%3A+up%2C+support%2C+Snow+pushing+up%0Aforce+P%3A+right%2C+60%2C+Rope+pulling%2C+adjust+0-120+by+10%0Afriction%3A+40%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-10/free-body/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask which direction they balanced first, and why it didn't matter. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 25px; }</style>

---

<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?

* They balance in both directions. Steady speed in a straight line means no change in motion, so no net force (Newton's first law): lift balances weight, and thrust balances drag.

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

---

<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 goes on it

**What goes on a free body diagram?**

- Every object in the scene and all the forces between them
- One object, and every force acting on that object
- Only the forces that make the object move

* Answer: One object, and every force acting on that object

<!-- Speaker note: One object, every force on it, each as an arrow starting at the object, labelled with what causes it and its size. -->

<!-- Speaker note: Teacher: the two number checks and the lift question are the success criteria. -->

---

<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>

## The skydiver

**A 600 N skydiver has 450 N of air resistance acting up on her. What is the net force on her, in newtons, downwards?**

* Answer: 150

<!-- Speaker note: 600 − 450 = 150 N downwards: she is still speeding up. -->

---

<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>

## The box

**A 2 kg box is pushed right with 14 N against 6 N of friction. What is its acceleration, in m/s²?**

* Answer: 4

<!-- Speaker note: Net force 14 − 6 = 8 N right. a = F ÷ m = 8 ÷ 2 = 4 m/s². -->

---

<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>

## The lift

**A lift's cable pulls up with exactly its weight. The lift is moving up. What is happening?**

- It must be stopped
- It is speeding up, because it is moving up
- It moves up at a steady speed: balanced forces, no change in motion

* Answer: It moves up at a steady speed: balanced forces, no change in motion

<!-- Speaker note: First law: balanced forces keep the motion the same, here steady upward motion. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 28px; }</style>

---

<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>

## Elaborate: four forces on a plane

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-10%2Ffree-body%2Fstep-4&code=widget%3A+forces%0Aobject%3A+Plane%0Amass%3A+1%0Aforce+W%3A+down%2C+6000%2C+Weight%0Aforce+L%3A+up%2C+4000%2C+Lift+from+the+wings%2C+adjust+0-10000+by+500%0Aforce+T%3A+right%2C+2000%2C+Thrust%2C+adjust+0-4000+by+250%0Aforce+D%3A+left%2C+1500%2C+Drag%2C+adjust+0-4000+by+250%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-10/free-body/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Balancing two directions at once with four forces is the full multi-force analysis in a real context. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 25px; }</style>

---

<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>

## More thrust

**The pilot now increases thrust a little and leaves everything else the same. What happens?**

- The plane climbs straight up
- The plane speeds up: a net force forward
- Nothing changes

* Answer: The plane speeds up: a net force forward

<!-- Speaker note: Thrust bigger than drag gives a net force forward, so the plane accelerates (until the extra speed raises the drag to match). -->

---

<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: the skydiver's fall

**As a skydiver falls faster, air resistance grows. What happens to her free body diagram over time?**

- The arrows stay the same throughout
- The upward arrow grows until it equals her weight; then she falls at a steady speed (terminal velocity)
- Her weight arrow shrinks

* Answer: The upward arrow grows until it equals her weight; then she falls at a steady speed (terminal velocity)

<!-- Speaker note: Net force downward shrinks to zero, so the acceleration does too. Opening the parachute makes air resistance jump, and she slows to a new, lower steady speed. What this step is for: A changing force (air resistance growing with speed) shows why free body diagrams change during a motion. -->

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 28px; }</style>

---

## Evaluate: look back

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

**Revision summary**

- A **free body diagram** shows one object and every force acting on it, as labelled arrows from the object.
- Work out the net force in each direction separately (up–down, left–right).
- Net force zero in every direction: no change in motion (still, or steady speed in a straight line).
- A net force: the object accelerates in that direction, with a = F ÷ m.

<style scoped>pre, marp-pre { font-size: 0.5em; line-height: 1.25; } section { font-size: 26px; }</style>

---

<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 1000 kg car has 3000 N of driving force and 1000 N of drag. Find the net force and the acceleration.**

* 2000 N forward; a = 2000 ÷ 1000 = 2 m/s².

<!-- 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: problems using all three laws (node 7) and real applications (node 8) are class tasks. In class: force meters on a block dragged at steady speed; drawing free body diagrams for photos (a climber, a kite, a car braking). -->