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# What is the net force?

Year 10 Physics · Newton's laws

<!-- Speaker note: Students represent forces as vectors and add or subtract forces along one line to find the net force, then say what the net force does to motion (AC9S10U05, node 1). About 24 minutes (Engage and Explore 7 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 4 min). -->

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## Today

**We are learning to** a force is a vector: it has a size and a direction. Forces along one line add if they point the same way and subtract if they point opposite ways. What is left is the net (resultant) force, and that is what changes motion.

- I can explain why force is a vector.
- I can find the net force from several forces along one line.
- I can say whether a net force will change an object's motion.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It revisits Year 7 force arrows (AC9S7U04) with numbers, and leads straight into Does a moving thing need a push? (first law, node 2). Not a hinge node, but the slip it targets returns in every later lesson: adding all the forces as numbers, ignoring direction. The prediction's 2800 N catches it. Board notes: the board shows horizontal forces only (and weight and support when set); the net force is horizontal, positive to the right. In the Wider step the car moves along a track with a speed–time graph. Real air resistance grows with speed; here it is a fixed force. Teacher: In your own words: the scaled force diagram with the calculation is the evidence. -->

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## Why it matters

> A car's engine pushes it forward with 2000 newtons. So why doesn't it keep speeding up for ever?

<!-- Speaker note: In Year 7 you drew forces as arrows. This lesson adds them up. -->

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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

**A car's driving force is 2000 N forward. Air resistance is 500 N and rolling resistance 300 N, both backward. What is the net force on the car?**

- 2800 N forward
- 1200 N forward
- Zero: the forces cancel
- 2000 N forward

<!-- Speaker note: What happens: 1200 N forward. The two backward forces add to 800 N; opposite to the 2000 N forward, so they subtract: 2000 − 800 = 1200 N, in the direction of the bigger side (forward). Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally 1200 / 2800 / 2000 / zero. The 2800s added every force. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-10%2Fnet-force%2Fstep-2&code=widget%3A+forces%0Aobject%3A+Car%0Amass%3A+1200%0Aforce+T%3A+right%2C+2000%2C+Driving+force%2C+adjust+0-4000+by+100%0Aforce+D%3A+left%2C+500%2C+Air+resistance%2C+adjust+0-3000+by+100%0Aforce+R%3A+left%2C+300%2C+Rolling+resistance%0A&fresh=1}}

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

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: ask what one click on a force did to the net force (changed it by the same amount). -->

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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?

* 1200 N forward. The two backward forces add to 800 N; opposite to the 2000 N forward, so they subtract: 2000 − 800 = 1200 N, in the direction of the bigger side (forward).

<!-- 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>

## Size and direction

**Why is force called a vector?**

- It can only act in a straight line
- It has a size and a direction, and both matter
- It is measured in newtons

* Answer: It has a size and a direction, and both matter

<!-- Speaker note: A vector has size and direction. 10 N left and 10 N right are different forces, and they cancel. -->

<!-- Speaker note: Teacher: the two number checks are the success criteria; the skydiver links zero net force to steady motion. -->

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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>

## A push and friction

**Two people push a stalled car forward with 300 N and 250 N. Friction acts backward with 400 N. What is the net force, in newtons? (forward is positive)**

* Answer: 150

<!-- Speaker note: Forward: 300 + 250 = 550 N. Backward: 400 N. Net: 550 − 400 = 150 N forward. -->

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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>

## Tug of war

**In a tug of war, Team A pulls left with 1800 N and Team B pulls right with 2100 N. What is the size of the net force, in newtons?**

* Answer: 300

<!-- Speaker note: Opposite directions: subtract. 2100 − 1800 = 300 N, to the right (Team B's way). -->

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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>

## Balanced

**A skydiver weighs 700 N. Air resistance on her is 700 N up. What is happening to her motion?**

- She must be stopped in the air
- Nothing is changing: net force zero, so she falls at a steady speed
- She is speeding up, because weight is pulling her down

* Answer: Nothing is changing: net force zero, so she falls at a steady speed

<!-- Speaker note: Balanced forces give a net force of zero: the motion doesn't change. She keeps falling at the same speed (terminal velocity). -->

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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 happens next

**What will the motion do?**

Groups: Change (speed up, slow down or turn) · Stay the same

Net force 500 N forward · Net force 200 N backward on a moving bike · Forward 900 N, backward 900 N · A puck sliding on ice with no friction

* Answer: Net force 500 N forward → Change (speed up, slow down or turn); Net force 200 N backward on a moving bike → Change (speed up, slow down or turn); Forward 900 N, backward 900 N → Stay the same; A puck sliding on ice with no friction → Stay the same

<!-- 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>

## Touching or not

**Contact or non-contact force?**

Groups: Contact force · Non-contact force

Friction between the tyres and the road · Air resistance on a cyclist · A table pushing up on a book (the normal force) · Gravity pulling a skydiver down (her weight) · A magnet pulling a paperclip from a few centimetres away

* Answer: Friction between the tyres and the road → Contact force; Air resistance on a cyclist → Contact force; A table pushing up on a book (the normal force) → Contact force; Gravity pulling a skydiver down (her weight) → Non-contact force; A magnet pulling a paperclip from a few centimetres away → Non-contact force

<!-- Speaker note:  -->

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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>

## Elaborate: a four-person tug of war

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-10%2Fnet-force%2Fstep-4&code=widget%3A+forces%0Aobject%3A+Rope%0Amass%3A+1%0Aforce+A%3A+left%2C+400%2C+Ana%2C+adjust+0-800+by+50%0Aforce+B%3A+left%2C+300%2C+Ben%0Aforce+C%3A+right%2C+350%2C+Cai%2C+adjust+0-800+by+50%0Aforce+D%3A+right%2C+250%2C+Dee%0A&fresh=1}}

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

<!-- Speaker note: What this step is for: Four forces, two adjustable, make students combine same-direction forces before subtracting, the full one-axis method. -->

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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>

## Dee's pull

**Ana pulls 400 N and Ben 300 N to the left. Cai pulls 450 N to the right. How hard must Dee pull right for the net force to be zero?**

- 150 N
- 250 N
- 700 N

* Answer: 250 N

<!-- Speaker note: Left: 400 + 300 = 700 N. Right must also be 700 N: 450 + 250 = 700. -->

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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>

## Elaborate: driving at a steady speed

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-10%2Fnet-force%2Fstep-5&code=widget%3A+forces%0Aobject%3A+Car%0Amass%3A+1200%0Aforce+T%3A+right%2C+1800%2C+Driving+force%2C+adjust+0-3000+by+100%0Aforce+D%3A+left%2C+1200%2C+Air+resistance%0Aforce+R%3A+left%2C+300%2C+Rolling+resistance%0Amotion%3A+on%0Aspeed%3A+20%0Atrack%3A+400%0Atime%3A+10%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-10/net-force/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Running the board with a zero net force shows the car keeps its speed, the bridge to Newton's first law next lesson. -->

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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>

## Why no speeding up

**The car's engine is still pushing with 1500 N, but it isn't speeding up. Why?**

- The engine's force is used up
- Moving things always slow down unless pushed harder
- Air and rolling resistance add up to 1500 N backward, so the net force is zero

* Answer: Air and rolling resistance add up to 1500 N backward, so the net force is zero

<!-- Speaker note: Net force zero: no change in motion. The engine only needs to cancel the resistance to keep a steady speed. -->

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## Evaluate: look back

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

**Revision summary**

- A force is a **vector**: it has a size (in newtons) and a direction.
- Along one line, forces in the same direction **add**; forces in opposite directions **subtract**. Choose one direction as positive.
- What is left is the **net force** (resultant force), in the direction of the bigger side.
- A net force changes motion (speeds up, slows down, turns). A net force of zero leaves the motion unchanged: still, or at a steady speed.

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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 cyclist pedals with a forward force of 120 N. Air resistance is 70 N and friction 20 N. Find the net force and say what happens to her speed.**

* 120 − (70 + 20) = 30 N forward: she speeds up.

<!-- 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: Does a moving thing need a push? (Newton's first law, node 2). In class: force meters on a trolley pulled in opposite directions; a tug of war with spring balances; drawing force diagrams to scale on grid paper. -->