---
title: "Same push, different mass"
subtitle: "Year 7 Physics · Forces and motion · Lesson 5"
---

# Same push, different mass

**Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}}

> **Key question:** A rugby front-rower and a winger sprint off the same line with the same push from the ground. Who gets going faster, and why?


:::teacher-note

**In one sentence:** Students state the qualitative relationship between force, mass and acceleration, predict which object accelerates more under the same force, and read the pattern from data (AC9S7U04, node 5, an integration node).

**Students should be able to say:** I can say what happens to acceleration when the force or the mass changes. I can predict which of two objects accelerates more under the same force. I can read a pattern from force data.

**Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet.

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## Engage


:::keep-together

**You give an empty trolley and a full trolley exactly the same steady push. Which one speeds up more?**

☐ They speed up equally

☐ The empty one: a smaller mass accelerates more for the same force

☐ The full one: it has more mass to push

☐ Neither: a steady push only gives a steady speed

My prediction, and why:

{{lines-3}}

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:::teacher-note

**What happens:** The empty one. For the same unbalanced force, a bigger mass means a smaller acceleration. Mass resists changes in motion.

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## Explore

Try it on the board in the tutorial. Was your prediction right? What was different?

{{lines-2}}

## Explain: the pattern


:::keep-together

### Which accelerates more

**The same unbalanced force acts on a 2 kg object and an 8 kg object. Which accelerates more?**

☐ The 2 kg object

☐ The 8 kg object

☐ Both the same

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:::teacher-note

**Answer:** The 2 kg object

For the same force, the smaller mass accelerates more.

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:::keep-together

### The relationship

**Which statement is right?**

☐ A bigger mass gives a bigger acceleration

☐ A bigger unbalanced force gives a bigger acceleration, and a bigger mass gives a smaller one

☐ Force and mass don't affect acceleration

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:::teacher-note

**Answer:** A bigger unbalanced force gives a bigger acceleration, and a bigger mass gives a smaller one

More force, more acceleration. More mass, less acceleration.

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:::keep-together

### Force data

**Same trolley, 2 kg. Pushing with 2 N gives 1 m/s², 4 N gives 2 m/s², 6 N gives 3 m/s². What acceleration would 10 N give (m/s²)?**

Answer: {{field-15}}

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:::teacher-note

**Answer:** 5

The acceleration goes up by the same amount for each extra newton: 10 N gives 5 m/s².

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:::keep-together

### Mass data

**Same push of 8 N. A 1 kg trolley accelerates at 8 m/s², 2 kg at 4 m/s², 4 kg at 2 m/s². What acceleration would 8 kg give (m/s²)?**

Answer: {{field-15}}

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:::teacher-note

**Answer:** 1

Each time the mass doubles, the acceleration halves: 8 kg gives 1 m/s².

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## Mix it up


:::keep-together

**Looking back at unbalanced forces: a box slides to the right. Friction pulls it left with 6 N and nothing pushes it. What does the box do?**

☐ It keeps sliding at a steady speed

☐ It speeds up to the right

☐ It slows down

☐ It stops straight away

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:::teacher-note

**Answer:** It slows down

The net force is 6 N to the left, against the motion, so the box slows down.

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:::typical

## Elaborate: hit a target


:::keep-together

### Double the mass

**You change the mass to 10 kg and keep the same push. What must happen to the acceleration?**

☐ It doubles

☐ It stays the same

☐ It halves

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:::teacher-note

**Answer:** It halves

Doubling the mass halves the acceleration for the same push.

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:::wider

## Elaborate: rockets and roads


:::keep-together

**A rocket burns fuel, so its mass falls while the thrust stays the same. What happens to its acceleration?**

☐ It increases

☐ It stays the same

☐ It decreases

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:::teacher-note

**Answer:** It increases

The same force on a smaller mass gives a bigger acceleration. This is one reason rockets are built in stages that fall away.

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## Evaluate: look back

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

**Revision summary**

- For the **same unbalanced force**, a **larger mass** gives a **smaller acceleration**.
- For the **same mass**, a **larger unbalanced force** gives a **larger acceleration**.
- Data show the pattern: acceleration rises with force and falls as mass rises.

## Exit ticket

:::keep-together

**The same push is given to a 3 kg box and a 6 kg box. Which accelerates more, and by how much compared with the other?**

{{lines-3}}

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:::teacher-note

**Answer:** The 3 kg box. It accelerates twice as much, because it has half the mass.

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