[Leejohnston]

## Work–Energy Theorem — Why Work Changes Speed

### 1. The Equation

Work done = Change in kinetic energy

W = ΔKE

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### 2. What Each Symbol Means

- W = work (joules)
- ΔKE = change in kinetic energy

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### 3. What the Equation Is Telling Us

Whenever work is done on an object, its **speed must change** (unless energy is transferred elsewhere).

Work is the mechanism that changes motion.

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### 4. Where It Comes From (Intuition)

Pushing an object over a distance transfers energy into it.

More work → more kinetic energy → more speed.

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### 5. Worked Example

If a force does:
- 500 joules of work

Then the object’s kinetic energy increases by **500 joules**.

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### 6. Real-World Applications

- Braking systems
- Engine acceleration
- Sports performance
- Crash analysis

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### 7. Common Misconceptions

- Force alone changes speed → false
- Work always increases speed → false (can remove energy)
- Energy and momentum are interchangeable → false

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### Try It Yourself

Why does a longer stopping distance reduce injury in a crash?