[Leejohnston]

## Acceleration — How Speed Really Changes Over Time

### 1. The Equation

Acceleration = Change in velocity ÷ Time

Often written as:
a = Δv ÷ t

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### 2. What Each Symbol Means

- a = acceleration
- Δv = change in velocity
- t = time

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### 3. What the Equation Is Telling Us

Acceleration measures **how quickly speed or direction changes**.

You can accelerate by:
- speeding up
- slowing down
- changing direction

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

Acceleration describes how motion evolves.

A constant speed means **zero acceleration**, even if the object is moving fast.

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### 5. Worked Example

If a car:
- increases speed from 0 to 20 m/s
- in 5 seconds

Acceleration = 20 ÷ 5
= **4 m/s²**

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### 6. Real-World Applications

- Car safety and braking
- Roller coasters
- Space launches
- Sports performance

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### 7. Common Mistakes

- Confusing speed with acceleration
- Forgetting direction matters
- Assuming acceleration always means speeding up

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### Try It Yourself

What acceleration do you feel when:
- a car brakes suddenly?
- an elevator starts moving?