Chapter 1: Concepts of Motion

1. Why We Need "Concepts of Motion"

Physics starts by answering one question: "How is something moving?" To describe that movement precisely we first agree on a handful of building-block ideas—time, position, velocity, and so on. This chapter introduces those ideas and shows how they fit together.

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2. Seeing Motion: Motion Diagrams & the Particle Model

Tip: Imagine a flip-book cartoon. Each page is a dot in a new spot. The set of dots is a motion diagram.

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3. Scalars vs. Vectors—Two Kinds of Numbers

Vectors are drawn as arrows. Arrow length = size, arrow head = direction.

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4. The Core Motion Quantities

4.1 Time ($t$)

• A clock reading ("3.0 s since the stopwatch started").
• Scalar; SI unit = $\text{s}$ (seconds).

4.2 Position ($r$)

• Where the object is relative to an origin (your chosen "zero" point).
• Vector; SI unit = $\text{m}$ (metres).

4.3 Displacement ($\Delta r$)

• The straight-line "from-start-to-finish" arrow: final position – initial position.
• Vector.

4.4 Distance (d)

• "How much ground you actually covered," following all twists and turns.
• Scalar; always positive.

4.5 Velocity ($\vec{v}$)

• Rate of change of position: $v_{\text{avg}} = \dfrac{\Delta r}{\Delta t}$.
• Vector (speed + direction). SI unit = $\text{m/s}$.

4.6 Speed ($v$)

• "How fast?" ignoring direction: $v_{\text{avg}} = \dfrac{d}{\Delta t}$.
• Scalar; SI unit = $\text{m/s}$.

4.7 Acceleration ($\vec{a}$)

• Rate at which velocity changes: $a_{\text{avg}} = \dfrac{\Delta \vec{v}}{\Delta t}$.
• Vector; SI unit = $\text{m/s}^2$.
• Picture acceleration as the "pull" controlling how velocity arrows grow or shrink.

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5. How Acceleration Affects Motion

• Same direction as velocity → object speeds up.
• Opposite direction → object slows down. Key warning: The sign of acceleration (+/-) alone does not tell you speeding-up vs. slowing-down—you must compare it with the velocity's sign.

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6. Reading a Motion Diagram—Rapid Check

The slides include a series of multiple-choice "which statement is correct?" questions on position, velocity, and acceleration arrows. When you face one:

1. Look at dot spacing.

   • Closer dots = slower speed.
   • Wider spacing = faster speed.

2. Add tiny arrows.

   • Draw velocity arrows from each dot to the next; see how they grow, shrink, or flip direction.

3. Compare arrow changes.

   • If arrows get longer in the same direction → positive acceleration (speeding up).
   • If arrows shorten or reverse → acceleration opposes velocity (slowing or turning).

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7. Example Walk-Through: The Stop-and-Go Cyclist

Imagine a cyclist moving 10 m/s → stop → 15 m/s forward (this is the scenario on the last slide).

1. Before the light: velocity arrow points right (10 m/s).
2. Stopping phase: arrows shrink to zero; acceleration arrow points left (opposite motion).
3. Starting again: new arrows grow rightward; acceleration now points right.

Result: one continuous motion diagram where spacing first tightens (slowing) then widens (speeding).

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8. Putting It All Together—Four Quick Take-Home Points

1. A motion diagram + arrows is your visual toolbox for any motion problem.
2. Vectors tell two things at once—size and direction—so always include a sign or arrow.
3. Displacement vs. distance: straight-line vs. path-length. Keep them distinct.
4. Acceleration's direction relative to velocity decides whether you speed up or slow down.

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Mini-Self-Check

1. If a car's velocity is -20 m/s and its acceleration is -5 m/s², is it speeding up or slowing down?

2. A runner completes one lap of a 400 m track in 50 s. What is the runner's average speed? What is the average displacement and average velocity?