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2.0Representing Motion

Overview: Representing Motion: Diagrams, Position, Position–Time Graphs and Velocity — American System Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 2 of 5 Unit quiz

📚 American System Physics Hub / Topical Revision
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Lesson summary

Draw motion diagrams.Two lines crossing on a position–time graph means the objects are at the same place at the same time.Instantaneous velocity is the slope of the tangent at a point.Velocity can be negative; speed cannot.Displacement can be zero even if distance is not.Vectors …

  • A car at x = 10 m moves at 15 m/s. Position after 4 s?
  • Particle model
  • Coordinate system
  • Average velocity
  • Identify / Describe (FRQ)
  • Explain / Justify (FRQ)

Section 1 of 7Overview: Representing Motion: Diagrams, Position, Position–Time Graphs and Velocity

1
2.1 Representing Motion: Diagrams, Positi… concept map: A car at x = 10 m moves at 15, Motion diagram, Particle model, Coordinate system, Position
Visual guide to the key ideas explained in this lesson.
2
Average speed = total distance ÷ total time.
3
A motion diagram with equally spaced dots shows constant velocity.
4
Increasing spacing shows speeding up.
5
Time interval Δt = tf − ti.
6
Always include direction for vectors.

Section 2 of 7Process & sequence

1
Choose an origin and positive direction.
2
Record positions at each time.
3
Plot position (y-axis) vs time (x-axis).
4
Slope of the line = velocity.
5
Use x = xi + vt for constant velocity.

From motion to graphwrite these steps in this order; each step is a marking point.

  1. Choose origin
  2. Measure positions
  3. Plot x vs t
  4. Find slope
  5. State velocity

Section 3 of 7Compare

Scalar vs vector
ScalarVector
DistanceDisplacement
SpeedVelocity
TimeAcceleration
MassForce
Reading position–time graphs
GraphMeaning
Horizontal lineAt rest
Straight line, positive slopeConstant positive velocity
Straight line, negative slopeConstant velocity toward origin
Steeper lineFaster
CurveChanging velocity

Section 4 of 7Worked examples

A runner goes 400 m around a track and returns to the start in 80 s. Find average speed and average velocity.
  • Speed = 5.0 m/s; velocity = 0 (zero displacement).
A car at x
10 m moves at 15 m/s. Position after 4 s?
  • x = 10 + 15 × 4 = 70 m.

Section 5 of 7Key terms

Section 6 of 7Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 7 of 7Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Motion diagram · Particle model · Coordinate system · Position · Distance · Displacement · Average velocity · Speed
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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