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3.0Accelerated Motion

Overview: Accelerated Motion: Acceleration, Constant-Acceleration Equations and Free Fall — 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

Define and calculate acceleration.In free fall, all objects accelerate equally (ignoring air resistance).At the top of a vertical throw, velocity is zero but acceleration is still g.Area under a v–t graph gives displacement.Deceleration is acceleration opposite to velocity.Air re…

  • Take up as positive
  • A car goes from 0 to 24 m/s in 6.0 s. Find a and distance
  • A ball is dropped from 20 m. Speed on hitting the ground?
  • Velocity–time graph
  • Uniform acceleration
  • Acceleration due to gravity (g)

Section 1 of 7Overview: Accelerated Motion: Acceleration, Constant-Acceleration Equations and Free Fall

3.1 Accelerated Motion: Acceleration, Con… concept map: Take up as positive, A car goes from 0 to 24 m/s i, A ball is dropped from 20 m., Acceleration, Velocity–time graph
Visual guide to the key ideas explained in this lesson.
Take up as positive
Take up as positive a = −9.8 m/s².
Time up equals time down for symmetric vertical motion.
v–t slope zero means constant velocity.
Units of acceleration are m/s².
Reaction time adds distance before braking begins.

Section 2 of 7Process & sequence

1
Choose a positive direction.
2
List vi, vf, a, t, Δx (know three).
3
Pick the equation that lacks the unknown you do not need.
4
Solve symbolically, then substitute.
5
Check sign and units.

Vertical throwwrite these steps in this order; each step is a marking point.

  1. Launched upward
  2. Slows at g
  3. v = 0 at top
  4. Falls, speeding up
  5. Returns with same speed

Section 3 of 7Compare

Constant-acceleration equations
EquationMissing variable
vf = vi + atdisplacement
xf = xi + vit + ½at²final velocity
vf² = vi² + 2aΔxtime
Δx = ½(vi + vf)tacceleration
Sign of acceleration
VelocityAccelerationResult
++Speeding up
+−Slowing down
−−Speeding up (backward)
−+Slowing down

Section 4 of 7Worked examples

1A car goes from 0 to 24 m/s in 6.0 s. Find a and distance.
A car goes from 0 to 24 m/s in 6.0 s. Find a and distance.
  • a = 4.0 m/s²; Δx = ½ × 4.0 × 36 = 72 m.
2A ball is dropped from 20 m. Speed on hitting the ground?
A ball is dropped from 20 m. Speed on hitting the ground?
  • v = √(2 × 9.8 × 20) = 19.8 m/s.

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
Acceleration · Velocity–time graph · Uniform acceleration · Free fall · Acceleration due to gravity (g)
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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