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Physics notebook Cambridge IGCSE 0625
Contains the answers

Answers · Speed and motion graphs

Speed and motion graphs — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. MR1 · Return

Model answer. (a) Speed is the distance travelled per unit time; velocity is speed in a given direction. (b) 400 m ÷ 80 s = 5.0 m/s. (c) Gradient = 150 m ÷ 30 s = 5.0 m/s. (d) Average speed = 3000 m ÷ (5.0 × 60 s) = 3000 m ÷ 300 s = 10 m/s. (e) A flat line: at rest. A straight falling line: moving back towards the starting point at a steady speed. A curve that gets steeper: speeding up (accelerating). (f) Axes time / s and distance from the wall / m: flat at 5 m from 0 to 10 s; a straight rise to 5 m + 1.5 m/s × 20 s = 35 m at 30 s; flat at 35 m from 30 s to 40 s.

A full answer includes

  • (a) Speed is distance travelled per unit time; velocity is speed in a given direction.
  • (b) 400 m ÷ 80 s = 5.0 m/s.
  • (c) (250 − 100) m ÷ (40 − 10) s = 5.0 m/s.
  • (d) 3000 m ÷ 300 s = 10 m/s.
  • (e) Flat: at rest. Straight falling: moving back towards the start at a steady speed. Curve getting steeper: speeding up.
  • (f) Labelled axes (time / s, distance from the wall / m); flat at 5 m from 0 to 10 s; a straight line rising to 35 m at 30 s (1.5 m/s × 20 s = 30 m further); flat at 35 m to 40 s.

Also accept

  • (b) 5.0 m/s; (c) 5.0 m/s; (d) 10 m/s

Watch for

  • “(c) 250 m ÷ 40 s = 6.3 m/s.” — The line starts at 100 m, not at 0 m. Use the changes between the two points: 150 m in 30 s.
  • “(d) 3000 m ÷ 240 s = 12.5 m/s.” — The minute spent waiting is part of the total time.

2. MR2 · Return

Model answer. (a) Along the axis: at rest. Flat above it: constant speed. Rising: speeding up (accelerating). Falling: slowing down (decelerating). (b) The distance travelled. (c) The shape is a triangle: ½ × 6.0 s × 15 m/s = 45 m. (d) Plot time / s across and speed / m/s up; the points rise in a straight line from 0 to 4.0 m/s at 4 s (speeding up evenly), then stay at 4.0 m/s (constant speed). Distance = triangle + rectangle = ½ × 4 s × 4.0 m/s + 4 s × 4.0 m/s = 8 m + 16 m = 24 m.

A full answer includes

  • (a) At rest; constant speed; speeding up; slowing down.
  • (b) The distance travelled.
  • (c) ½ × 6.0 s × 15 m/s = 45 m.
  • (d) Labelled axes (time / s, speed / m/s), five crosses joined part by part: a straight rise from 0 to 4.0 m/s at 4 s, then flat. Distance = ½ × 4 s × 4.0 m/s + 4 s × 4.0 m/s = 8 m + 16 m = 24 m.

Also accept

  • (a) rest, constant, speeding up, slowing down; (b) distance travelled; (c) 45 m; (d) 24 m

Watch for

  • “(c) 15 m/s × 6.0 s = 90 m.” — The car started from rest, so its speed was below 15 m/s most of the time: the shape is a triangle, take ½.
  • “(a) A flat line means at rest.” — On a speed–time graph a flat line above the axis means a constant speed.