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GCSE Physics 03 — Forces and Motion

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Topics include Speed, Velocity and Acceleration, Interpreting Motion Graphs, Newton’s Laws of Motion and Inertia, Newton’s Third Law: Interaction Pairs, Resultant Forces and Vector Diagrams, Free-Body Diagrams and Resolving Forces, Terminal Velocity, and Momentum and Conservation.

Physics EN
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Interpreting Motion Graphs

Analyzing distance–time and velocity–time graphs to determine speed, acceleration, and distance travelled.

Key points

  • Distance–Time Graphs: Gradient = Speed. Horizontal line = Stationary. Curved line = Accelerating.
  • To find speed from a curved distance–time graph, draw a tangent and calculate its gradient.
  • Velocity–Time Graphs: Gradient = Acceleration. Horizontal line = Constant velocity. Area under graph = Distance travelled.
  • A linear relationship (straight line) on a graph indicates a constant rate of change.
  • Area under a velocity–time graph can be calculated by splitting the shape into rectangles and triangles.

Worked example

Question

A velocity–time graph shows a cyclist accelerating from rest to 8 m/s8\,m/s in 10 s10\,s, then maintaining that velocity for 20 s20\,s. Calculate the total distance travelled.

Solution

1. Identify the shapes under the graph line.
2. Shape A (Triangle): Base = 10, Height = 8. Area = 12×10×8=40 m\frac{1}{2} \times 10 \times 8 = 40\,m.
3. Shape B (Rectangle): Base = 20 (from 10s to 30s), Height = 8. Area = 20×8=160 m20 \times 8 = 160\,m.
4. Total Distance = Sum of areas = 40+160=200 m40 + 160 = 200\,m.
5. Interpretation: The cyclist travelled 200 meters in total.

Common pitfalls

  • Confusing the gradient rules (e.g., thinking gradient of distance–time is acceleration).
  • Thinking a flat line always means 'stopped' (on a velocity–time graph, it means constant velocity).
  • Forgetting to calculate the area of the triangle part (12×b×h\frac{1}{2} \times b \times h) when finding distance.

Prerequisites

  • Calculating gradients of straight lines
  • Calculating areas of rectangles and triangles
  • forces.kinematics_basics
Further resources
  • Motion Graphs Explained

    Detailed breakdown of graph shapes and their physical meanings.

    The Physics Classroom · article