4PH1

Movement & Position

Forces & Motion · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 12% of your exam marks.

stable
High
Stable12%

Distance-time graphs, speed calculations and velocity appear in nearly every series.

Reading the slope

  • A velocity-time graph plots an object's velocity (y-axis) against the time elapsed since the start of the motion (x-axis)
  • The slope of the line tells you about the acceleration:
    • A straight line with a positive gradient means constant acceleration, with velocity climbing at a steady rate
    • A straight line with a negative gradient means constant deceleration, with velocity falling at a steady rate
    • A steep line signals a large magnitude of acceleration; a gentle line signals a small one
    • A flat horizontal line means zero acceleration, so the velocity stays at a single fixed value (a constant velocity)
A velocity-time graph with velocity on the y-axis and time on the x-axis, showing a line that rises with a positive gradient (labelled constant acceleration), levels off into a flat horizontal section (labelled constant velocity), then falls with a negative gradient (labelled constant deceleration)
Source: Velocity time graph by Save My Exams

Gradient = acceleration

  • The of an object equals the of its velocity-time graph:

a = Δy / Δx = Δv / Δt

  • For a straight-line section, the gradient is the same everywhere, so draw a large gradient triangle and divide the rise by the run, taking care to read the axes in their stated units (m/s and s)
Exam tip

Reading a velocity–time graph: gradient and area

What comes up: the exam asks what the gradient of a velocity–time graph represents, or asks you to use the gradient to find acceleration.

Write: acceleration = gradient = (change in velocity) ÷ (change in time). State this explicitly before substituting numbers — the mark scheme awards the first mark for linking acceleration to the gradient.

Watch out: a common slip is to confuse the two graph types: on a distance–time graph the gradient gives speed; on a velocity–time graph the gradient gives acceleration. The area under the line is not acceleration — it is the distance travelled.