Velocity-time graphs

    OCR
    GCSE

    Velocity-time graphs provide a vector-based graphical representation of motion, plotting velocity on the y-axis against time on the x-axis. The gradient of the line corresponds to the acceleration, where positive gradients indicate acceleration and negative gradients indicate deceleration or acceleration in the opposite direction. Crucially, the area enclosed between the graph line and the time axis represents the displacement of the object. Mastery of this topic requires distinguishing between scalar speed and vector velocity, and interpreting features such as horizontal lines representing constant velocity rather than a stationary state.

    0
    Objectives
    3
    Exam Tips
    4
    Pitfalls
    4
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for calculating the gradient of the line to determine acceleration, showing correct substitution of change in velocity over time
    • Credit the calculation of the area under the graph (splitting into rectangles and triangles) to determine distance travelled
    • Award 1 mark for identifying that a horizontal line with a non-zero y-value represents constant velocity
    • For curved graphs, credit the drawing of a tangent at a specific time to determine instantaneous acceleration (Higher Tier only)
    • Award 1 mark for correct units (m/s²) when calculating acceleration

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the gradient as acceleration, but check your units — acceleration is m/s², not m/s"
    • "Careful with the horizontal section — on a velocity-time graph, this means constant speed, not stopped"
    • "Good calculation of the rectangular area, but you missed the triangular section above it when finding the total distance"
    • "Excellent use of the tangent for the curve. To secure the mark, ensure your triangle for the gradient calculation is large enough to be accurate"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for calculating the gradient of the line to determine acceleration, showing correct substitution of change in velocity over time
    • Credit the calculation of the area under the graph (splitting into rectangles and triangles) to determine distance travelled
    • Award 1 mark for identifying that a horizontal line with a non-zero y-value represents constant velocity
    • For curved graphs, credit the drawing of a tangent at a specific time to determine instantaneous acceleration (Higher Tier only)
    • Award 1 mark for correct units (m/s²) when calculating acceleration

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When calculating distance, physically draw lines on the graph to split the area into clear rectangles and triangles to ensure no section is missed
    • 💡Check the y-axis label immediately; if it reads 'Velocity', a horizontal line indicates constant speed, whereas on a 'Distance' graph it indicates stationary
    • 💡For 'Estimate the acceleration' questions involving curves, draw your tangent as long as possible to maximize the accuracy of your gradient calculation

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing velocity-time graphs with distance-time graphs, specifically interpreting a horizontal line as 'stationary' rather than 'constant velocity'
    • Incorrectly calculating the area of triangular sections by omitting the 0.5 factor in the area formula
    • Calculating the mean of the start and end velocities instead of calculating the gradient when asked for acceleration
    • Misinterpreting a negative gradient as 'moving backwards' rather than deceleration (unless velocity itself is negative)

    Study Guide Available

    Comprehensive revision notes & examples

    Read Study Guide

    Key Terminology

    Essential terms to know

    Gradient as acceleration
    Area under graph as displacement
    Interpretation of stationary, constant velocity, and constant acceleration states
    differentiation between linear and non-linear motion

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Describe
    Explain
    Determine
    Estimate

    Practical Links

    Related required practicals

    • •{"code":"PAG P1","title":"Investigation of motion","relevance":"Using light gates and trolleys to generate real-time velocity-time data for analysis"}

    Ready to test yourself?

    Practice questions tailored to this topic