Topic Three (1.3): Motion Graphs

Introduction

This topic is about interpreting graphs, particularly those relating to motion. The movement of objects can be described using motion graphs and numerical values. These are both used to help in the design of faster and more efficient vehicles.

You will need to be able to describe the motion of an object by looking at displacement-time (DT) graphs or velocity-time (VT) graphs. You need to focus on the steepness of the lines in the graphs and what they mean. For example, a horizontal line on a DT graph means the object is stationary but a horizontal line on a VT graph means constant velocity.

Reading

Edexcel International AS/A Level Physics Student Book 1 pages 12 to 14

Edexcel International A Level Physics Lab Book pages 6 to 10

Tasks

Complete the following:

  • Read pages 12 to 14 of the textbook.
  • Revise straight line graphs, using the following videos:

WATCH – Corbett Maths: Equation of a Line (YouTube) Links to an external site.

WATCH – Corbett Maths: Gradient of a Line (YouTube) Links to an external site.

WATCH – MathsGenie: Velocity-Time Graphs (YouTube) Links to an external site.

WATCH – Velocity vs. Time Graph (YouTube) Links to an external site.

SIMULATION – Motion with Constant Acceleration (Walter Fendt Sim) Links to an external site. Apply a force to the car in the SIM and observe the motion graphs. Calculate the acceleration from the gradient in the velocity - time graph.

PRINT, or DOWNLOAD, and SAVE the ‘Graph that Motion’ graphs (Physics Classroom) Links to an external site.

Practical Knowledge

  • Read Practical Skills box on page 14.

WATCH – ‘Measuring Acceleration Using Multi-flash Photography’ (YouTube) Links to an external site.

  • If possible, try to plan and conduct your own free-fall experiment, using the Doing Experiments at Home Download Doing Experiments at Home document to help you get started. For example, you could use a stopwatch (or any hand-operated timer) with large drop heights (and careful attention to safety!). Or you could try a slow-motion video app on a smartphone (try to work out the true time from the video replay, or else you may be able to include a running stopwatch in your picture).
  • Work through pages 6 to 10 in the Lab Book.

Top Tips

Make sure that you understand the following key points:

  • Speed and velocity are often used interchangeably, However, speed is a scalar quantity (has a size only), whereas velocity is a vector quantity (it has a direction as well as a size).
  • First check the graph LaTeX: yy-axis. Is it displacement or velocity on the LaTeX: yy-axis?
  • In a displacement-time graph, gradient gives velocity, curvature gives acceleration, negative values are behind the start.
  • In a velocity-time graph, gradient gives acceleration, area under the graph gives distance, zero is stationary, negative values are moving backwards.
  • Distance travelled is the ‘area under the graph’, make sure you are clear how to calculate the area of triangles LaTeX: ½ bh½bh, and the area of trapeziums LaTeX: ½ b(h₁ + h₂)½b(h₁+h₂).
  • When you calculate a gradient, make sure the triangle that you draw on the graph is large enough to provide an accurate answer. It is advisable to use more than half the length of the line to determine your gradient.

Key Terms

Add the following key terms with definitions to your word list:

  • Displacement-time (DT) graph
  • Velocity-time (VT) graph