Calculating Acceleration from Speed-Time Graphs (Cambridge O Level Physics)

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Interpreting Speed-Time graphs

  • If there is a change in an object's speed, then it is accelerating

  • An object may accelerate at a steady rate, this is called constant acceleration
    • On a speed-time graph this will be a non-horizontal straight line

Constant acceleration

cie-1-2-5-constant-acceleration-graph

  • An object may accelerate at an increasing rate
    • On a speed-time graph this would be an upward curve

Increasing acceleration

cie-1-2-5-increasing-acceleration-graph

  • An object may accelerate at a decreasing rate
    • On a speed-time graph this would be an upward curve with a decreasing gradient

new-1-2-5-decreasing-acceleration

  • An object is said to decelerate if its speed is decreasing over time, i.e. its acceleration is negative 
    • On a speed-time graph this would be a downward line
      • If the line is a non-horizontal straight line, deceleration is constant
      • If the line is a curve with an increasing gradient, deceleration is increasing
      • If the line is a curve with a decreasing gradient, deceleration is decreasing

Calculating Acceleration

  • The acceleration of an object can be calculated from the gradient of a speed-time graph

 a c c e l e r a t i o n space equals space g r a d i e n t space equals fraction numerator space r i s e over denominator r u n end fraction

Gradient of a speed-time graph

1-2-5-gradient-speed-time-graphs-cie-igcse-23-rn

How to find the gradient of a speed-time graph

Worked example

Tora is training for a cycling tournament.

The speed-time graph below shows her motion as she cycles along a flat, straight road.

1-2-5-worked-eg-graph-1-cie-igcse-23-rn

(a) In which section (A, B, C, D, or E) of the speed-time graph is Tora’s acceleration the largest?

(b) Calculate Tora’s acceleration between 5 and 10 seconds.

Part (a)

Step 1: Recall that the slope of a speed-time graph represents the magnitude of acceleration

  • The slope of a speed-time graph indicates the magnitude of acceleration

    Therefore, the only sections of the graph where Tora is accelerating is section B and section D

  • Sections A, C, and E are flat – in other words, Tora is moving at a constant speed (i.e. not accelerating)

Step 2: Identify the section with the steepest slope

  • Section D of the graph has the steepest slope

    Hence, the largest acceleration is shown in section D

Part (b)

Step 1: Recall that the gradient of a speed-time graph gives the acceleration

  • Calculating the gradient of a slope on a speed-time graph gives the acceleration for that time period

Step 2: Draw a large gradient triangle at the appropriate section of the graph

  • A gradient triangle is drawn for the time period between 5 and 10 seconds below:

1-2-5-worked-eg-graph-2-cie-igcse-23-rn

Step 3: Calculate the size of the gradient and state this as the acceleration

  • The acceleration is given by the gradient, which can be calculated using:

acceleration = gradient = 5 ÷ 5 = 1 m/s2

  • Therefore, Tora accelerated at 1 m/s2 between 5 and 10 seconds

Examiner Tip

Use the entire slope, where possible, to calculate the gradient. Examiners tend to award credit if they see a large gradient triangle used - so remember to draw 'rise' and 'run' lines directly on the graph itself!

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Ashika

Author: Ashika

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.