Speed-Time Graphs (Cambridge (CIE) O Level Physics): Revision Note

Speed-Time Graphs

• A speed-time graph shows how the speed of a moving object varies with time

  • The red line represents an object with increasing speed

  • The green line represents an object with decreasing speed

Increasing and decreasing speed

Increasing and decreasing speed represented on a speed-time graph

Acceleration on a Speed-Time Graph

• Speed-time graphs also show the following information:

  • If the object is moving with a constant acceleration or deceleration

  • The magnitude of the acceleration or deceleration

• A straight line represents constant acceleration

• The slope of the line represents the magnitude of acceleration

  • A steep slope means large acceleration (or deceleration) - i.e. the object's speed changes very quickly

  • A gentle slope means small acceleration (or deceleration) - i.e. the object's speed changes very gradually

  • A flat line means the acceleration is zero - i.e. the object is moving with a constant speed

Acceleration, deceleration and constant speed

This image shows how to interpret the slope of a speed-time graph

Using Speed-Time Graphs

• The distance travelled by an object can be found by determining the area beneath the graph

Area under a velocity-time graph

The distance travelled can be found from the area beneath the graph

• If the area beneath the graph forms a triangle (the object is accelerating or decelerating) then the area can be determined using the formula:

 $\mathrm{area}=\frac{1}{2}\times \mathrm{base}\times \mathrm{height}$

• If the area beneath the graph is a rectangle (constant velocity) then the area can be determined using the formula:

 $\mathrm{area}=\mathrm{base}\times \mathrm{height}$