Distance-Time Graphs (Edexcel IGCSE Physics)

The Edexcel IGCSE Physics specification states that you need to be able to explain distance-time graphs and to be able to plot them. 

Plotting graphs is an important skill in physics and usually comes up in at least one exam paper. If you are not confident in plotting graphs, you should definitely practice before your exam! 

A distance-time graph is drawn below for part of a train journey. The train is travelling at a constant speed.

Calculate the speed of the train.

Answer:

Step 1: Draw a large gradient triangle on the graph 

• The image below shows a large gradient triangle drawn with dashed lines

• and are labelled, using the units as stated on each axes

Step 2: Convert units for distance and time into standard units

• The distance travelled  = 8 km = 8000 m

• The time taken  = 6 mins = 360 s

Step 3: State that speed is equal to the gradient of a distance-time graph

• The gradient of a distance-time graph is equal to the speed of a moving object:

Step 4: Substitute values in to calculate the speed

A student decides to take a stroll to the park. They find a bench in a quiet spot and take a seat, picking up where they left off reading a book on Black Holes. After some time reading, the student realises they lost track of time and runs home.

A distance-time graph for for the student's trip is drawn below:

a) How long does the student spend reading the book?

b) Which section of the graph represents the student running home?

c) What is the total distance travelled by the student?

Answer:

Part (a)

• The student spends 40 minutes reading the book

• The flat section of the line (section B) represents an object which is stationary - so section B represents the student sitting on the bench reading

• This section lasts for 40 minutes - as shown in the graph below

Part (b)

• Section C represents the student running home

• The slope of the line in section C is steeper than the slope in section A

• This means the student was moving with a larger speed (running) in section C

Part (c)

• The total distance travelled by the student is 0.6 km

• The total distance travelled by an object is given by the final point on the line - in this case, the line ends at 0.6 km on the distance axis. This is shown in the image below:

Use the entire line, where possible, to calculate the gradient. Examiners tend to award credit if they see a large gradient triangle used - so remember to draw these directly on the graph itself!

Remember to check the units of variables measured on each axis. These may not always be in standard units - in our example, the unit of distance was km and the unit of time was minutes. Double-check which units to use in your answer.