On a distance time graph, a straight line indicates that an object is moving at a constant speed. 

A non-moving stationary item can alternatively be thought of as a body travelling at a constant speed. 

Remember that this graph does not contain a downward-sloping line because every moving object "increases" its length with time. 

It does, however, if an object returns to its original location. 

On the distance time graph, different lines represent different speeds and motions. 

As a result, in addition to understanding what a straight line entails, students must also be aware of different deviations in order to grasp the ideas.