Estimate Gradients of Graphs

Estimate Gradients of Graphs


With a curved graph the gradient changes continuously. To estimate the gradient of a curved line, draw the graph accurately. Then draw a tangent to the graph at the point at which the gradient is required.


1. A train moves away from a station with an acceleration of 0.2 ms-2. By drawing a graph of time against distance, estimate the speed of the train after 5 seconds.

The distance travelled by the train is given by `s = frac(1)(2)at^2`, where s is distance, t is time and a is acceleration.

Answer: 1 m/s

The velocity of the train is given by the gradient of the graph at the point in time in question.

Take the tangent to the graph at x = 5. Extend the line to measure a reasonable gradient. Gradient measures as `frac(5-0)(7.5-2.5)` = `frac(5)(5)` = 1.


acceleration of a train

2. Using the graph, above, indicate whether the speed of the train is faster at 2 seconds or at 8 seconds.

Answer: Faster at 8 seconds

The gradient of the curve is steeper at 8 seconds compared to the gradient at 2 seconds: the train is therefore moving more quickly.