Here is the velocity-time graph for a car for the first 20 s of a journey - Edexcel - GCSE Physics - Question 5 - 2013 - Paper 1

Acceleration is defined as the change in velocity over time. Using the change in velocity found in part (i) and the time interval of 5 seconds:

$\text{Acceleration} = \frac{\text{Change in velocity}}{\text{Time}} = \frac{8 \text{ m/s}}{5 \text{ s}} = 1.6 \text{ m/s}^2$

From the graph, we see that between 10 s and 15 s, the velocity remains constant at 12 m/s. Thus, the acceleration is 0 m/s², which implies that the resultant force acting on the car is also 0 N due to:

$F = m \cdot a; \text{ where } a = 0 \text{ m/s}^2$

So the resultant force is:

$\text{Resultant force} = 0 \text{ N}$