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 can be calculated using the formula:

$a = \frac{\text{Change in velocity}}{\text{Time}}$

Substituting the values we calculated:

$a = \frac{8 \text{ m/s}}{5 \text{ s}} = 1.6 \text{ m/s}^2$

During the time interval between 10 s and 15 s, the graph indicates that the car travels at a constant velocity of 8 m/s. With no change in velocity, the acceleration is 0 m/s². Therefore, using Newton's Second Law:

$F = m \times a$

If we assume the mass of the car is 1200 kg:

$F = 1200 \text{ kg} \times 0 = 0 \text{ N}$

Thus, the size of the resultant force is 0 N.