Question

https://isaacphysics.org/questions/gcse_ch2_14_q6?board=eae16c47-ddad-49d3-9dd4-4e2af0506bd3

https://isaacphysics.org/questions/gcse_ch2_13_q13?board=92398d11-6559-45dd-a194-62c9e4f264e3

Answer 1

1) According to Newton's second law:

$$\Sigma F=ma,$$

i.e. the resulting force (vector sum of all forces) equals mass times acceleration. For the given problem the driving force accelerates the lorry while the drag force decelerates the lorry. Hence:

$$a=\frac{\Sigma F}{m}=\frac{F_{driving}-F_{drag}}{m}=2.4\text{ m/s}^2.$$

2) This problem can solves the same way. Our goal is to find the driving force from the last equation in problem 1:

$$F_{driving}=ma+F_{friction}=3250 \text{ N}.$$