Answer
35.3 Newtons
Work Step by Step
To be on the verge of moving means that the applied force is equal to Ff,max. To find Ff,max or the maximum frictional force we need to find the normal force, N. To do this we balance the forces in the y direction and see that there is only the weight of the block downwards and the normal force upwards. The two must balance since the box is not moving in the y direction so N = mg = 8kg*9.8 = 78.4 Newtons. Then, this means that Ff, max = 78.4N * 0.450 (coefficient of friction given) = 35.3 Newtons. If the force is directed horizontally then it needs to exceed 35.3 Newtons.
