(3-7)

①  work  =  force x distance travelled  =  2 x (40 – 10)  =  2N x 30m  =  60 J

           This is the area under the line on the graph, between these two limits.

②  Since the dependence of force on distance is linear, the mean force applied over the range between 10m and 40 m distance is just the force at the midpoint of this range 

= 2.5 N

           So work = force x distance =  2.5 x 30  = 75 J.   You can easily verify that this is again equal to the area under the line in this range.

③   we just add up the areas of each slice:   (10 x 3.5) + (10 x 2.5) + (10 x 1.5)  =  75 J.