Answer:
In the last diagram the object is being acted upon by forces not adding up to zero.
Explanation:
(Assuming force 1=force 2 in terms of magnitude)
Let forces acting towards the left = -1
Let forces acting toward the right = +1
In the first diagram, force1 is acting towards the left and force2 is acting towards the right:
Total force = force1+force2=-1+1=0
In the second diagram, force1 is acting towards the left and force2 is acting towards the right:
Total force = force1+force2=-1+1=0
In the third diagram, force1 is acting towards the right and force2 is acting towards the left:
Total force = force1+force2=+1-1=0
In the fourth diagram, force1 is acting towards the left and force2 is acting towards the left:
Total force = force1+force2=-1-1=-2. Here, the forces do not add up to zero.