Force diagrams like the one shown depict the direction and strength of forces by vector arrows. Longer arrows mean stronger forces. One can determine if the forces are balanced by looking at the arrow size. Equal length arrows in opposite directions are required for forces to be considered balanced. Such situations are consistent with constant speed.
 

In these situations there is only one horizontal force. For horizontal forces to balance, every rightward force must be countered by an equal-strength leftward force. This is not the case in this situation. Thus the forces are unbalanced and an acceleration will occur.


Accelerations can involve either speeding up or slowing down motions. To determine which is occurring in this situation, first determine which way the objet is moving (read the first sentence). An object moving to the right will speed up if there is more force to the right than to the left. Objects moving to the right will slow down if there is more force to the left than to the right. Similar reasoning can be applied to leftward-moving objects.