Forces in equilibrium

If two tug-of-war teams pulling on a rope are well matched, there may for a while be no movement, just a lot of shouting and puffing! Both teams are exerting the same amount of force on the two ends of the rope. The forces are therefore in equilibrium and there is no change of momentum. There are,

however, other more common occurrences of forces in equilibrium. If you push down on an object at rest on a table, the table will resist the force with an equal and opposite force of reaction, so the forces are in equilibrium. Of course, if you press too hard, the table might break, in which case the forces will no longer be in equilibrium, and a sudden and unwanted acceleration will occur.

As another example, consider a glider being towed behind a small aircraft as in Fig. 1.1. If the aircraft and glider are flying straight and level at constant speed, then the pulling force exerted by the aircraft on the tow-rope must be exactly balanced by an equal and opposite aerodynamic resistance or drag force acting on the glider. The forces are in equilibrium.

Some people find it hard to believe that these forces really are exactly equal. Surely, they say, the aircraft must be pulling forward just a bit harder than the glider is pulling backwards; otherwise, what makes them go forward? Well, what makes them go forward is the fact that they are going forward, and the law says that they will continue to do so unless there is something to alter that state of affairs. If the forces are balanced then there is nothing to alter that state of equilibrium, and the aircraft and glider will keep moving at a constant speed.