Equilibrium - a question of balance
Physics Narrative for 5-11 11-14
You saw a situation where the hand was pulled down by a single force, the tension in the shopping bag. It is quite possible for an object to be acted upon by more than one force. A team of nine husky dogs pulling an Arctic sledge or a commuter being squeezed into a busy rush-hour train by a bunch of people pushing on all sides are examples where many forces act on one object. If these forces all push in the same direction the result will be a rapid change – perhaps the Arctic sledge will race away or the commuter will be shoved along the carriage.
However, it could be that all of the forces acting on the object balance each other out. The object is then said to be in equilibrium. All of the forces acting on an object which is in equilibrium add to zero. In other words, one force acting upwards is balanced by another force acting downwards; one force acting to the left is balanced by a similar force acting to the right, and so on. All the forces that are present add up to produce no overall force at all. Here's another way of saying this.
Teacher: Objects in equilibrium have no unbalanced forces acting on them. The resultant force is zero.
Most objects are in equilibrium
Look around you and notice how most things are doing nothing. The coffee cup just sits on the table, the picture hangs from the wall, the shirt lies on the chair. The cup, picture and shirt all have forces acting on them but the result for each one is a balanced or equilibrium state. There are no unbalanced forces acting on each one of these objects. The resultant force is zero.