Force equals motion: no motion equals no force
Teaching Guidance
for 5-11
11-14
Forces on stationary objects
Wrong Track: It's just stood there. There are no forces acting.
Right Lines: Stationary objects that appear to be affected by no force might have several forces acting on them which all add to zero.
Forces in equilibrium
Thinking about the learning
Early introductions to forces in equilibrium will be via static situations. However, rather than identifying forces which sum to zero, pupils often describe such static situations as having no forces. This is an understandable conclusion and one that is almost correct. In equilibrium there is no resultant force.
Thinking about the teaching
You can subvert this natural tendency by identifying the forces acting from looking at the physical situation – an approach that is built into this SPT: Forces topic. We'd suggest not using equilibrium as a means of identifying forces. However it may be useful, in tricky situations, as a backup, and as a means of nudging individuals towards looking again at the situation because you believe that there is another force yet to be identified.
