Force changes motion but does not set motion
Teaching Guidance for 14-16
Forces on objects moving at constant speeds
Wrong Track: It's just stood there: no forces are acting on it by the environment.
Wrong Track: The speed is steady: no forces are acting.
Right Lines: Objects that appear to be affected by no force might have several forces acting on them which all add to zero.
Talking about forces acting on objects in equilibrium
Thinking about the learning
Many situations used to discuss forces, particularly when first introducing the action of adding force arrows acting on objects to replace the interactions with their environment, are of objects in equilibrium. Very often these objects are stationary.
Such forces are often said to cancel out, reinforcing the idea that the forces were never really there in the first place.
Thinking about the teaching
We'd suggest explicitly going through the process of:
- Identifying and isolating the object.
- Noticing interactions of the object with its surroundings and replacing these with forces acting on the object, exerted by its environment.
- Drawing a diagram showing all these forces.
- Adding these forces as vectors, tip to tail, to show the resultant force.
- If the resultant is zero, saying so, and adding the explanatory phrase:
because all the forces add to zero.
It's particularly important to keep forces and velocities separate. The resultant force changes the velocity. Even though both are represented by arrows (because they're both best represented using vectors), they're very different things. Here we suggest that you show only force arrows on the diagrams. There's much more on representing different quantities with arrows throughout this topic.