Quantifying forces by looking at motion

Thinking about the teaching

In the SPT: Forces topic we suggested that you look at the physical interactions of the objects with the surroundings as a way of identifying the forces acting. Once identified, you can begin to think about the relative values of the forces. Ascertain the type of motion and from that deduce the relative value of the forces acting:

If the object is speeding up then…

… the driving force(s) is/are greater than the retarding force(s).

If the object is slowing down then…

… the driving force(s) is /are less than the retarding force(s).

If the object has a constant speed then…

… the driving force(s) is/are equal to the retarding force(s).

Or, in terms of resultant forces:

Object speeding up  →  resultant force is not zero (driving force is greater than retarding force)

Object slowing down  →  resultant force is not zero (driving force is less than retarding force)

Object at constant speed  →  resultant force is zero (driving force is equal to retarding force)

Teacher Tip: Don't use these relationships to identify the forces that are acting. Instead use them to begin to quantify those forces.