I emphasize these chains of reasoning involving kinematics, forces, and work after students learned them whenever possible. I believe students will benefit if they apply them to problems.
Here is one example: Objects at rest / moving at constant speed:
- Acceleration is zero
- Net force is zero
- The forces acting on the object are balanced /
Sum of forces pointing right = sum of forces pointing left /
Sum of forces pointing up = sum of forces pointing down - Kinetic energy remains constant
- Net work done is zero.
This chain of reasoning is very powerful for objects moving in 1D. It helps me to check the consistency in my answers and solve problems easily.
Let us expand this chain of reasoning to objects which are speeding up and slowing down.
Objects speeding up while moving to the right:
- Acceleration is pointing right (same direction as the velocity)
- Net force is pointing right (same direction as the acceleration)
- Sum of forces pointing right > sum of forces pointing left (in magnitude)
- Kinetic energy increases
- Net work done is positive
Objects slowing down while moving to the right:
- Acceleration is pointing left (in opposite direction to the velocity)
- Net force is pointing left (same direction as the acceleration)
- Sum of forces pointing right < sum of forces pointing left (in magnitude)
- Kinetic energy decreases
- Net work done is negative
In cases when the object is speeding up and slowing down, there are different situations to look at, such as speeding up while moving down or moving up, slowing down while moving down or moving up, and so on. The directions of acceleration and net force can still be determined accordingly.
Every line is in accordance with the definition of the physical quantity. For example, acceleration is defined as change in velocity / change in time, net force is mass times acceleration, and so on.
Doing problems while applying these chains of reasoning will constantly remind students of the consistency of physics principles they encountered in learning mechanics.
I strongly recommend physics teachers teach students these chains of reasoning.
Chang Physics Class 