1. Newton’s Third Law

2. Newton’s Laws of Motion
Newton’s first law: NL1- A body will remain at rest or will continue to move with a constant velocity unless external forces cause it to do otherwise.
Newton’s second law: NL2- The acceleration produced by a resultant applied force is proportional to the mass of the body on which it acts.
Newton’s third law: NL3- Whenever two bodies interact, they exert forces on each other which are equal in magnitude and opposite in direction.

3. Newton’s Laws of Motion
Newton’s first law: NL1- Resultant force causes change in speed or direction of motion.
Newton’s second law: NL2- F = ma
Newton’s third law: NL3- Equal and opposite forces.

4. Equilibrium
Students confuse equilibrium from NL1 with pairs of forces from NL3.
Equilibrium occurs when the resultant force on a body is zero.
Sometimes (but not always) this is due to a pair of equal and opposite forces acting on the same body, e.g. weight and normal reaction force.
Pairs of forces from NL3 do not act on the same body

5. Mechanics in Action Chapter Four: Misconceptions, p.44 (p.50)
Chapter Five: Practical Investigations
Bathroom Scales and a Broom, p.61 (p.67)
Worksheets, after p. 166 (p. 173)