Knowledge Organiser: Newton's Laws of Motion
Key Terms
- Inertia: Resistance to change in motion
- Resultant force: Net combined force
- Third Law pair: Equal, opposite forces on different objects
- Acceleration: Rate of change of velocity (m/s²)
The Three Laws
- 1st: No resultant force → constant velocity
- 2nd: Resultant force causes acceleration (F = ma)
- 3rd: Equal and opposite forces on different objects
Key Equations
- F = ma
- a = F ÷ m
- m = F ÷ a
- Units: F in N, m in kg, a in m/s²
Exam Tips
- F in F = ma is RESULTANT force
- Third Law pair: name both forces and both objects
- Weight ≠ normal force (not a Third Law pair)
- Constant speed = balanced forces (not zero forces)
Common Mistakes
- Using total force instead of resultant force in F = ma: The F in F = ma is the resultant (net) force — always subtract opposing forces before calculating acceleration
- Describing Newton's Third Law pairs incorrectly: The two forces must act on different objects, be equal in magnitude, opposite in direction, and of the same type — "the ground pushes back on you" not "your weight and the normal force"
- Confusing weight and normal force as a Third Law pair: Weight (gravity pulling you down) and normal force (ground pushing you up) act on the same object — they are NOT a Newton's Third Law pair
- Saying constant speed means no forces: Constant speed means zero resultant force (forces are balanced) — individual forces are still acting
- Forgetting units: Force in Newtons (N), mass in kg, acceleration in m/s² — using grams or cm/s² gives wrong answers
Memory Aid
- The Three Laws as a Story:
- Law 1 — "Stay Lazy": Objects are lazy — they stay as they are (rest or constant motion) unless a force makes them change. Inertia is physics laziness!
- Law 2 — "F = ma": Force equals Mass times Acceleration. Remember the triangle: F on top, m and a on the bottom. Cover the one you want to find.
- Law 3 — "Bouncer Rule": Every force gets a matching force back, on the OTHER object. Like a bouncer pushing you out — you push back on the bouncer with equal force, but on a DIFFERENT object.
- Third Law pairs checklist — DOSE: Different objects, Opposite directions, Same magnitude, Equal type of force.