This topic summary covers Knowledge Organiser: Newton's Laws of Motion within Newton's Laws of Motion for GCSE Physics. Revise Newton's Laws of Motion in Forces for GCSE Physics with 24 exam-style questions and 15 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 15 of 15 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
Practice questions for Newton's Laws of Motion
According to Newton's First Law, what happens to an object when there is no resultant force acting on it?
A spaceship is travelling through deep space far from any planets. The engines are switched off. Explain what will happen to the motion of the spaceship and why.