IGCSE Physics Forces & Motion Notes
Forces change how objects move. Learn the motion equations and Newton's three laws and you can explain and calculate any everyday motion.
Key concepts
- Force
- A push or pull measured in newtons (N). A resultant force changes an object's velocity.
- Resultant force
- The single force that has the same effect as all forces acting together. Zero resultant means constant velocity (or rest).
- Acceleration
- The rate of change of velocity, measured in m/s². Produced whenever there is a resultant force.
- Weight = mass × gravitational field strength (g ≈ 9.8 N/kg on Earth, often taken as 10).
- Balanced forces give zero acceleration; unbalanced forces cause acceleration.
- The gradient of a distance–time graph is speed; the gradient of a velocity–time graph is acceleration.
Key formulas
Newton's second law
F = m × a
resultant force (N) = mass (kg) × acceleration (m/s²).
Weight
W = m × g
g ≈ 9.8 N/kg on Earth.
Acceleration
a = (v − u) ÷ t
change in velocity ÷ time.
Required practicals
- Investigate the relationship between force, mass and acceleration using a trolley and light gates.
- Determine acceleration due to gravity by timing a falling object.
Worked example
A resultant force of 20 N acts on a 4 kg mass. Find the acceleration.
- 1Write the equation: F = m × a.
- 2Rearrange for a: a = F ÷ m.
- 3Substitute: a = 20 ÷ 4.
- 4Calculate: a = 5 m/s².
Answer: a = 5 m/s²
Common mistakes
✗ Confusing mass (kg) with weight (N).
✓ Mass is the amount of matter; weight is the force of gravity on it (W = mg).
✗ Using total force instead of resultant force in F = ma.
✓ Work out the resultant (net) force first by combining all forces.
✗ Forgetting units on the final answer.
✓ Always quote N, m/s² or kg — answers without units lose the final mark.
Quick check
Try these, then reveal the answers.
State Newton's second law as an equation.Reveal answer
F = m × a.
A 2 kg mass accelerates at 3 m/s². What resultant force acts on it?Reveal answer
F = 6 N
What does the gradient of a velocity–time graph represent?Reveal answer
Acceleration.
Exam tips
- Rearrange the equation before substituting numbers — it earns a method mark.
- Read graph axes carefully: distance–time vs velocity–time behave very differently.
- For 'explain' questions, refer to the resultant force and Newton's laws by name.
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Frequently asked questions
What is the difference between speed and velocity?+
Speed is how fast something moves (a scalar); velocity is speed in a stated direction (a vector).
What does a resultant force of zero mean?+
The forces are balanced, so the object stays at rest or continues at constant velocity.
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