Knowledge Organiser: Forces and Their Effects
Key Terms
- Force: Push or pull — vector, measured in N
- Mass: Amount of matter — scalar, kg, constant
- Weight: Gravitational force — vector, N, varies
- Resultant force: Net single equivalent force
- Contact force: Requires physical touch
- Non-contact force: Acts at a distance
Key Facts
- Contact: friction, tension, normal force, air resistance, upthrust
- Non-contact: gravity, magnetic, electrostatic
- Zero resultant = constant velocity (or stationary)
- Non-zero resultant = acceleration
- g = 10 N/kg on Earth, 1.6 N/kg on Moon
Key Equations
- W = mg (Weight = mass × g)
- Forces same direction: add magnitudes
- Forces opposite direction: subtract magnitudes
Exam Tips
- Weight always in Newtons, never kg
- Free body diagram: one object, all forces on it
- Balanced ≠ no forces — just zero resultant
- State contact/non-contact type in every answer
Common Mistakes
- Giving weight in kg instead of Newtons: Weight is a force measured in Newtons — mass is in kg; W = m × g converts between them
- Saying balanced forces means no forces acting: Balanced forces means the resultant is zero — forces are still present and acting, they just cancel out
- Drawing forces from the wrong point: In free body diagrams, all forces must be drawn as arrows from the centre of the object (or its surface), not floating nearby
- Confusing mass and weight: Mass (kg) is the amount of matter; weight (N) is the gravitational force on that mass — weight changes on different planets, mass does not
- Forgetting to include all forces: Always check for weight (downward), normal/contact force, friction, and any applied forces — missing one force means the resultant is wrong
Memory Aid
- Contact vs Non-contact: "FANT vs GME"
- Contact forces: Friction, Air resistance, Normal contact, Tension (FANT)
- Non-contact forces: Gravity, Magnetic, Electrostatic (GME)
- For Weight vs Mass: "Mass is Mine, Weight is Where" — your mass is the same whether you're in London, on the Moon, or on Mars. Weight depends on WHERE you are (the local gravitational field).