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Cheat Sheet: Forces, Motion & Turning Effects (O Level / IGCSE Physics)

1.5.1 Balanced and Unbalanced Forces

1. Types of Forces

• Weight (gravitational force) – force pulling objects toward Earth
• Friction – opposes motion between surfaces
• Drag / Air resistance – fluid friction (air or liquid)
• Tension – force in stretched strings/ropes
• Electrostatic force – between charged objects
• Magnetic force – between magnets or magnetic materials
• Thrust – driving force (e.g. engine force)
• Contact force – force from touching surfaces

2. Free-Body Diagrams

• Show all forces acting on a single object using arrows
• Arrow length = magnitude; arrow direction = direction of force

3. Newton’s First Law

• An object remains at rest or in uniform motion unless a resultant force acts on it

4. Effect of Force

• A force can change:
  • Speed
  • Direction
  • Or both → causes acceleration

5. Resultant Force (same line)

• If forces are in same direction: add
• If in opposite directions: subtract
  Resultant = larger force − smaller force
  (use + for forward, − for backward)

6. Newton’s Second Law

• Equation: F = m × a
  F = resultant force (N)
  m = mass (kg)
  a = acceleration (m/s²)

7. Newton’s Third Law

• If A exerts a force on B, then B exerts an equal and opposite force on A
• Forces are:
  • Same type
  • Equal in size
  • Opposite in direction
  • Act on different objects

1.5.2 Friction

1. Definition

• Friction opposes motion and produces heat

2. Motion with and without Drag

• Without drag: object accelerates due to net force
• With drag: motion is resisted → slower acceleration or constant speed

3. Terminal Velocity

• At start: weight > drag → accelerates
• As speed increases: drag increases
• Terminal velocity = weight = drag → no acceleration

4. Stopping Distances

• Thinking distance: distance travelled during driver’s reaction time
• Braking distance: distance travelled while brakes are applied
• Stopping distance = thinking + braking distance

5. Factors Affecting Stopping Distance

• Thinking distance:
  • Speed
  • Tiredness
  • Alcohol / drugs
• Braking distance:
  • Speed
  • Load
  • Tyre condition
  • Road surface (wet, icy, etc.)

1.5.3 Elastic Deformation

1. Effect of Forces

• Can change shape or size of objects (stretch, squash, twist)

2. Spring Constant

• Equation:
  k = F / x
  k = spring constant (N/m)
  F = force applied (N)
  x = extension (m)

3. Load–Extension Graph

• Procedure:
  • Hang spring, attach weights incrementally
  • Measure extension each time
  • Plot load (force) vs extension
  • Straight line = obeys Hooke’s Law
  • Curve = limit of proportionality exceeded

4. Limit of Proportionality

• Point beyond which extension is no longer proportional to force
• Identified where graph deviates from straight line

1.5.4 Circular Motion

• Object moving in circle experiences centripetal force (perpendicular to motion)

(a) If force increases, with constant mass & radius → speed increases

(b) If force increases, with constant mass & speed → radius decreases

(c) If mass increases, with constant speed & radius → force must increase

1.5.5 Turning Effect of Forces (Moments)

1. Moment

• Turning effect of a force around a pivot
  Examples: doors, levers, spanners

2. Moment Equation

• Moment = force × perpendicular distance from pivot
  M = F × d
  M = moment (Nm), F = force (N), d = distance (m)

3. Principle of Moments

• For an object in equilibrium:
  Clockwise moments = Anticlockwise moments

4. Experimental Verification

• Use a meter rule balanced on a pivot
• Hang known weights at measured distances
• Adjust until balanced
• Check: clockwise moment = anticlockwise moment

1.5.6 Centre of Gravity

1. Definition

• The point where the whole weight of the object acts vertically downward

2. Finding Centre of Gravity (Plumb Line Method)

• Suspend a lamina (flat shape) from a point
• Hang plumb line and draw vertical line
• Repeat from another point
• Intersection of lines = centre of gravity

3. Effect on Stability

• Lower centre of gravity = more stable
• Wider base = more stability
• Objects with high centre of gravity tip over more easily when tilted