Home » Handbook of O Level Physics » UNIT 3: FORCES


  • Force: It is a push or pull that one object exerts on another which produces, or tends to produce motion, stops or tends to stop motion.

    • Unit: N

    • Examples of forces are:

      weight; tension; magnetic force; electric force; contact force; friction; resistance;

  • Addition of vectors

    • Simply add / subtract if vectors are acting in a line.

    • Law of Parallelogram:

    • Head-to-tail Rule:

  • Newton’s Laws of Motion

    • First Law: Every object continues in its state of rest or uniform motion in a straight line unless a resultant force acts on it to change its state.

    • Second Law: Acceleration of a mass is directly proportional to the resultant force.

      F ∝ a

      F = m a

      Note: Acceleration has the same direction as the resultant force.

    • Third Law: To every action there is an equal and opposite reaction.

  • Balanced forces

    • When there is no resultant force

      • acceleration will be zero (2nd Law)

      • no change in state of motion (1st Law)

  • Unbalanced forces

    • When there is a resultant force

      • acceleration will NOT be zero (2nd Law)

      • change in state of motion(1st Law)

  • Friction: It is the force that opposes motion.

    • It has advantages and disadvantages.

    • Friction is affected by

      • lubrication / oiling

      • how hard the surfaces are rubbed together

      • nature of the surfaces in contact

    • Types

      • Static friction is the friction when a body is not moving.

      • Limiting friction is the (maximum static) friction when a body is just about to move.

      • Dynamic friction is the friction when a body is moving.

      Note: Dynamic friction is always less than limiting friction.

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