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 (2^nd Law)

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

• Unbalanced forces

  • When there is a resultant force

    • acceleration will NOT be zero (2^nd Law)

    • change in state of motion(1^st 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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