Force and Friction Notes

11.2 Force

  • Force is a push or a pull on an object.
  • Forces can be represented by arrows.
    • The arrow starts where the force acts and points in the direction of the force.
    • The length of the arrow indicates the magnitude of the force; a longer arrow means a larger force.
  • Examples of forces:
    • The force to pull a paper towel.
    • The force to push a box on a table.

Effects of Force on Motion

The effects of force on the motion of objects are:

  • Changing the speeds of objects:
    • Causing moving objects to move faster or slower.
    • Causing objects at rest to start moving.
    • Causing moving objects to stop.
  • Changing the moving direction of objects.

Measuring Force

  • Force can be measured using a spring balance or a force sensor connected to a data-logger.
  • The unit of force is newton (N).

Types of Forces

  • Contact forces:
    • Act on objects only when the objects are in contact.
    • Example: pulling force.
  • Non-contact forces:
    • Act on objects without any contact between them; they can act at a distance.
    • Example: magnetic force.

Examples of Contact and Non-Contact Forces

  • Contact:
    • The force to pull a dog.
  • Non-contact:
    • Magnetic forces (bookmark magnets).

Free-Body Diagrams

  • A free-body diagram shows all the forces acting on an object.
  • Example: A man holding a paper bag.
    • Forces acting on the bag: force applied by the person (upward) and force of gravity (downward).

Balanced and Unbalanced Forces

  • Balanced forces:
    • When balanced forces act on an object, the object will stay at rest or in uniform motion.
  • Unbalanced forces:
    • When unbalanced forces act on an object, the motion of the object will change.

11.4 Friction and Air Resistance

Friction

  • Friction acts on an object and opposes the object's sliding motion.
  • It is a contact force.

Air Resistance

  • Air resistance opposes the motion of an object moving in air.
  • The higher the speed, the larger the air resistance.
  • Example: air resistance opposing the motion of a bicycle rider in air.

Methods of Reducing Friction or Air Resistance

  • Using lubricants.
  • Separating surfaces using an air cushion.
  • Using rolling objects.
  • Streamlining the shapes of objects.

Usefulness of Friction and Air Resistance

  • Walking.
  • Holding objects in position.
  • Preventing slipping.
  • Slowing down moving objects.