Contact & non-contact

• Contact & Non-Contact Forces

  • A force is defined as:A push or a pull that acts on an object due to the interaction with another object
  • Forces can have an effect on bodies in several different ways:
  • They can change their speed
  • They can change their direction
  • They can change their shape
  • Some examples of these changes are:
  • An engine can increase the speed of a vehicle due to a force called thrust
  • A comet’s direction can be affected by gravitational attraction
  • A spring can have its shape changed by the force from a heavy load
• • Important forces to be familiar with:
  • Weight – force of gravity on a mass
  • Electrostatic force – force between two charged objects
  • Thrust – force causing an object to move (force from a rocket engine)
  • Air resistance (or drag) – friction of the air on a moving object
  • Upthrust – force of a fluid (e.g. water) pushing an object upwards
  • Compression – forces that act inward on an object
  • Tension – force transmitted through a cable or a string when pulled on by forces acting on its opposite ends
  • Reaction force (or the normal force)– a force due to contact between two objects
• FREE BODY DIAGRAMS:

  A) Air resistance

  B) Thrust

  C) Upthrust

  D) Weight

  E) Compression

  F) Tension

  
• • All forces can be categorised into one of two types:
  • Contact forces
  • Non-contact forces
• Contact Forces: A force which acts between objects that are physically touching

  • Friction
  • Air resistance
  • Tension
  • Reaction force / Normal force
  • Friction: opposes motion when objects rub against each
  • Air resistance: type of friction that occurs when an object moves through air
  • Tension: pulls two objects connected by a length that occurs when a force is applied to the length
  • Normal force: pushes touching objects apart that occurs when objects are supported by a surface
• Non-Contact Forces: A force which acts at a distance, without any contact between bodies, due to the action of a field

  • Gravitational force
  • Electrostatic force
  • Magnetic force
  • Gravitational attraction: two objects with mass
  • For example, the force between a planet and a comet
  • Electrostatic force: experienced by charged objects which can be attractive or repulsive
  • For example, the attraction between a proton and an electron
  • Magnetic force: experienced between magnetic poles that can be attractive or repulsive
  • For example, the attraction between the North and South poles of magnets