Forces

• Particle

  1)Mass of the object concentrated at a single point

  2) Rotational forces and air resistance can be ignored
• Rod

  1)Mass is concentrated across a line

  2)No thickness

  3) Rigid(does not bend or buckl)
• Lamina

  1)Mass is distributed across a flat surface
• Uniform body


  Mass of the object is concentrated at the centre of mass
• Light object

  1)Treat object as having zero mass

  2) Tension the same at both sides of the light string
• Inextensible string


  1)Acceleration is the same in objects connected by a taut inextensible string
• Smooth surface

  No friction between surface and an object on it
• Rough surface

  Objects in contact with the surface experience friction if they are moving or acted on by a force