Assumptions

• Particle

  Dimensions of the object are negligible
• Particle Assumptions

  • Mass of the object is concentrated at a single point
  • Rotational forces and air resistance can be ignored
• Smooth Surface Assumption

  Assume there is no friction between the surface and any object on it
• Uniform body


  Mass is distributed evenly
• Uniform body assumptions

  Mass of the object is concentrated at a single point at the geometrical centre of the body (centre of mass)
• Light Object

  Mass of the object is small compared to other masses like a string or pulley
• Light Object Assumptions

  • Treat object as having zero mass
  • Tension is the same at both ends of a light string
• Inextensible string


  A string that does not stretch under load
• Inextensible string assumptions


  Acceleration is the same in objects connected by a taut inextensible string
• Rough Surface


  If a surface is not smooth it is rough
• Rough Surface Assumptions

  • Objects in contact with the surface experience a frictional force if they are moving or acted on by a force
• Smooth and Light Pulley

  All pulleys will be smooth and light
• Pulley Assumptions

  • Pulley has no mass
  • Tension is the same on either side of the pulley
• Air resistance


  Resistance experienced as an object moves through air
• Air resistance

  Usually modelled as being negligible
• Gravity

  Force of attraction between all objects. Acceleration due to gravity is denoted by g (9.8 ms-2)
• Gravity assumptions

  • Assume all objects with mass are attracted towards the earth
  • Earth's gravity is uniform and acts vertically downwards
  • g constant and is taken as 9.8 ms-2 unless otherwise stated in the questions
• Weight

  Acts vertically downwards
• Normal Reaction


  Force which acts perpendicular to a surface when an object is in contact with the surface
• Friction

  Force which opposes the motion between two rough surfaces
• Tension


  If an object is being pulled along by a string, the force acting on the object is tension
• Thrust


  If an object is being pushed along using a light rod the force acting on the object is called the thrust
• Buoyancy


  The upward force on a body that allows it to float or rise when submerged in a liquid when submerged in a liquid.