Models are required when explaining a theory so to create a representation you can mathematically work out
You need to become acustomed to the models below

Particle - an object. To make things simple a particle is considered to be located purely at one point and not over a large distance like would occur in reality
Rod - a long object that has one noticable length
Lamina - a 2D object like a piece of paper
Uniform Body - An object where the mass in evenly distributed
Light Object - An object with negligible mass in comparison to other objects around it
Inextensible String - String that is inelastic and therefore won't stretch
Smooth surface - A surface with negligible friction
Rough surface - A surface with friction
Wire - Rigid thin length of metal. Can be smooth or rough
Smooth and Light pulley - The pulley is negligible in calculations as having an effect on an action as it has negligible mass and negligible friction
Bead - A particle which can be threaded
Peg - A support or point of rest for an object
Air resistance - Friction due to air
Gravity - The force on objects . Their acceleration downwards. It is 9.8 m/s^2