Density is the mass per unit volume . Common units for density are g/cm3 or kg/m3 . SI unit is kg/m3 . Density of a pure substance is fixed, regardless of it’s size! Floating and Sinking A denser (heavier) substance will sink in liquid which is less denser (lighter). A less dense (lighter) substance will float in a liquid which is denser . If substance has the same density as the liquid, it will be suspended in the liquid.

For Combined Science Physics Olevel, when constructing vector diagram, a method called  Parallelogram Method is commonly used. Parallelogram Method is used when 2 forces are known , and the resultant (net) force R , can be found. Below show 5 different variations of questions when can be solved by using Parallelogram Method: (A) Body has acceleration , forces are unbalanced , there is a resultant force R (Newton’s 2nd law) (B) Body at rest , all forces are balanced , there is no resultant force R (Newton’s 1st law) (C) Body moving at constant velocity , all forces are balanced , there is n o resultant force (Newton’s 1st law) (D) Finding unknown force (with  resultant force R  + another known force are given ) (E) Vector diagram is applicable to all vector quantities (with magnitude and direction). Not just for forces.

Work done W = F x d Only when there is distance moved in the direction of the force , there is work done. (i.e. look for the distance parallel to the force)