The video shows how to calculate weight if the mass is given and vice versa.Later on in this unit you will learn about Newton's laws of motion and Newton's second law of motion gives us a way to change mass into weight and vice versa.This law will tell you what is happening to objects when the net force on the object is not equal to zero and the easy way to say what will happen is that the object will accelerate.But how much the object accelerates depends on two things,firstly the mass of the object(how heavy the object is) and secondly force(how much you push or pull it).From this simple idea we can say
where a is acceleration,F is force and m is the mass of the object.
Or in other words acceleration is directly proportional to the force in which you push or pull and inversely proportional to the mass.So the larger the object mass the smaller the acceleration.
If force is the force of gravity then its weight.So we get equation relating weight,acceleration and mass.Therefore
weight=mass X acceleration of planet you are on.
On earth,acceleration is 9.81 in most books without units.Approximating it we get acceleration is 10.So if you know mass in kilograms,ten times the mass gives you the weight.
If my mass is 70kg then my weight will be 700N.If mass of book is 0.5kg then its weight is 5N.If mass of pencil is 100th of a kilogram then its weight is 0.1N.Its easy!