Why do we multiply mass by acceleration due to gravity to get the weight of the object?
Gravity is the force by which one mass attracts another, and vice-versa. Even though an object is at rest and motionless, there is a gravitational force acting upon it. Without that gravitational force, the object would essentially start floating around. Now, to calculate the force of the object on the earth, and the force imparted by the earth on that object, take the equation F = (G me m2) / (r^2) where G is the universal gravitational constant, me is the mass of the earth, m2 is the mass of your object, and r^2 is radius of the earth, squared. For the earth, G me / (r^2) = 9.8 m/(s^2). So, F = m a. Similarly, you can consider Newton’s law of motion where the sum of all forces on an object will equal its mass * its acceleration. F = m a. In case you still don’t see why there’s acceleration involved, consider that the earth is rotating at high speeds. At any instantaneous point in time, the velocity vector of an object on the earth will point in the direction tangential to the earth’s
