



 
James, The volume of a cylinder is given by π r^{2} h cubic units where r is the radius, h is the height and π = 3.1416. Suppose R is the radius of the roll, r is the radius of the core and h is the height of the roll. For your example R = 76/2 = 38, r = 9/2 = 4.5 and h = 7.5, all in centimeters. The volume of film on the roll is then
Suppose the mass is W units (for your example W = 7.13 kilos). The density of the film is then
For your example this is
Suppose that, after a use, the radius of the film remaining on the roll is s units. The volume of film remaining on the roll is then
Since the density is [W/(original volume)] mass units/(cubic length units) the mass of the reduced roll will be
For your example that would be kilos. Harley  


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