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A solid metallic sphere of radius 12┬аcm is melted and recast in the form of small spheres of radius 2┬аcm. How many small spheres are formed?
96
216
48
864
216
Since the volume is proportional to the cube of the radius (VтИЭr3), the number of small spheres formed is simply the cube of the ratio of their radii: (rRтАЛ)3=(212тАЛ)3=63=216.
Radius of the solid metallic sphere R=12┬аcm, radius of each small sphere r=2┬аcm.
Number┬аof┬аsmall┬аspheres=Volume┬аof┬аone┬аsmall┬аsphereVolume┬аof┬аlarge┬аsphereтАЛ=r3R3тАЛ
Since the volume is proportional to the cube of the radius (VтИЭr3), the number of small spheres formed is simply the cube of the ratio of their radii: (rRтАЛ)3=(212тАЛ)3=63=216.
Students often mistakenly divide the radii linearly (12/2=6) or use the surface area ratio instead of the volume ratio.
Identify given parameters
The radius of the large solid metallic sphere is R=12┬аcm and the radius of each small sphere is r=2┬аcm.
R=12┬аcm,r=2┬аcm
Set up volume conservation equation
When a solid sphere is melted and recast, the total volume remains constant. Thus, the volume of the large sphere equals n times the volume of one small sphere: VlargeтАЛ=n├ЧVsmallтАЛ.
34тАЛ╧АR3=n├Ч34тАЛ╧Аr3
Simplify and solve for n
Cancelling out 34тАЛ╧А from both sides, we get n=(rRтАЛ)3. Substituting the values: n=(212тАЛ)3=63=216.
n=63=216
B is correct because the ratio of the volumes gives n=(12/2)3=63=216 small spheres formed.
This volume conservation principle also applies when melting solid cylinders into cones or spheres, where VinitialтАЛ=тИСVfinalтАЛ.