Sun January 15, 2012 By:

sir i wanna ask you the procedure of the activity given in the CBSE textbook on pag 223 for finding the formula of surface area of sphere ?

Expert Reply
Mon January 16, 2012
In this activity:The procedure is as follows:
(1)Take a Rubber ball and drive a nail into it.
(2)Taking support from the nail ,start winding string/thread around the ball.
(3)Use pins to keep the string in place on the ball and continue winding the string ,till you have completely covered the ball.
(4)Mark the starting and end points on the string and slowly unwind the string from the surface of the ball.
(5)Measure the diameter of the ball and get the radius.
(6)On a sheet of paper.draw 4 circles with radius equal to the radius of the ball.
(7)Then fill each of the circles with the string,one by one.
From,this activity ,we observe that the string that covered the surface area of the ball ,is used completely to fill the 4 circles ,all of the same radius.
This,leads us to the conclusion that the curved surface area of a sphere=4  times the area of a circle with same radius.
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