The length of a taut belt around the two pulleys is 79.3 cm.
Length around the pulley
The length of a taut belt around the two pulleys is calculated as follows;
Shapes formed within the two circles of the pulley.
From top to bottom, a rectangle, a right triangle and a trapezium.
Length of the rectangle
The height of the right triangle is equal to length of the rectangle
base of the right triangle = radius of big circle - radius of small circle
base of the right triangle = (0.5 x 12 cm) - (0.5 x 6 cm) = 3 cm
tan α = height/base
tan (72.54) = h/3
h = 3 tan(72.54)
h = 9.54 cm
Length of trapezium at bottom
The length of the trapezium at bottom is equal to length of rectangle at top, L = h = 9.54 cm
Angles and length of belt in each circle
Portion of belt in contact with circumference of small circle is subtended by an angle = 2 × 72.54 = 145.08°
Length of belt in contact with circumference of smaller circle
= 2πr (θ/360)
= (2 x 6 cm)π x (145.08/360)
= 15.19 cm
Portion of belt in contact with circumference of big circle is subtended by an angle = 360 - 145.08° = 214.92⁰
Length of belt in contact with circumference of smaller circle
= 2πr (θ/360)
= (2 x 12 cm)π x (214.92 / 360)
= 45.01 cm
Length of a taut belt around the two pulleys
= 9.54 cm + 9.54 cm + 15.19 cm + 45.01 cm
= 79.28 cm
= 79.3 cm
Thus, the length of a taut belt around the two pulleys is 79.3 cm.
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