A boy is playing with a ball of mass 72g attached to a string. He is moving it at constant speed in a horizontal circle of radius 0.9m. a. State the net work done on the ball in one complete revolution. (1) b. The tension in the string is 60N. Determine the angular velocity of the ball. (4) c. Calculate the time it takes for the ball to complete 20 revolutions. (2)

Explanation:

radius of circle r = 0.9 m.

(a ) In a motion on circular path , work done is zero because force ( centripetal force ) acts perpendicular to displacement .

( b )

Tension in string T = m ω²r

Putting the values

60 = .072 x ω² x 0.9

ω² = 926

ω = 30.4 rad /s

angle made in 20 revolutions θ = 20 x 2π = 126.6 rad

time taken = θ / ω

= 126.6 / 30.4

= 4.16 s .

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