A plastic beach ball has radius 20.1 cm and mass 0.11 kg, not including the air

Question

A plastic beach ball has radius 20.1 cm and mass 0.11 kg, not including the air inside.
(a) What is the weight of the beach ball including the air inside? Assume the air density is 2.3 kg/m3 both inside and outside.


(b) What is the buoyant force on the beach ball in air? The thickness of the plastic is about 2 mm—negligible compared to the radius of the ball.


(c) The ball is thrown straight up in the air. At the top of its trajectory, what is its acceleration? [Hint: When v = 0, there is no drag force.]


I answered part a (1.84N) but not sure how to find parts b and c. If you could please show how you got your answer, that would be great! Thank you!

Explanation / Answer

(b). the buoyant force on the beach ball in air F = Vg where = density of air = 2.3 kg / m^ 3            V = volume of the ball = ( 4/ 3) r ^ 3            r = radius of the ball = 20.1 cm = 0.201 m            V = ( 4/ 3) (0.201) ^ 3                = 0.03401 m^ 3             g = 9.8 m / s^ 2 F = 2.3 * 0.03401 * 9.8         = 0.7667 N (c). The ball is thrown straight up in the air. At the top of its trajectory, its acceleration  a = g                                                                      = 9.8 m / s^ 2 Since at top velocity is zero,v = 0,therefore there is no drag force.

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