PhysicsLAB AP Free Response Question
2011 B4 Form B

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A helium-filled balloon is attached by a string of negligible mass to a small 0.015 kg object that is just heavy enough to keep the balloon from rising. The total mass of the balloon, including the helium, is 0.0050 kg. The density of air is air = 1.29 kg/m3, and the density of helium is He = 0.179 kg/m3. The buoyant force on the 0.015 kg object is small enough to be negligible.
(a) On the dot below that represents the balloon, draw and label the forces (not components) that act on the balloon.

 (b) Calculate the buoyant force on the balloon. If you need to draw anything other than what you have shown in part (a) to assist in your solution, use the space below. Do NOT add anything to the figure in part (a).

 (c) Calculate the volume of the balloon.

(d) A child holds the string midway between the balloon and the 0.015 kg object. The child gets into a car, brings the balloon and the 0.015 kg object into the car, and holds the string so that neither the balloon nor the 0.015 kg object touches any surface. The car then begins to move forward, accelerating in a straight line. What behavior does the 0.015 kg object exhibit when the car accelerates? Explain your reasoning.

_____ It swings toward the front of the car.
_____ It swings toward the back of the car.
_____ It swings toward the right side of the car.
_____ It swings toward the left side of the car.
_____ It remains vertical below the child’s hand.

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