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Sample learning object 2
- 1. Physics 101 Learning Object 2: Buoyancy Explanation Question Three beakers are filled with water and various objects and placed on a weighing scale. Beaker 1 contains nothing, Beaker 2 contains a wooden ball, and Beaker 3 contains a metal ball suspended from the roof with a string. Question: Compare and contrast the three beakers and identify all of the important forces on the ball. Which weighing scale reads a greater value? __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ Ceiling 1 2 3
- 2. Physics 101 Learning Object 2: Buoyancy Explanation Question Question: Compare and contrast the three beakers and identify all of the important forces on the ball. Which weighing scale reads a greater value? Answer: Important forces are drawn on the ball in each case. All three beakers weigh the same. Observations: - All three beakers are filled to the same level, but the first beaker has the most water while beakers 2 and 3 have the same amount of water - All three beakers are in an equilibrium situation - all the forces balance out in each case - The diameter of the metal and wooden ball is the same - The metal ball in beaker 3 weighs the most, and has the highest density (the string is taut) but it’s not sinking because the ball is suspended from the ceiling - The wooden ball in beaker 2 floats on the surface of the water, and has a lower density than water - The amount of water displaced in beaker 2 and 3 due to the wooden and metal balls (respectively) is the same and consequently, the weight of this water is the same as well Explanation: The most important thing to note in this question is that the wooden ball is floating on the surface of the water, while the metal ball is suspended in the same position by a string suspended from the ceiling. Thus, the weight of the metal ball is reduced because of the tension in the string. Then, the weight of the water displaced by the wooden and metal balls is equivalent to the weight of the ball (in the case of the metal ball, the displaced water weighs the same as the reduced weight of the metal ball). Given that the water levels are the same in all three beakers (there is more water in beaker 1 though), the weight of the remaining water adds to the weight of the displaced water in beakers 2 and 3 - thus, the total weight of all three beakers is the same. Ceiling TFB Fg FB Fg