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Lo3 explanation2

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Lo3 explanation2

Question from the physics 101 learning object tutorial at UBC.

Question from the physics 101 learning object tutorial at UBC.

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Lo3 explanation2

1. 1. Physics 101 Learning Object:! Buoyancy Explanation Question 2B ! Beaker 1 contains a wooden ball (which would normally ﬂoat to the top) attached to a massless string and Beaker 2 contains just water. Both beakers are ﬁlled with water to the same level, and put on a weighing scale. ! ! Question: Compare the two beakers and determine which is heavier. Explain.! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! __________________________________________________________________________! ! 1 2
2. 2. Physics 101 Learning Object:! Buoyancy Explanation Question 2B ! Question: Compare the two beakers and determine which is heavier. Explain.! ! Answer: Beaker 2 (right) is heavier than Beaker 1 (left). The free body diagram on the beaker is below (arrow lengths not to scale)! ! ! ! ! ! ! ! ! ! ! ! ! ! Observations:! ! - Both beakers are ﬁlled to the same level, but Beaker 2 has more water in it because of the wooden ball present in Beaker 1! - The wooden ball in beaker 1 would normally ﬂoat on the surface of the water because it has a lower density than water but in this case, it’s held submerged by the string! ! Explanation 1: Think about the concept of ‘effective densities’. Both beakers are ﬁlled to the same water level but the only thing that’s different is a certain percentage of the beaker contents in Beaker 1 have a different density. The wooden ball has a lower density than water (it has a tendency to ﬂoat) and ultimately, this results in the fact that Beaker 1 has a LOWER effective density than Beaker 2. From the density = Mass / Volume formula, we can see that if one object has a lower Density but the same Volume, then the Mass must be lower. ! ! Explanation 2: Another way to consider this question is to think about what happens when the string is cut. Will the beaker weigh the same before it is cut and after it is cut? The answer is yes - cutting the string will have no overall impact on the weight of the beaker. It will have an impact on the level of the water of the beaker. When the string is cut, the ball ﬂoats to the top, and the water level will decrease. Then, it is easier from there to see that the weight of the ball plus the weight of the remaining water is less than the weight of the beaker on the right.! ! 1 2 FB FgT