act-dog

Introduction A teacher performed a demonstration on forces. She placed a beaker on a digital scale and added water until the combined weight (the force of gravity on an object) of the beaker and water was 10.00 newtons (N). She then covered the display panel and held a solid steel rod at rest, partially submerged in the water such that it was not in contact with the beaker (see Figure 1). Figure 1 before after The teacher then asked her students, “The rod has a weight of 5.00 N. How much of that weight is supported by my hand, and how much force is exerted on the scale?” Four students responded. Student 1 The rod displaces some water, producing 2 simultaneous effects: • An upward buoyant force, B, is exerted on the rod. Regardless of the rod’s density, B is equal in magnitude to the weight of the water that is displaced. • The depth of the water increases, which causes the water pressure at the bottom of the beaker to increase. As a result, a downward force—equal in magnitude to B— is exerted on the bottom of the beaker. Therefore, the teacher’s hand is supporting a weight equal to (5.00 N) − B, and the force exerted on the scale is equal to (10.00 N) + B. Student 2 Student 1 is correct that B is exerted on the rod and that water depth increases. However, depth has no effect on water pressure. Therefore, the teacher’s hand is supporting a weight equal to (5.00 N) − B, and the force exerted on the scale is 10.00 N. Student 3 Student 1 is correct that B is exerted on the rod and that water depth increases. However, the rod is less dense than water, so B is equal in magnitude to the weight of the rod. Further, Student 2 is correct that depth has no effect on water pressure. Therefore, the teacher’s hand is supporting 0.00 N, and the force exerted on the scale is 15.00 N. Student 4 Student 1 is correct that water depth increases. However, the rod is denser than water, so B is zero. Further, Student 2 is correct that depth has no effect on water pressure. Therefore, the teacher’s hand is supporting 5.00 N, and the force exerted on the scale is 10.00 N.