![]() Here we assume that the plastic cup adds a negligible amount of weight, and it is comparable to the weight of the string on the grey ball. Where m 1 is the mass of the grey ball and m 2 is the mass of the red ball and cup. We can describe the forces F 1 and F 2 as follows: We know (according to Newtons Second Law) that F = ma. We know that mass 1 equals mass 2, allowing us to calibrate the scales appropriately. This depicts the forces F1 and F2 when neither of the balls are submerged in water. Once this additional mass is added, the red and grey balls are equal weights on the scale (shown by the scale balancing itself).įollowing are force diagrams showing the forces acting in each of the three situations that happen above: We can show that the weight of the displaced water is equal to the buoyant force by adding the displaced volume of water to the cup above the red ball. ![]() ![]() Because we can see the red ball floating on the water, we know that it must be experiencing some buoyant force that causes the effective weight of the red ball to be less than the effective weight of the grey ball. When both the red and grey balls are on the scale and the red ball is placed in water and allowed to come to equilibrium, we see that it does not sink completely into the water, but rather floats on top of the water in the container. This part of the demo shows that the weight of the displaced water is equal to the buoyant force acting on the red ball. Show the students that the submerged red ball with water in the cup is in equilibrium with the gray ball (Pic 4).Reattach the gray ball to the scale and pour collected displaced water into the cup (Pic 3).Collect all displaced water in the empty glass (Pic 2). Detach the gray ball from the scale and then allow the red ball to sink into dyed water.Show the students that the weight of the red ball and cup on one side of the scale is equal to the weight of the gray ball on the other side (Pic 1).Hollow copper sphere attached to a plastic cup by a rigid wire.Red pool ball attached to a plastic cup by a string.Plastic cylinder with a nozzle, filled with dyed water. ![]()
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