The Boat & Rock (i.e. weight) activity explores the buoyancy that results from the pressure in a fluid. The weight of fluid makes the pressure higher as you go deeper. For an object in the fluid, the bottom of the object is lower and will therefore have a higher pressure on it than the top of the object. Therefore, there will be a greater force upward on the bottom of the object than downward on the top of the object. The net force from the water (which is the difference in pressure between the top and bottom) is an upward force — in the opposite direction of gravity. That force is called buoyancy. If an object is floating, the buoyancy equals the weight of the object. Buoyancy can be calculated with Archimedes’ principle: An object immersed in a fluid is buoyed up with a force equal to the weight of the fluid displaced by the object. This means that the displaced water in the "Rock the Boat" exploration should have a mass equal to that of the boat plus the rock (or in this case, the weight).
The weight doesn’t float because its density is greater than the water’s density. The combined boat and rock displace water from the container such that the weight of the displaced water is equal to the weight of the boat and mass. When the rock is moved into the water, two things happen: (1) The rock displaces an amount of water with the same volume as the mass. That would make the water level in the container rise a little. (2) At the same time, the boat rises up in the water (i.e., has less draft) because the weight of the rock is no longer causing additional water to be displaced. This makes the water level go down. So now the question is, which is greater — the amount the water goes up because of (1) or the amount that the water goes down because of (2)? The amount of water no longer displaced (as described in ) had a weight equal to the weight of the rock. Since the rock is denser than water, this amount of water — having weight equal to the weight of the rock — must have a greater volume than the rock. So the amount by which the water level goes down due to (2) is greater than the amount the water level rises due to (1), and the net effect is that the water level in the container goes down.
Source: Bobrowsky, M., M. Korhonen, and J. Kohtamäki. 2014. Using physics gadgets and gizmos, grades 9–12: Phenomenon-based learning, pp. 112–113. Arlington, VA: NSTA Press. Reprinted with permission from the publisher.
2 100g masses
Plastic container with drain tube
Plastic overflow container