Effect of Boat and Anchor on Water Level
Brief Description
This is a demonstration of Buoyant Forces and liquid displacement. We start with a fish tank full of water and place a "boat" with an "anchor" in it, so it floats at the top. The water level will be marked. To figure out the displacement caused by the boat and anchor, we can use Archimede's Principle; the buoyant force is equal to the weight of the displaced fluid. A simple force diagram can tell us that the buoyant force must also be equal to mg where m is the mass of the boat and anchor. Thus, the total displaced water is the amount of water that has an equal weight to the boat and anchor.
When the anchor is dropped and the water level checked again, we'll find that the water level is lower. This is because when the anchor is submerged, the amount of water it displaces is only equal to its own volume. Since the anchor is more dense than water, this is less displacement than before.
Materials Needed
Fish Tank with water, Neoprene Rubber sheet, tupperware "boat", brick "anchor", towel, tape, marker, and water pump.
Setting Up & Performing the Demo
Before filling up the fish tank with water, place the rubber mat on the bottom of the tank. This is where we'll place the anchor to minimize chances of cracking the tank. After filling up the fish tank with water, set the boat afloat on top, and carefully place the brick inside. The brick is our anchor. Stick a piece of tape to the tank and mark the water level.
After asking the class to predict what will happen to the water level after "dropping the anchor", take the brick out and gently place it on the mat. Do not actually drop the brick. You can now mark the new water level and see that it is lower than before.
Safety and Other Notes
Water is heavy. Please be careful when handling the fish tank. Have the tank on a cart when you fill it so it can be easily transported. When placing performing the demo do not drop the brick onto the glass bottom. Gently place it onto the neoprene mat. When emptying the tank, bring the cart to either the sink in the outreach room or the one in 108 and use the pump to siphon the water out. Place one end of the pump in the tank and the other in the sink. Make sure the end in the sink is lower than the tank so it will siphon the majority of the water by itself. Squeeze the pump to get the water going.