The Idea
Does iron float? Clearly a cube of iron, which is much denser than water, will sink. Then, how is it possible for a boat made of iron (or iron alloy) to float? In this experiment, you investigate the forces that counteract the force of gravity to allow objects that are denser than water to float.
What You Need
- "boat": a shallow plastic cup (such as a ¼ pound coleslaw container). You can also use a piece of wood as your boat.
- "lake": a plastic tray or fish tank filled with water deep enough and wide enough to float the "boat"
- "cargo": small weights, pennies (each penny has a mass of 2.7 g)
- 100 mL graduated cylinder
Method
- Measure the volume of the boat. You can do this by geometry, if you are so inclined, or you can do it by filling the cup with water and measuring the amount of water to do this.
- The number of grams of water that occupy the volume of the boat equals the number of grams of cargo it can carry just before it sinks.
- Test this by adding the amount of weight you predicted. Don't forget to include the weight of the boat itself. If you use pennies, count each as 2.7 grams (or weigh them). As you add weight, be careful not to tip the boat or you will capsize it prematurely.
- Compare your prediction with the amount of cargo your boat could actually carry. See Figure 38-1.
- We are taking a slight liberty here for the sake of clarity by focusing on the mass. What holds the boat up is a buoyant force, which is measured in newtons. The buoyant force equals the weight of the water (also measured in newtons) displaced by the floating object.
Expected Results
As a rule of thumb, for every 1 mL that an object is held submerged below the surface of the water, there is a buoyant force capable of supporting 1 gram of mass.
An equivalent way of expressing this is an object will float if the density of the entire boat, considering its entire volume, is less than the density of the same volume of water. The density of water is 1 gram/cubic centimeter or 1000 kilogram/cubic meter.
Why It Works
The buoyant force on an object is given by the weight of the fluid it displaces.
The buoyant force exerted on a floating object equals the volume of that object that is submerged (m3) times the density of water (1000 kg/m3) times the gravitational acceleration (9.8 m/s2).
Other Things to Try
- Take a weight of known volume. Either calculate the volume using geometry or determine how much water it displaces. Measure the weight on a spring scale in the air. Immerse the weight in the water. How is the weight affected by being immersed in water? How does this compare with the weight of the water that would fill the volume of the submerged object?
- Predict how much weight a boat can support based on the weight of the water that it can hold. Test your prediction.
- Predict where a float line will be based on the volume contained below the float line set equal to the weight to be added to the boat.
The Point
A buoyant force is exerted on an object that is floating or submerged in a liquid. An object will float if it is less dense than the liquid it is floating in. The buoyant force exerted on an object floating in water equals the weight of water that would occupy the volume of the object that is underwater.
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