8.8.c Students know the buoyant force on an object in a fluid is an upward force equal to the weight of the fluid the object has displaced.

8.8.d Students know how to predict whether an object will float or sink.

Questioning Script

Materials

2 clear plastic tumblers

water

Procedure

Start with two clear plastic disposable tumblers.

Fill one nearly to the top then ask the students if they think that the empty second glass will float in the first glass filled with water. Most will agree that it can and will be pleased that they were correct when you demonstrate it to them.

Then pour about 1/3 of the water into the second glass and repeat the question again. Will the glass I/3rd filled float in the glass that is 2/3rds filled? Again, most will agree that it will. When it is demonstrated, the cup does float as expected.

Finally, pour another third of the first cup into the second and repeat the question. Will the second cup which is now 2/3rds filled be able to float in the first cup that is now only 1/3rd filled? Most of the students will agree that it will not but watch the surprise on their faces when you actually try it!

Prior knowledge & experience:

Students will come with the experience that some objects sink while others float.

Students may also recognize that a "heavier" object will sink while "lighter" objects float.

Most will not yet understand the concept of density and will not yet know how to accurately predict whether a particular object sinks or float based on Archimedes' Principle.

Root question:

Predictions 1. Will the empty tumbler float in the full tumbler of water?

2. Will the 1/3 full glass float in the 2/3 full glass?

3. Will the 2/3 full glass float in the 1/3 full glass?

Post-Demonstration 1. Were your predictions correct or were you surprised by the floating cups? (Most students will predict that the empty and 1/3 full cup float. Most will be surprised that the 2/3 full cup also floats)

2. What did you notice about the level of water in Cup 1 each time Cup 2 is placed in it? (Students should recognized that each time Cup 2 is placed in Cup 1, the water level rises to the top of the cup)

3. Why doesn't the 2/3 full cup sink in the 1/3 full cup? (The 2/3 full cup sinks until the water level equals that in the 1/3 full cup because the buoyant force is acting on it)

4. What does Archimedes' Principle tell us? (The buoyant force on an object is a fluid is an upward force equal to the weight of the volume of fluid that the object displaces)

5. Is there any amount of water we could put in Cup 2 to make it sink to the bottom of Cup 1? (Yes, only if Cup 2 is full will it sink to the bottom of Cup 1)

Target response:

This demonstration can really help with an understanding of Archimedes' principle if a line is drawn at the starting level of the water in the first cup. Because the disposable tumblers are so light no appreciable water will be displaced when the second empty tumbler is placed on top of the water and floated. Therefore, the water level remains basically in the same place. During the second try when the second cup is 1/3rd filled, students should be able to observe that the water in the first cup is displaced (rises) to its original position AND that the second cup will sink until its water level is essentially equal to the original mark and the water in the first cup. Finally, when the second cup is filled with 2/3rds of the original water, students should notice again that the water in the first cup will rise to its original level and the second cup should sink until its water level is even with the water level in the first cup.

This is a good vivid example that floating objects are buoyed up with a force equal to the weight of the water that they displace. Since the disposable tumblers are nearly weightless compared to the water that we are working with, the weight of the floating object is equal to the weight of water it contains. Therefore, it always floats so that the water level inside is equal to the water level outside. Since the volume of water that has been removed from the first cup is the difference between the present water level and the original mark, the water will always rise to its original level when the other glass is floated on top.

Common Misconceptions:

Most students will incorrectly believe that a cup full of "lots" of water will always sink