PHYS 1020 LAB 8: Buoyancy and Archimedes Principle Note: Print and complete the separate pre-lab assignment BEFORE the lab. Hand it in at the start of the lab. Pre-Lab While at home, put one ice cube (made of ice, not a frozen plastic object) into a glass of water. Mark the level of liquid with a sticky note, marker, or other method. Cover the top with a piece of plastic and set it somewhere out of direct sunlight to melt. 1. Before the ice cube melts, write down what you think the level of water will be after the ice cube melts and, most importantly, why you think this is true. 2. After the ice cube melts, observe what actually happened and write it below. Why do you think this happened? How does the result compare with your initial guess? 3. Does water exert a force on the ice cube? Why or why not? Draw a force diagram or reason it out in words.
Lab Logistics: (continuous reminder) You and your group will work together to complete the lab. Remember, everyone will need to assume a different role from last week. For instance, if you were the manager last week, then you should either be the recorder or the skeptic for this lab. Again, everyone should be helping with the hands-on stuff. Again if you are part of a group of two, you should be switching each week who is the manager and who is the recorder. NO ONE SHOULD BE RECORDER TWO LABS IN A ROW. The manager: This person is responsible for making sure that the group follows the lab procedure and completes everything that is asked for in the lab. The collector recorder: This person is responsible for keeping the lab notebook for the day, recording the observations observed by the group and the group s answers to the questions asked in the lab. The skeptic: This person is there to question the results of the lab. Is everything making sense? Are we taking the data correctly? Are the results and conclusions reasonable? Did we skip a step? Begin each lab report by titling the lab, listing your lab partners who are present, and listing the jobs that each lab partner has assumed for the lab. Remember, your lab report should give an explanation of all of your observations and measurements. Also, you need to think of and try one additional experiment for either Part 1, 2, or 3 of this lab that will further test your explanation of the results that you found. (If time permits) Lab Description: Part I: Does water exert a force? Do you think water exerts a force? Why or why not? How could you test this? With the open end facing down, push the empty cup into the water until it is submerged. What do you feel? Why do you think this happened? What would happen if you removed your hand? Check your prediction. With the open end facing up, push the empty cup into the water until it is submerged. What do you feel? Why do you think this happened? What would happen if you removed your hand? Check your prediction.
Gently hold the water-filled baggie ( neutral ) in one hand and submerge it. What do you feel? Why do you think this happened? What would happen if you removed your hand? Check your prediction. With the rock ( sinker ) on your palm, submerge your hand. What do you feel? Why do you think this happened? What would happen if you removed your hand? Check your prediction. Does water exert a force? How does that force change from object to object? Draw a force diagram for each of the four experiments above while the objects are in motion. Reflect on the prelab activity. Why do you think the ice cube floats?
As you ve found, water exerts a force. That force has a name: the buoyant force. This force points towards the surface of the water. If it is smaller than the gravitational force of an object, the object sinks; if it is larger, the object floats. Part II: How strong is the buoyant force? Using a balance measure the mass of each of the three objects. Then measure the weight of each of the three objects using the force scale in the air. Next, place each of the objects in water and weigh the objects in the water. (Make sure the objects do not touch the bottom of the beaker) Sinker Neutral Floater Mass Weight in Air Weight in Water Why do you think the weight changed in the water? (Hint: What does the spring scale measure (you are calling this the weight)?) Why should the floater not included in the chart (hint: if you put a small mass on the floater would its weight in water change?)? What was the change in the object s weight? Can you guess what the strength of the buoyant force is? Do not continue until you make a guess.
Buoyant force = weight of object - apparent weight of object in water Part III: Archimedes Principle Measure the mass of the large container and write it down below. Now fill the smaller container completely full of water (so that it about to overflow). Place the container in the larger one to catch spilled water. Wipe the sinker off with a paper towel and hang it from the force scale. Gently place it in the water and make sure it does not touch the bottom. Let the water overflow into the second container. Weigh the large container with water in it. Mass of empty container: Weight of the sinker in air: Weight of the completely submerged sinker: Mass of container with water: How do you get mass from weight? Divide the weight in Newtons by 9.8 using your calculator. Weight of empty container: Weight of container with water: Now find the difference between the two (subtract them): What does this difference measure the weight of? How do the weight of the water and the difference in the weight of the object in air and water from before compare? How does the strength of the buoyant force (calculated above) compare to the weight of the overflowed water? When the sinker is underwater, the strength of the upward buoyant force equals the weight of the water that is displaced. This is called Archimedes Principle. How do you think the buoyant force relates to how the object behaves in water (sink or float)?
Part 4: How does the buoyant force determine if the object will sink or float? While the sinker is sinking, how does the buoyant force on it compare to its weight in air? How does the volume of water that overflows compare to the volume of the sinker? At the large tub at the front of the class, place the large boat into the water. Put the brick in the boat. How does the buoyant force on the boat compare to the weight of the boat in air? What about how it compares to the weight of the brick? Both brick and boat? Why does the boat float even when it has a heavy brick in it? How could you make sure it sinks? Once you ve made your prediction, sink the boat - call upon the power of Poseidon if you must. Explain what you did and why it worked (or didn t). When the brick is on the boat is the volume of water displaced equal to, less than, or greater than the volume of the brick? Think of a method to answer the question and do it. Questions to answer in the conclusion: Does water exert a force? What is this force called? What direction does it point? Why do things feel lighter (weigh less) in water than in air? How does Archimedes Principle apply to everyday life? Where would you use it in the real world? Why did the volume of water in the glass remain constant after the ice cube melted?