Introduction: (Initial Observation)
When we talk about water supporting heavy objects, we are talking about floatation. You may have seen many floating objects so far. Heavy ships, birds and insects are possibly some of the floating objects that you may remember; however, that is not all and many other objects can be floating too. In this project we will study different factors that may make an object float.
Find information on floatation. Read books, magazines or ask professionals who might know in order to learn about the factors that may affect floatation of heavy objects on the surface of liquids. Keep track of where you got your information from.
Following are samples of information that you may find.
Water can support heavy objects based on two different physical laws, or properties. One is surface tension and the other is buoyancy.
For sample information about surface tension click here.
The cohesive forces (intermolecular attractive forces) between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface “film” which makes it more difficult to move an object through the surface than to move it when it is completely submersed.
Cohesion and Surface Tension
The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.
Surface Tension Examples
Walking on water
Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.
Floating a needle
If carefully placed on the surface, a small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.
Don’t touch the tent!
Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through.
Soaps and detergents
Help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.
Clinical test for jaundice
Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile.
Washing with cold water
The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unnecessary.
Surface tension disinfectants
Disinfectants are usually solutions of low surface tension. This allow them to spread out on the cell walls of bacteria and disrupt them. One such disinfectant, S.T.37, has a name which points to its low surface tension compared to the 72 dynes/cm for water.
Can you think of another?
For sample information about buoyancy click here.
Following table shows the density of some different material:
|Ammonia – liquid||0.68||g/cm^3|
|Argon – liquid||1.39||g/cm^3|
|Copper – pure||8.90||g/cm^3|
|Gold – pure||19.32||g/cm^3|
|Helium – liquid||0.0125||g/cm^3|
|Hydrogen – liquid||0.07||g/cm^3|
|Lead – pure||11.34||g/cm^3|
|Methane – liquid||0.42||g/cm^3|
|Nickel – pure||8.90||g/cm^3|
|Nitrogen – liquid||0.80||g/cm^3|
|Nylon (kind of plastic)||1.70||g/cm^3|
|Polycarbonate (kind of plastic)||1.30||g/cm^3|
|Polyethylene (kind of plastic)||2.30||g/cm^3|
|Silver – pure||10.5||g/cm^3|
|White pine (kind of wood)||0.5||g/cm^3|
By looking at the above table, predict which of the material will float onto the surface of the water.
Some things can float on top of water because of what we call “surface tension.”
What is surface tension? When the molecules of water inside the bowl attract and pull on the molecules of water on the top surface. This is called “surface tension.” It is as though the water molecules below the surface are pulling and tugging on the water molecules at the top.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.
The purpose of this project is to demonstrate the conditions that make objects to float (Surface tension and buoyancy).
My specific question is:
How does the weight of an object affect its floatation?
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
For objects that may float because of buoyancy:
- Independent variable (also known as manipulated variable) is the object weight.
- Dependent variable (also known as responding variable) is the portion of the object that goes into the water. (If this portion is less than 100%, the object floats, otherwise it sinks.)
- Controlled variables are water temperature, water type, and all other experiment procedures and environmental conditions that may affect floatation or sinking of an object.
For objects that may float because of the surface tension of water:
- Independent variable is the cutting edge of the object.
- Dependent variable is the ability of the object to float.
- Constants are the object weight, type of water.
When we place a greasy paperclips, a hair or any other water repellent object on the water, this object needs to cut the water surface to enter the water. The length of this cutting line is what we call it cutting edge. For example the cutting edge of paperclip is the length of the wire used to make that paperclip.
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
Water can support heavy objects (objects will float) if the upward force (Buoyancy) that water enters onto the object is more than the objects weight. We can also say that water can support any object that has lower density than water.
Water can also support water repellant objects if the weight of object is not able to cut through the water surface.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”
Experiment 1: How does the weight of an object affect its floatation?
In this experiment we float an object like a boat to see how the weight of the object affects the portion of object that goes into the water.
For this experiment, use the metric system for all calculations (Grams as the unit of weight and cc as the unit of volume). It is easier that way.
1. Small empty box
2. Some latex paint and paint brush
3. Some pennies or other type of weights
Instead of a box, you may use a cubic or cylindrical plastic container. In this way you will not need latex paint to make it water proof.
Do not use large box. With a large box the submerged height will be very small.
Step 1: Paint the box on the outside and let it dry. This will make the box water resistance, so you can use it as a boat.
Step 2: Weight the box and measure the area of it’s bottom and it’s volume in cc or cubic centimeter.
Step 3: Float the box and see how much of the box is in the water. Record it.
Step 4: Add some weight to the box. Record the total weight of box with added weight. Record the height of box in the water.
Step 5: Continue adding more weight to the box until one half of it’s height is in the water. Record the total weight of the box and the height of box in water.
Step 6: Start adding weight and record until the box sinks. Record the final total weight. Your results table may look like this:
|Boat weight with contents ( in grams)||submerged height
(in cubic centimeters)
Draw a graph:
You can make a graph to show the relation between the weight and the submerged height. You can use a graph template to simplify your graph creation process. Use the horizontal line (x- axis) for the boat weight and the vertical line (Y- axis) for the submerged height. Below the X axis write the numbers 1, 2, 3, … for weight. On the left of Y axis write the number 1, 2, 3,… for the submerged height.
Experiment 2: Demonstrate floatation caused by surface tension
In this experiment, we try to float a light object with a density higher than water to see if the surface tension can prevent the object from sinking.
bowl of water
Part 1: We drop a paper clip in a cup of water to see what happens?
Part 2: Tear off a piece of paper towel that is slightly larger than the paper clip. Place the piece of paper towel on top of the water. Gently place another paper clip on the piece of paper towel. Wait a few seconds. Now what happens?
(Paper towel will make it possible for the paper clip to touch the water while being horizontal. In this way the pressure of the paperclip is distributed evenly along its side. For more details learn about pressure and find out why a needle can easily enter a layer of cardboard from its tip but it can not enter from its end or its side)
Part 3: Try to float a paper clip on the water without using paper towel.
Part 4: Put a piece of waxed paper on the water. Carefully place some pennies on it to see how many it can hold and for how long?
A greasy paper clip will stand on water easier because water molecules do not stick to it and do not make it heavier.
Another possibility is doing the same experiment with sewing needles. You may repeat the test with different sizes of needles and measure the amount of time each needle can stand on the surface of water.
Need a graph?
When testing needles, you may also record the length and the mass of each needle and report which needles remained on water. You may draw a line graph to show the relation between the needle length and its mass. On the graph you may mark the needles that remained on water with a different color.
Materials and Equipment:
List of material can be extracted from the experiment section.
Results of Experiment (Observation):
Experiment 1 Results:
If you drop a paper clip in the water, the paper clip sinks. However, if you put the paper clip on a piece of paper towel, the paper towel sinks and the paper clip floats. This is because water particles are attracted to each other in all directions, making them “stick” together. However, because there are no water particles above them, the water particles at the surface “stick” only to particles next to and below them. This makes the surface act as if it had a thin “skin”. This is called surface tension. The paper towel helps you to lower the paper clip onto the surface gently without breaking the surface tension. If you’re very careful, you can float the paper clip on the water without using the paper towel.
Experiment shows that if the paper clip is more greasy (For example by more touching) it will float easier and longer.
Experiment 2 Results:
Write it as it happens.
The submerged volume (the last column of your results table) must be calculated. To do that you can multiply the submerged height in centimeter by the area of it’s bottom (in square centimeters).
Summery of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Now compare the final total weight with the box volume.
Remember that box volume also shows the weight of equal volume water. The reason is that one cc water weighs one gram. For example, if your box volume is 200 cc, it means that the same volume water weighs 200 grams.
Since Buoyancy force is equal to the weight of displaced water, our 200 cc box can displace 200 cc water (at most) so the buoyancy force should be 200 grams at most.
It this way we expect our boat to sink as soon as the total weight exceeds 200 grams.
Can you determine how much a boat weighs by knowing how much of it has submerged?
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.
If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.
List of References