Science Buddies: "Ask an Expert"

My son (12 yrs. old) is asking, “How does a boat float?” Using your project, “How Much Weight Can Your Boat Float?” as a resource, he has made some changes, and I am wondering if he has an acceptable hypothesis and project.

He wants to use clay to show that clay sinks. He then wants to make 1-2 boats that sink and a 3-4 boats that float. The boats that float he then wants to add additional weight onto them until they reach their threshold and sink. All these boats he wants to calculate the density of them (with an open hull and with a loaded hull) and using the volume of the boats calculate the water displacement. In the end, he wants to create the graphs for buoyancy, density, and water displacement; and discuss how each related to whether a boat will sink or float.

He isn’t necessarily interested in comparing the boats to one another so he didn’t want to create a hypothesis that would compare which boat floats better or which one holds more weight.

This is what he's come up with:
Hypothesis: If the weight inside the boat is equal to the water that has been displaced, then it will sink.
Independent Variable: Boat Hulls (different volumes & shapes created with the same amount of clay)
Dependent Variable: Measuring weight with pennies and/or small pre-weighed sand bags and observing the point at which the boat sinks.
Controlled Variables: Tap water with a density of 1 g/cm3.

Could you please give us feedback on this project idea and let us know if it is an acceptable project and testable hypothesis for a science fair?
Thank you!

Hi rmedwards,
It sounds like you son have a very good start with the project!

I would suggest revising the hypothesis though. As it is written now, it sounds like he's trying to prove "Archimedes' principle" https://en.wikipedia.org/wiki/Archimedes%27_principle This is fine but I am not seeing how "boat hull" (the independent variable) is part of this hypothesis.

Please take a look at the following: https://www.sciencebuddies.org/science- ... hypothesis

Finally, in regards to whether this is an acceptable project / testable hypothesis for a science fair, this will depend on the requirements / guidelines of the specific science fair.

Good Luck and please post again if there's anything else we can do to help.
Willey

Thank you Willey for your reply, but I think I need more help. I honestly am quite frustrated with this project, as is my son, and I cannot think of any more ways to help him rewrite his hypothesis.

The independent variable, "boat hulls," is not directly worded in the hypothesis, but unspoken "boat hulls" will be the change in shape and volume because the change in boat hulls will be determining if the boat floats. Archimedes principles explains the water displacement but isn't focused on density. Is there a relationship between density and the amount of water being displaced? The more dense an object is compared to the density of the fluid, the more water will be displaced. I guess until it hits it's threshold and sinks.

I thought his hypothesis would be interesting for him and would make him think. He hypothesized, "If the weight inside the boat is equal to the water that has been displaced, then it will sink." Because he keeps thinking about the weight he is going to load into the boat, he's not thinking about the mass of the boat itself and that has to be included in the equation too. He knows it has to be when he looks at the equation, but he's not acknowledging it when he's trying to visualize it. The first part of his hypothesis does seem to appear to focus more on Archimede's principle, but whether the boat floats or sinks will be a discussion including density, and overall, the relationship between water displacement and density will determine whether a boat will float or sink.

I probably rambled a little because I had to get all my thoughts out so you know where I'm coming from. We homeschool and physics is not one of my strengths, but I do love science and my son loves boats, especially sailboats. Because we homeschool, we do not have strict guidelines for science projects. We are just advised to use your website as a guide, which we have been doing and we've read the links you supplied and we've filled out the worksheet for creating a hypothesis. But it sounds like we are still missing something, but I don't know what. I can't figure it out. Could you please give me a little more guidance and maybe even a suggestion on how to reword the hypothesis?

Thank you!

Hi rmedwards,
I am very sorry to hear that you and your son is getting frustrated with this experiment / hypothesis.
I would like to suggest the following for you and your son to consider:

Starting with your son's original hypothesis: "If the weight inside the boat is equal to the water that has been displaced, then it will sink." I would suggest modifying it to something like this: "If the weight inside the boat is equal to the water that has been displaced, then it will sink independent of the shape or configuration of the boat hull."

With this, I feel like it's a more complete hypothesis (not to say whether the hypothesis is right or wrong) with all of the variables this experiment deals with. At this point, we can run experiments to prove / disprove this hypothesis with all of these elements. After analyzing the results, the hypothesis can then be revised / tested against new experiments.

(Just a little bit of background - I am just an engineer by trade (and a volunteer on this forum) so writing hypothesis is not something that I deal with on a regular basis. I have contacted one of the moderator and see if they can get someone with a more suitable background to advise on this but I will continue to respond as best I can.)

Good Luck!
Willey

Thank you Willey for responding again! Your help is very much appreciated! I understand the addition at the end of the hypothesis. I knew it was suppose to involve a variable but had no clue how to include it. We are going to move forward with the project using the new hypothesis, but if you receive a response from the moderator that you contacted and have more helpful information please let us know. I will continue throughout December checking the message board for any responses. My son's project is due in January so I'm hoping he'll get it completed before the end of the year.

Thank you!

Ok, sounds great! Best of luck to you and your son with the experiment!
Willey

Willey gave you great advice in thinking about this project and how to set up the hypothesis. It sounds like your son is well on his way to a project that he likes. Kudos to all of you for working through the frustrating moments.

I’ll take a minute to offer some additional clarification that might help with the variables and hypothesis, too.

Let's start out by listing the variables.

Your son clearly has the dependent variable figured out: the mass at which the boat sinks. He’s also identified a control variable (something he needs to keep the same): tap water with density 1g/cm3. There’s another control variable that he’s figured out though, even if he hasn’t labeled it as one: the quantity of clay used to create the boats. He’s also realized that if he uses the same quantity of clay, the boats have the same starting mass (without a load). So, the second control variable is really: the same quantity of clay used to create the boats, and thus the same starting mass for each boat. Which brings us to independent variables. Independent variables are the things you change as an experimenter. What is he changing? 1) hull shape 2) volume of the hull 3) water displacement.

So there are 3 independent variables (which is fine for a more sophisticated project). However, it would also be okay to simplify down to one independent variable: density of the loaded boat. In that case, the hypothesis becomes: If the density of the boat is ______, then it will sink. Your son should choose what to put in the blank (less than, equal to or greater than) based on his background research.

If your son is truly interested in examining multiple variables, things get a little bit more complicated in the experimental design. With several independent variables, you need to determine what the effects are of each one. So, if the hypothesis is: "If the mass inside the boat is equal to the water that has been displaced, then it will sink independent of the shape or volume of the boat hull." Then you are already saying “hey, I think displacement is really important and these other two variables are not.” To test that, your son needs to make sure to try boats that have the same shape and different volume as well as different shape and same volume. For example, let's say you have volumes A and B and shapes square and circle. You would need to make and test these boats:

A square, B square, A circle, B circle

Looking at the data in various ways tells you which variable is important to determine when the boat sinks
A square, B square vs A circle, B circle (tells you whether shape alone is the determining factor)
A square, A circle vs B square, B circle (tells you whether volume alone is the determining factor)

He can have more boats than that, but at the minimum, given the hypothesis, he needs to test whether shape or volume are enough to drive the dependent variable. Does that make sense?

As a side note, you’ll see that I said “mass” rather than “weight” throughout. Although we use the term “weight” in everyday English, in physics it has a different meaning because it takes into account gravity and is reported in a unit called Newtons. Since your son is measuring in grams and isn’t concerned here about the effects of gravity, he means “mass” as far as physicists are concerned. Not a big deal, just a bonus for him to use the right physics terms in this case!

If more questions or areas of confusion come up during the testing and writing, post back to this thread so that Willey can continue assisting.

Remember that science is like a puzzle, it often raises lots of questions – but that’s the fun of it!

Amy
Science Buddies