After seeing all of the fun building materials recommended by my favorite kid bloggers earlier this month, I felt inspired to help the kids build something extra special. We out pulled some craft sticks, lids, baking dishes, foil, and pipe cleaners and saw the makings of boats.
You can use whatever materials you have in your house for this activity.
I let the kids explore the materials to see what they could make before giving them suggestions about what a boat might need. Pie discovered that she could tape a straw to a foil baking dish as a mast and that our craft sticks would make a good raft.
They practiced using tape to put their pieces together as they made whatever boats they could imagine.
We created the frame for a foil raft using straws and pipe cleaners, and we built floats for a baking dish, kind of like you’d find on a seaplane.
Time for testing!
By the end of our building session, we built a whole fleet of boats to test in the bathtub.
Of course, we needed something to ride in the boats. Peas and Pie decided to bring their cars along for a bathtub boat ride.
As we added cars to the boats, we sang a little song:
Our initial results were promising.
First we tested out all of the boats alone and discovered that all of them floated.
I explained that boats float due to buoyancy, or the force that causes floating. An object will do one of two things in the water: sink or float.
The boats floated because the materials they were made from were less dense (or the molecules in them were spread out more than) the water.
You just know the cars had to go in the tub next, and guess what…
Interesting. This called for further testing.
We wanted to know what would happen if we put the cars in the boats.
Even though the cars sank on their own, the boat was able to hold them and still float. (Technically, the boats sank just a little more, but they were still floating.)
What’s going on here?
In a nutshell, there are a few factors at work with buoyancy, like density. The cars were made of material that was more dense (or the molecules were packed more tightly together) than the water.
There are also a few forces (or pushes and pulls) that contribute to buoyancy.
The first is displacement. This is the force that pushes water out of the way.
Think about what happens when you put your hand in water vs. what happens when you out your hand over a stack of fluffy feathers. When you put your hand in the water, you end up pushing the water out of the way. It doesn’t squish down the way the stack of fluffy feathers would.
This is the same thing that happens when the water level rises when you get into the tub.
All of the boats and the cars displaced the water a little, but displacement doesn’t work alone.
The second force is upthrust. This is the force of the water pushing up against the objects in the water. Even though the boats were pushing water out of the way, the water was pushing back and helping to hold them up.
But even those two aren’t working alone!
There’s also weight, or the force of gravity pulling an object toward Earth. Basically, this is how heavy something is.
Without anything on our boats, the weight of the boat equals the weight of the water it displaces AND the boat’s weight and upthrust (or the force of water pushing back) are equal and opposite forces. And this means it will float.
But wait a minute, didn’t we find that the cars sank? Why didn’t they make the boat sink?
Long story short, the boat has a much larger surface area than the car which means that it pushes a lot more water out of the way than the car, and that means that it also has more water holding it up. It will take a lot of weight to push the boat to the point where it can’t displace any more water. At that point, the weight of the boat would be more than the force of water holding it up, and it would sink.
We discovered this ourselves after Peas and Pie decided to make big waves in the tub, filling the boats with water.
What happened next was a sinking ship!
The water weighed down the boat enough to make it sink.
Did you know that’s the way submarines sink or float? They fill up water tanks to weigh themselves down enough to sink in the water. Maybe next time we’ll try building a submarine!
Everyone had a great time building and testing boats, and we learned a lot about forces!
What will you discover when you make boats of your own?