Let's Have a Ball!-Size vs. Density
Lesson 2 of 11
Objective: Student will be able to explain that density, not the size of the object is the main determining factor in whether an object will sink or float.
Students often think that the size of an object is what impacts its ability to float or sink. The goal of this lesson is to get students to really understand that it is an objects density that impacts its ability to float, not the overall size of the object. To set the stage for the learning, I start out with a discussion.
We begin with a summary of our previous investigation in which the students dropped a variety of things into a container of water to see whether they would sink or float. To guide this opening discussion, I asked the students the following questions:
What did we explore yesterday?
What did we find out from our exploration?
What is density?
How does density impact whether something sinks or floats?
After we work our way through those questions, I ask them the question that I want them to focus on during our investigation in this lesson.
Does the size of an object impact its ability to sink or float?
Some of the students answer "yes". Some answer "no".
It sounds to me like you are not sure. What could we do to figure out the answer to that question? That's right!!! Another experiment!!! (Cheers and excitement!!!)
Materials Needed for Each Work Station:
- Large container full of water
Materials Tip: I use empty pretzel and animal cracker containers. You are able to get a large amount of water in them and the students can view the investigation through the clear sides See photo.
- Eight different balls (marbles, ping pong, golf, foam, etc)
Materials Tip: Make sure you have small ones that have a high density (like marbles) and larger ones with a low density (inflatable, etc.). See photo.
- Recording Sheet that is included as a PDF with this lesson.
During this section, the students will carry out an experiment. The students do not completely design the experiment by themselves. They are given support by me to assist in the planning of this experiment. This scaffolding is an appropriate for Science and Engineering Practice 3, Planning and Carrying Out Investigations.
I say to the students, I have a bunch of different balls here. I want to figure out if they sink or float. How could I do that (solicit answers). So, you think that we should drop each one in the water, testing it to see what happens. We have talked many times about how scientists make predictions about what may happen during an experiment or investigation. What do you think we should do before we conduct our experiment? That's right! We will make a prediction. I am going to hold up the ball that we will be testing and I want you to make a prediction about whether it will sink or float. Make sure you tell why. Then record your prediction on your recording sheet.
The students begin making their predictions (see video). After everyone has had a chance to make a prediction, we will count off and drop the ball in the water. The students then record what happened to the ball on their sheet. We continue in this fashion until all of the balls have been tested.
After we tested all of the balls, we come together to discuss what they learned. It is important for the students to understand the importance of the data that is collected during an investigation. They often get so excited about the experiment that they forget that actual purpose of it is to collect data so it can be analyzed. I guide the discussion to help them make this connection. I begin by asking them some questions that help lead our discussion .
What balls floated?
What balls sank?
The marble was the smallest ball we tested. Why didn't it sink?
The ping pong ball was larger than the marble. It floated. Why was that?
So, you are saying the physical size of the ball does not matter. Let's think about what we learned about density yesterday. If the material has a low density, with those molecules spread out, it will float. If it has a high density, with the molecules close together, it will sink. So, it is all about the density, not its size.
I am curious, what happened with the whiffle ball (I hold it up for the students to see). The plastic seems kind of like the plastic that they other ball that floated was made out of. Why did the whiffle ball sink when the other one didn't it. I lead the students toward the discussion of how the water is able to get into the whiffle balls through the holes. It allows them an opportunity to think critically about what happened during the investigation and apply their knowledge to this scenario.
To wrap up our lesson, I ask the students to turn to their neighbor and explain why the marble did not float, but the ping pong ball did, even though the ping pong ball is bigger than the marble. I then asked them if they could think of something really big that floats. At first the students came up with things like beach balls. I challenged them to talk to their neighbor to see if they could come up with some things even bigger than a beach ball. It took awhile, but pretty soon I heard things like boats, ships, etc. I was happy to hear them thinking on a larger scale about buoyancy. They will have the opportunity to explore the buoyancy of larger things in a future experiment.