# The Wagon and The Ball

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## Objective

SWBAT analyze and compare different types of motion

#### Big Idea

Students explore a situation that may be familiar from childhood â what happens to a ball sitting in a wagon when the wagon moves? Simple? Not necessarily.

## Getting Started

Exploring motion

The focus of this uint is on motion and changes in motion. In this lesson students explore changes in motion in two ways:

1. They'll explore a situation that may be familiar from childhood—what happens to a ball sitting in a wagon when the wagon moves? The answer to this question may seem simple, but there is actually a lot of interesting complexity to investigate in this situation.They'll consider how a physicist would study this motion, and use that perspective in their own investigation.
2. They will begin to adopt a physicist's view of the world by identifying places in their life where motion occurs and changes. They will choose and describe one surprising motion in their life that they have not noticed before.

Goals for this lesson:

1. Perform, document, analyze, and compare different types of motion
2. Become more aware of the motions that are constantly taking place around you

Materials needed

• Wooden cart
• tennis ball

Science and Engineering Practices

• SP1: Asking Questions
• SP2: Developing and Using Models
• SP6: Constructing Explantions

Core Ideas

MS-PS2 Motion and Stability: Forces and Interactions

Credit: Lessons in this unit have been adapted from Investigating Physics; Copyright (c) 2001 by TERC (Cambridge, MA) and Lesley University (Cambridge, MA).  Permission is granted for classroom use.

## Engage

10 minutes

Imagine and predict

To engage student thinking, begin with a scenario that most of them have experienced or are familiar with, playing with a ball and a wagon. Use the following prompt to get them started:

Remember the  wagons that many of us played with as children? Imagine standing on a flat playground with one of these wagons—the kind made of red metal with a black handle.

Now imagine placing a ball in the center of the wagon's flat bed. Imagine pulling the wagon forward. What happens to the ball? Imagine stopping the wagon. What happens to the ball now? Take a few minutes to predict.

In your journal: Write down your predictions in words and/or drawings. Include an explanation about why you made these predictions.

In this activity students are using both mental models and physical models. Check out the video below for more thoughts on Science Practice 2: Developing and Using Models.

## Explore Part 1

10 minutes

Use the following prompt:

After you've made your predictions, observe what actually happens in real-life. How do the wagon and the ball move? Will they always move in the same way?

Start with the wooden cart and the tennis ball and place them on a table. With the cart turned flat side up, center the ball on the wood so that the cart wheels do not interfere with its movement.

Pull the cart forward, observing the ball as carefully as you can. Stop the cart abruptly. Push the cart backwards and stop it. Move the cart back and forth. Roll it at a constant speed.

How do your experiences with the ball and cart differ from your predictions? In what ways are your experiences similar to what you imagined? Why do you think your predictions matched (or didn't match) what happened?

Explore other ways of moving the ball on its cart. For example, what happens to the ball when you move the cart quickly, slowly, speeding up, or slowing down?

In your journal: Record your observations of the ball and cart.

I stress to my students the importance of documenting their observations and thoughts using their science journals. In the examples in this and all subsequent lessons in this unit take time to notice how the students use both images and words to convey their ideas. I spend time at the start of this lesson discussing the use of journals to record qualitative and quantitative observations.

## Explore Part 2

20 minutes

In this part of the lesson, students focus on four signature motions, speeding up, slowing down, constant speed and crashing in order to make it easier to discern patterns of motion. They will explore what new insights emerge when they employ scientific sensibilities in observations of these categories

In their journals: Using words, sketches, and annotated diagrams, document what happens when they explore the following:

1. Speeding up: What happens to the ball when the cart speeds up - either from a stop or from a slower constant speed? Can you make the cart accelerate rapidly? Gradually? Do different things happen to the ball in these cases? Where does the ball move with respect to the table?
2. Slowing down: What happens to the ball when the cart slows down from moving at a constant speed? Can you make the cart slow down rapidly? Gradually? Do different things happen to the ball in these two cases? Where does the ball move with respect to the table?
3. Constant speed: What happens to the ball when the cart is moving at a constant speed? What happens if the cart is moving slowly? More quickly? (Note: you may need to hold the ball on the cart until you achieve the constant speed.) Where does the ball move with respect to the table?
4. Crash: Crash your cart into a wall or something else that won't move, such as a pile of books. Try crashing the cart when it is moving at a low speed and a high speed and see if there is any difference in what happens to the ball. Where does the ball move with respect to the table?

Student Sample A.

Student Sample B.

## Explore Part 3

20 minutes

The motions that we are studying happen very fast --a lot  goes on in just a few seconds. Many important details are only visible if the action is slowed down. Fortunately, with digital technology it is possible to view a video frame by frame, seeing clearly how a motion changes in a fraction of a second and analyzing the event in more depth. Appearances can be deceiving—sometimes our preconceived notions influence what we think we see. As you analyze the videos, you may even find that what actually happened is different from what you thought you saw.

For this section, your students look at videos of the cart to see what details they can observe. Have them take their time and record their observations in detail. They should view the videos, both at normal speed, and at one frame at a time. Students examine the motion of each of the three objects -- ball, cart, and table -- with respect to one another, then answer the following questions:

1. How does the ball move in relation to the cart?
2. How does the ball move in relation to the table?
3. What kinds of additional information can you gather from these videos?

In your journal: Record the additional things you noticed when you looked at the video one frame at a time. What details would you like to explore in more depth? Did you see anything on the video that was different from what you saw when you did the motions yourself?

Note: I share these videos with my students through my class website.

## Explain

10 minutes

Signature motions

Students have been organizing their observations into four categories and looking for patterns. The four categories of motion are:

1. Moving at a constant speed

2. Accelerating (going faster and faster)

3. Decelerating (going slower and slower)

4. Crashing (coming to an abrupt stop!)

Explain to your students that they will use these categories many times during this unit. We call them "signature motions" because they belong to a set of simple motions that can be combined in various sequences to form much more complex motion scenarios.

Once they can identify these basic motions, i.e., recognize their signatures and know how they work, they'll be well on their way toward understanding many of the motions they encounter. If these distinctions are not clear to your students right away, don't fret; they will crystallize soon. In this and the next two sessions, they'll study these motions in depth, learn how to represent them, and compose motion stories with them.

## Elaborate

20 minutes

Ask your students to report on their investigation experiences and results with one of the four signature motions. For each lab group, divide the motions between each of the students.

Make sure that each of the four motions is described by someone in each group. Ask each student to create a reflection in their journal.

Include the following information in their journal reflection:

1. Their original prediction and whether or not their ideas have changed.
2. What is the motion of the ball relative to the cart?
3. What is the ball's motion relative to the table?
4. How did they achieve the motion? What surprised them?
5. How does their motion differ from the other three signature motions? What might account for these differences?

They are the Experts:

Each person in the lab teams becomes an "expert" on their particular motion. Alert them to listen to the other teams' discussions on the motion that they have described. Synthesize all these ideas on their signature motion into a single, refined description, and write this in their journal.

## Extend

20 minutes

Motion and Acceleration in Your Life

Where do you see acceleration in your life?

Acceleration takes place all the time; we are constantly starting, stopping and changing speed as we move through the day. Objects around us also move.

If we take the time to observe our surroundings closely, we notice changes in speed happening to all sorts of objects in our environment. Leaves fall, clock hands move, beetles crawl, coins are tossed—all these motions involve acceleration. Often we also feel changes in motion that happen to us physically. Our stomachs lurch as the elevator picks up speed. We experience the inevitable painful crash as we fall off a bicycle.

In your journal: Look at the world around you, focusing on motions in your surroundings.

Use your journal to record information about at least three motions in your everyday life that you hadn't noticed before.

In Class: Share one of the motions you have observed this week. The challenge is to select and describe a motion from your real-life observations that will tell us more about you and the motions that make up your world.

Select one of the motions you recorded in your journal to write about. You may want to consider the following questions:

1.Where did the motion take place?

2.What did it look like? Did it speed up and/or slow down?

3.What might your motion look like from different perspectives (e.g., head on, top down)?

4.What puzzles you about your motion?

Next, you'll use these motion descriptions to gain deeper insight into the role motion plays in our lives.