Mass of Trash

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SWBAT measure classroom trash in grams and compare the weights and extend that to a year's worth of trash

Big Idea

If students understand what a mass of trash might look like, they may be more likely to reduce their own waste.

Before the Lesson

5 minutes

Before conducting this lesson, divide the room into 3 or 4 groups, and provide each group with a special bag to collect their trash for the day, up until the time of the math lesson. These special collection bags will be used by each group to sort and weigh their trash. 

Tell children that any trash they generate from the start of the day, until math, should be placed in their special collection bags. They should wash out any yogurt, pour out any juice from snack, but other (non liquid) food scraps can be included.

Introducing Recycling

20 minutes

To begin this lesson I show students a short video about recycling.

This short video introduces students to how recycling works. 

I follow this with a discussion with students about recycling and what they are already familiar with. I read them the story Edith and Little Bear Lend A Hand by Dare Wright, about a small doll and bear who try to clean up New York City. The message is that even someone very small can make a difference.

I ask students if they think we do a good job in the classroom reducing our own use of trash? I tell them that today we will look at how much trash we generate in a day, a week or a school year. 

Weighing the Trash

15 minutes

I have students gather in their trash groups and show them the recording sheet for weighing the trash. Explain that they will weigh each type of trash separately but that first you want them to predict which type of trash there will be the most of.

Spread a covering on each work station. Ask students to dump the trash onto the workstation and begin by sorting it into recyclable, non-recyclable and plant material. Tell them that we will weigh each category in grams using a spring scale. Discuss what to do if students can’t agree on a particular item. (Start an unknown category, vote and go with the most common vote, let 1 person decide, etc.)

Tell students to record the weights for each amount of trash on the recording sheet. When they are done they will write down their observations. 

I circulate from group to group, providing support, asking questions and observing student interactions.

Comparing the Weights

20 minutes

I bring students together on the rug and ask them to bring their recording sheets and sit with their trash groups. I write the categories (Recyclable, Nonrecyclable and Plant Material) on the easel. I ask each group to give me their weights in grams for each category. I record the amounts on the easel. Students have recorded numbers under 100 today for each category.

I ask students to think about how we might figure out which category of trash we had the most of. Here I am presenting students with a problem that I want them to figure out. (MP1 - make sense of problems). Students propose adding the numbers within the 4 groups. I ask for several students to come up and do this for us. I ask the students to explain what they did. They are now modeling with mathematics using the data from our measurements (MP4). We look to see if each person solved the problems the same way. If they are the same, I might ask for a volunteer to come up and show me another way that they solved the problem. We determine which type of trash we had the most of by mass, by comparing the sums of each category. 

Now I tell students that they will try to figure out how much trash we might generate in a week at school. I assign each trash group 1 category to check. I ask how many days are in a school week? I ask students how we might determine how much trash we would use in each category if today is a typical day. Students may suggest that we need to add the number 5 times (repeated addition). We agree that this is a good idea and I send each group back to their seats to figure out how much trash of their category we would use in a week. The numbers will be less than 1,000 in total so they are adding within 1,000.

Students use repeated addition to solve the problem, employing one of the addition strategies we have explored to find the answer. 

After about 5 minutes, when each group is done, I ask for a total. We look at how much trash we generate in a week at school. I remind students that we are weighing in grams. When we have a total I remind them that we are more used to measuring in pounds. I show them on a spring scale how grams and ounces compare. I ask if anyone knows how many ounces are in a pound. When we determine 16 ounces in a pound, I show how many grams in a pound (approximately 1000 grams to 2 pounds).  I draw a comparison of 1000 grams on a scale with 2 pounds on the other side. We add our grams and convert it to pounds together to determine how many pounds we generate in a week in total trash. 

While it is not imperative for students to convert the trash totals from grams to pounds, they are more familiar with pounds and their own weight in pounds, so the conversion just makes the mass of the trash more understandable. 

I ask students if they could figure out how many pounds we would generate in a month of school days. We count the number of school weeks in the month. I allow students to work with a partner to determine how many pounds of trash we generate in a month of school. 

Wrapping It Up

10 minutes

Students figure out how many pounds of trash we generate in a month of school. (for us it was about 2 pounds a week or 8 in a month)

We talk about how much room all that trash might take up. 

We close by thinking of a list of ways we might use less trash each week. We post the list in the classroom.