Illuminations: Links Away

Links Away

Finding the Balance

This lesson encourages students to explore another model of subtraction, the balance. Students will use real and virtual balances. Students also explore recording the modeled subtraction facts in equation form.

Learning Objectives
 
Students will be able to:
  • explore the balance model of subtraction
  • write the subtraction modeled on the balance in equation form
  • practice subtraction facts from 10
  • demonstrate understanding of the equals sign (=)

Materials
 
Pan Balance, or Balance Beam and Hanging Weights
Computer with Internet connection
Links or Connecting Cubes (in two or more colors)
Paper bags

Instructional Plan
In this lesson, the balance model for subtraction will be demonstrated using both an actual and a virtual pan balance. If you have only a balance with hanging weights, please modify the directions given as needed.

Display a pan balance and review with the students how it operates. Place 5 weights on one side of the balance and 3 weights on the other side. Discuss with students how to make the scale balance using take away. Relate this action to the equation 5 – ___ = 3. Explain that the equals sign is like the middle point of a balance scale — both sides must be equal. The Pan Balance Overhead can be displayed if you do not have access to a pan balance.

Pan Balance Overhead Pan Balance Overhead

Ask how many links need to be taken away so that the scale balances. Accept and model all student responses. When the corrent response [2] is given, complete the equation so that it reads 5 – 2 = 3. Repeat by having students place different weights on the balance, write an equation, and solve. Continue until the students are comfortable with the process.

 

Now show students the online Shape Balance and assign some children to work with this tool. Because the different colored on-screen weights have different values, tell the children that they should use only one color of weight in a problem. Have students place an unequal number of weights on each side (for example, 5 on one side and 3 on the other), then remove some from the heavier side until the beam balances. Have them write the equation suggested by their action — in this case, 5 – 2 = 3.

Pan Balance - Shapes Pan Balance - Shapes

Split the class into two or more groups for center activities. While some children are using the online balance, others can play "What’s in the Bag?" To start the game, give pairs of children a pan balance, a paper bag, and some links. Assign one child to go first, placing 10 links (or connecting cubes) on the right side of the balance. The same student then places up to 10 links (or connecting cubes) in the bag and places the bag on the left side of the scale. The other child is to take away from the right side until the scale balances. Then the students write a subtraction equation to describe the situation. Have them repeat the activity several times, switching roles each time.

Questions for Students
 
Place 5 weights on one side and 3 weights on the other side of a balance. Which side of the scale is heavier? How do you know?
[The side with 5 weights is heavier because it is lower than the side with 3 weights.]

If we want the sides to balance, which side will we have to take weights from? How many weights will we have to take away?

[We have to take 2 weights away from the side with 5, since it is heavier.]

Suppose you put 6 links on the left side of the balance and 9 links on the right side. How would you balance the scale? What equation tells what you did?

[Take 3 links away from the side with 9. The equation is 9 – 3 = 6.]

How did you find out how many links were in the bag?

[Take weights away until the sides balance.]

Assessment Options
 
  1. Have students respond to the following prompt in their math journals: "How can a balance help you find differences?"
  2. Have students write equations for balance problems. Display an unbalanced scale. For example, place 6 weights on one side and 2 weights on the other side. Ask students to write a subtraction equation for this balance. Repeat with other problems.

Extensions
 
  1. Give dice or spinners numbered 1‑6 and a pan balance to pairs of students. Assign each child one side of the pan balance and tell them to roll the die (or spin the spinner) and place that many links on their side of the balance. Then have them work together to make the scale balance by taking some links away, and to record the subtraction equation that shows what they did. Have them repeat the activity several times.
  2. For advanced students, have them use the online Number Balance instead of the shape balance.

Teacher Reflection
 
  • Which students met all the objectives of this lesson? What extension activities are appropriate for these children?
  • Which students did not meet the objectives of this lesson? What instructional experiences do they need next?
  • When the students worked in pairs, did both children contribute equally?
  • Which students are counting to find differences? Which students use known facts to solve balance problems?
  • What adaptations did you make for special needs students? Were these appropriate?

NCTM Standards and Expectations
 
Algebra Pre-K-2
  1. Use concrete, pictorial, and verbal representations to develop an understanding of invented and conventional symbolic notations.
This lesson prepared by Grace M. Burton.
  
1 period   

NCTM Resources

Principles and Standards for School Mathematics

 Activities
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