### Submitted by: Ariel Paul, PhET Interactive Simulations

Grade Levels: 3-5, 6-8, 9-12

In this lesson plan, students use an interactive simulation to describe the factors whether objects will balance each other, predict how changing the position of mass on a balance will affect the motion of the balance, and use a balance to find the masses of unknown objects.

### Students will:

1. Describe the factors that determine whether two objects will balance each other
2. Predict how changing the position of a mass on the balance will affect the motion of the balance
3. Use a balance to the find the masses of unknown objects

### Preparation:

This lesson features an interactive simulation titled Balancing Act, developed by our award-winning partner PhET through the University of Colorado Boulder.  In this simulation, students explore balance by predicting how objects of varying masses balance a plank and how changing positions of the masses affect the motion of the plank. The sim has four levels, with tasks growing more difficult as students move up in the levels. Photocopy a class set of the Teeter-Totter printable. Preview and play with the Balancing Act sim to plan how you will adapt it to your students’ needs. If students will be working in small groups, review tips on Setting Cooperative Gaming Expectations. Depending on your classroom routines and available technology, you may want to consider these grouping options:
• 1:1 with students and devices
• Two to three students sharing one device and swapping ideas and the device back and forth
• Station model where small groups rotate through using the devices.
Build background knowledge or reinforce topics with these BrainPOP movies: Force, Gravity, Levers, and Newton’s Laws of Motion.

### Lesson Procedure:

1. Invites students explore the Balancing Act sim with a partner. As they explore,encourage them to talk about what they discover with their partner (About 5 minutes).
2. Ask students if there is more than one way to get two objects with identical masses to balance. If they say, yes, how?
3. Now challenge students to try and balance two objects of different masses. Have them describe at least two different ways they were able to balance them. Then distribute the Teeter-Totter printable and have students draw the two ways on the first two sets of teeter-totters. Remind them to label the masses and the distance to each mass from the center (pivot point).
4. Now have students use the second set of teeter-totters on the Teeter-Totter printable to draw two examples of balancing a single mass on one side with two other masses on the other. Again, make sure they label the masses and the distance to each mass from the center (pivot point).  Then instruct students to draw the forces from each mass.
5. Using the last picture on the Teeter-Totter printable, have students draw what will happen next if the box on the left is 45 kg and the box on the right is 60 kg.

### Extension Activities:

Challenge!  What is the mass of mystery object F?  Explain how you determined the mass.
Filed as:  3-5, 6-8, 9-12, Balancing Act