# Balancing Bird

### Introduction

The balancing bird will yield hours of entertainment and learning. Students can even calculate the center of gravity of the balancing bird.

### Concepts

• Center of gravity

### Materials

Balance, electronic or triple-beam, 0.1 g
Balancing bird with stand
Calculator
Metric ruler

### Procedure

Demonstration

1. Hold the bird by the wing and show it to your students.
2. Place the beak on your finger and listen for comments like “How does it do that?”
3. Explain that the center of gravity is at the beak of the balancing bird because of some weights that are in the wings.

Experiment

1. Remove the weights from the wings by carefully popping open the plastic.
2. Mass the bird on the 0.1-g balance. Then mass each of the weights.
3. Find the center of gravity of the non-weighted bird. This is done by setting the bird on its stand in different positions until it balances.
4. Mark this point on the outline drawing of the balancing bird (see Figure 1). Label this point C. Mark the mass of the bird (without its weights) next to this point.
5. Mark the masses of the weights next to points A and B on the wings in the drawing.
6. Create an x–y grid (lines spaced one centimeter apart) around the bird on the page below and find the coordinates (in centimeters) of the three points that you have marked. This includes the center of gravity of the bird and the two points which represent the weights in the wings. Note: It doesn’t matter where you make (0, 0)—the mathematics will give the correct answer based on what coordinates you create.
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### Science & Engineering Practices

Using mathematics and computational thinking
Developing and using models
Constructing explanations and designing solutions

### Disciplinary Core Ideas

MS-PS2.A: Forces and Motion
MS-PS2.B: Types of Interactions
HS-PS2.A: Forces and Motion

### Crosscutting Concepts

Cause and effect
Structure and function
Patterns
Stability and change

### Performance Expectations

MS-PS2-2: Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object
MS-PS2-4: Construct and present arguments using evidence to support the claim that gravitational interactions are attractive and depend on the masses of interacting objects
MS-PS2-5: Conduct an investigation and evaluate the experimental design to provide evidence that fields exist between objects exerting forces on each other even though the objects are not in contact

### Discussion

In order to find the center of gravity of the bird with the weights in it, the center of gravity of the x-axis and the center of gravity of the y-axis must be found. These values can be represented by the following equations:

Xcenter = (xAmA + xBmB + xCmC)/mtotal
Ycenter = (yAmA + yBmB + yCmC)/mtotal

where xA is the x coordinate of point A and mA is the mass acting at point A (the weight). The total mass of the bird and weights is mtotal.

The center of gravity is then (Xcenter, Ycenter).

You should find that this center of gravity is just in front of the beak of the bird. This causes the tail of the bird to rise up a little more than if the center of gravity were at the point of the beak.

### References

Tipler, P. A. Physics for Scientists and Engineers; Worth: New York, 1991; pp 278–279.

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