Physics · 9th Grade

Active learning ideas

Center of Mass

Active learning helps students move beyond abstract formulas by giving them direct, tangible experiences with the center of mass. When students physically manipulate objects and observe balance points, they develop an intuitive grasp that supports later calculations and engineering applications.

• Common Core State Standards
• HS-PS2-1
• HS-ETS1-2

• 25–40 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Hexagonal Thinking
• 30 min · Small Groups

Hands-On Lab: Locating Center of Mass by Suspension

Small groups receive irregularly shaped cardboard cutouts of US state silhouettes. They suspend each shape from multiple points, draw the plumb-line for each, and find where the lines intersect to identify the center of mass. For at least one concave state shape, they verify the center of mass lies outside the solid region and discuss what physical meaning that has.

Why does a high jumper use the "Fosbury Flop" to clear the bar?

Facilitation Tip

During the suspension lab, have students test different pivot points with irregularly shaped cutouts to observe how the hanging thread lines intersect at the center of mass.

What to look for

Provide students with a diagram of three objects: a solid sphere, a hollow sphere, and a sphere with a dense core. Ask them to label the approximate location of the center of mass for each object and briefly justify their reasoning.

• Analyze
• Evaluate
• Create
• Self-Awareness
• Relationship Skills

Activity 02

• Think-Pair-Share
• 25 min · Pairs

Think-Pair-Share: Fosbury Flop Analysis

Pairs watch a slow-motion video clip of the Fosbury Flop and sketch the athlete's body position at the moment of peak height. They estimate where the center of mass lies and determine whether it clears the bar. The class then discusses how the same cleared height can require less energy than a traditional straddle jump.

How does the location of the center of mass affect the stability of a vehicle?

Facilitation Tip

In the Fosbury Flop analysis, ask students to model the jumper's body as a series of connected segments to see how mass shifts during the maneuver.

What to look for

Pose the question: 'Why is it easier to carry a long, thin rod by its center than by one of its ends?' Guide students to discuss how the center of mass relates to balance and the forces required to support an object.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 03

• Hexagonal Thinking
• 40 min · Small Groups

Engineering Challenge: Stable Tower Design

Groups build the tallest possible freestanding tower from index cards and tape, given the constraint that the structure must survive a lateral push. They discuss where the center of mass of each design sits, test their towers, and modify designs based on stability failure analysis before a final competition round.

How can the center of mass of an object be located outside of its physical body?

Facilitation Tip

For the stable tower design challenge, provide varying base materials so students must test and adjust designs to keep the center of mass low and centered.

What to look for

Present students with a scenario: a construction worker is lifting a heavy, irregularly shaped beam with a crane. Ask them to write two sentences explaining why knowing the beam's center of mass is crucial for safely lifting it.

• Analyze
• Evaluate
• Create
• Self-Awareness
• Relationship Skills

Activity 04

• Hexagonal Thinking
• 30 min · Pairs

Structured Calculation: Vehicle Rollover Threshold

Using provided diagrams of a sedan, SUV, and sports car with labeled dimensions and mass distributions, students calculate the center of mass height and track width for each. They then determine the critical tipping angle for each vehicle and rank them by rollover resistance, connecting their calculations to real NHTSA safety ratings.

Why does a high jumper use the "Fosbury Flop" to clear the bar?

Facilitation Tip

In the vehicle rollover calculation, model the effect of passenger weight distribution by having students adjust cargo placement in a toy car before computing thresholds.

What to look for

Provide students with a diagram of three objects: a solid sphere, a hollow sphere, and a sphere with a dense core. Ask them to label the approximate location of the center of mass for each object and briefly justify their reasoning.

• Analyze
• Evaluate
• Create
• Self-Awareness
• Relationship Skills

A few notes on teaching this unit

Teach this concept by starting with qualitative experiences before introducing formulas. Use open-ended labs where students discover patterns first, then formalize those patterns with calculations. Research shows students retain conceptual understanding better when they encounter misconceptions directly through hands-on exploration rather than lecture. Avoid rushing to the weighted-average formula; let students build intuition with balance boards and irregular objects first.

Students will confidently identify the center of mass for uniform and irregular objects, explain how mass distribution affects it, and apply the concept to real-world stability problems. They will also recognize when and why the center of mass lies outside an object's boundaries.

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Watch Out for These Misconceptions

• The center of mass of an object is always located inside the object.

  During the Hands-On Lab: Locating Center of Mass by Suspension, provide ring-shaped and L-shaped cutouts. Have students suspend each shape from multiple points and observe that the balance threads intersect at the geometric center of the ring, which lies in empty space. When students notice this during testing, ask them to pause and discuss how the hole affects the mass distribution and center of mass location.

• The center of mass is the same as the geometric center of any object.

  During the Structured Calculation: Vehicle Rollover Threshold activity, give students a meterstick with washers taped near one end. Ask them to balance it first by hand to find the new center of mass, then calculate it using the weighted-average formula. When students see the balance point shift from the midpoint, use their observations to clarify that mass distribution, not shape, determines the center of mass.

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Methods used in this brief

• Hexagonal Thinking
• Think-Pair-Share