Physics · 9th Grade

Active learning ideas

Tension and Normal Forces

Active learning works for tension and normal forces because students need to see, measure, and feel how forces change in real time. These concepts are often counterintuitive, so hands-on labs and demonstrations help students confront misconceptions directly.

• Common Core State Standards
• HS-PS2-1
• CCSS.MATH.CONTENT.HSG.SRT.C.8

• 20–45 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Problem-Based Learning
• 45 min · Small Groups

Lab Stations: Tension Angles

Set up stations with protractors, strings, weights, and spring scales. Students hang a mass from strings at 30, 45, and 60 degrees from vertical, measure tension, and plot angle versus force. Groups calculate expected values using trig and compare to data.

How does the angle of a support cable affect the tension it must withstand?

Facilitation Tip

During Lab Stations: Tension Angles, circulate with spring scales to ensure students measure tension at multiple points and discuss why the readings should match.

What to look for

Present students with a diagram of a mass hanging from two ropes at different angles. Ask them to draw the free-body diagram for the mass and write an equation representing the vertical equilibrium of forces, identifying which rope must have greater tension.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Activity 02

• Problem-Based Learning
• 30 min · Pairs

Pairs: Elevator Scale Demo

Partners take turns standing on a bathroom scale inside a cardboard elevator box. One lifts and accelerates upward while the other reads scale changes. Repeat for downward motion, then graph normal force versus acceleration.

Why does your "apparent weight" change when an elevator starts moving upward?

Facilitation Tip

During Pairs: Elevator Scale Demo, remind students to record scale readings before, during, and after motion to connect acceleration with normal force changes.

What to look for

Pose the scenario: 'You are standing on a bathroom scale in an elevator. Describe what happens to your apparent weight (the scale reading) as the elevator starts to move upward, and explain why using Newton's second law.'

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Activity 03

• Problem-Based Learning
• 40 min · Whole Class

Whole Class: Pulley Bridge Model

Suspend a central mass with two angled strings from a beam, mimicking a bridge cable. Class predicts and measures tensions as angles change by adjusting string lengths. Discuss results using free-body diagrams on the board.

How do rock climbers use physics to choose safe anchor points?

Facilitation Tip

During Whole Class: Pulley Bridge Model, assign roles so all students contribute to building and analyzing the model, ensuring everyone engages with the force components.

What to look for

Facilitate a class discussion: 'Imagine a heavy box is being pushed across a rough floor. How does the normal force change if the person pushing also applies a slight downward force? How does it change if they apply a slight upward force?'

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Activity 04

• Problem-Based Learning
• 20 min · Individual

Individual: Free-Body Sketches

Provide diagrams of climbers on ropes and elevator riders. Students draw force vectors for tension and normal forces, label magnitudes, and write equilibrium equations. Share one sketch per student with class feedback.

How does the angle of a support cable affect the tension it must withstand?

Facilitation Tip

During Individual: Free-Body Sketches, provide colored pencils and ask students to label each force with its source and direction before sharing with peers.

What to look for

Present students with a diagram of a mass hanging from two ropes at different angles. Ask them to draw the free-body diagram for the mass and write an equation representing the vertical equilibrium of forces, identifying which rope must have greater tension.

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

A few notes on teaching this unit

Teach this topic by starting with concrete experiences—labs and demos—before moving to abstract diagrams and equations. Avoid rushing to formulas; let students derive relationships from data first. Research shows that students retain force concepts better when they connect measurements to real-world contexts, like elevators or bridges.

Students will accurately predict and measure tension changes with angle, explain normal force variations during acceleration, and sketch free-body diagrams that reflect Newton’s laws. Success looks like students using evidence from labs to revise their initial ideas.

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Watch Out for These Misconceptions

• During Elevator Scale Demo, watch for students who assume the normal force always equals their weight no matter what the elevator is doing.

  During the demo, have students graph scale readings versus acceleration, then ask them to explain why the normal force changes using Newton’s second law and their data.

• During Lab Stations: Tension Angles, watch for students who believe tension is highest at the top of the string.

  In the lab, provide spring scales at multiple points along the string and ask students to compare readings. When they see tension is constant, guide them to explain why in their lab reports.

• During Lab Stations: Tension Angles, watch for students who think cable angle has no effect on tension.

  During the station, ask students to predict how tension will change as they adjust the angle, then measure and compare. Use their data to discuss how steeper angles require more tension to support the same load.

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Methods used in this brief

• Problem-Based Learning