Conservation of Energy: Non-Conservative Forces

Students often struggle to accept that energy isn't always conserved within a system when non-conservative forces are present, so active learning is essential. Hands-on experiments and collaborative analysis help them see that energy transforms rather than vanishes, building conceptual permanence beyond abstract equations.

• 12th Grade
• Physics
• 3 activities
• 25 min–55 min

Learning Objectives

1. Calculate the work done by non-conservative forces, such as friction, in a given scenario.
2. Compare the initial and final mechanical energy of a system when non-conservative forces are present.
3. Analyze the transformation of mechanical energy into thermal energy due to friction.
4. Predict the final velocity of an object moving on a surface with friction using energy conservation principles.
5. Evaluate the efficiency of energy transfer in systems involving non-conservative forces.

Inquiry Circle

⏱ 55 min·Small Groups

Inquiry Circle: The Friction Energy Budget

Groups release a block from a measured height down an inclined ramp and measure its speed at the bottom using a photogate. Students calculate expected speed from energy conservation, identify the discrepancy, and use temperature probes on the surface to estimate thermal energy generated. Groups reconcile their energy budgets.

Prepare & details

Explain how non-conservative forces alter the total mechanical energy of a system.

Facilitation Tip: During Collaborative Investigation: The Friction Energy Budget, ensure each group records temperature changes and displacement measurements simultaneously to connect friction’s work with thermal energy output.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Think-Pair-Share

⏱ 25 min·Pairs

Think-Pair-Share: The Ski Jump Problem

Present a skier going down a slope with a given friction coefficient and ask students to predict landing distance on a flat run-out. Pairs first estimate using perfect conservation, then revise with friction, comparing results. Whole-class discussion highlights how friction coefficient changes outcome non-linearly.

Prepare & details

Analyze the role of friction in converting mechanical energy into thermal energy.

Facilitation Tip: When running Think-Pair-Share: The Ski Jump Problem, ask students to sketch energy bar charts before and after the jump to make the energy loss tangible.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

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

Jigsaw

⏱ 50 min·Small Groups

Jigsaw: Real-World Friction Scenarios

Expert groups each research one application (car braking systems, conveyor belts, bicycle gears, turbine bearings) using a structured worksheet. Groups reassemble and teach each other how friction affects energy efficiency in their assigned system, then collaboratively rank systems by energy waste.

Prepare & details

Predict the final velocity of an object sliding down a ramp with friction, using energy principles.

Facilitation Tip: In Jigsaw: Real-World Friction Scenarios, assign each expert group a different friction context so they can compare how direction and magnitude affect mechanical energy.

Setup: Flexible seating for regrouping

Materials: Expert group reading packets, Note-taking template, Summary graphic organizer

• Understand
• Analyze
• Evaluate
• Relationship Skills
• Self-Management

Teaching This Topic

Teach this topic by starting with physical experiences, not equations, so students feel energy conversion before they formalize it. Avoid overemphasizing friction as always opposing motion; instead, use examples where friction drives motion, like conveyor belts, to build nuance. Research shows that when students manipulate variables in real time, their misconceptions about energy ‘loss’ diminish faster than with passive demonstrations.

What to Expect

Students will trace energy flow through real-world systems, correctly accounting for mechanical energy loss due to non-conservative forces. They will justify their reasoning using the modified energy equation and explain energy transformations verbally and in writing.

Watch Out for These Misconceptions

Common Misconception

During Collaborative Investigation: The Friction Energy Budget, watch for students who assume friction always slows objects down.

What to Teach Instead

Use the conveyor belt model in the lab: students place a book on a moving belt and measure its acceleration. Ask them to draw the friction force vector to show how friction can increase kinetic energy when aligned with motion.

Common Misconception

During Jigsaw: Real-World Friction Scenarios, watch for students who say energy ‘disappears’ when friction acts.

What to Teach Instead

Have students use temperature probes during their scenario tests. They must record a rise in surface temperature and explicitly link it to the mechanical energy lost, reinforcing that total energy is conserved in the universe.

Assessment Ideas

Quick Check

After Collaborative Investigation: The Friction Energy Budget, give each group a ramp scenario with friction. Students write the energy conservation equation including work by friction, label initial/final mechanical energy, and identify where thermal energy appears.

Discussion Prompt

After Think-Pair-Share: The Ski Jump Problem, facilitate a whole-class discussion where groups defend where the mechanical energy went during the jump, citing air resistance and thermal energy.

Exit Ticket

During Jigsaw: Real-World Friction Scenarios, ask students to complete an exit ticket listing two non-conservative forces acting on their scenario and explaining how each converts mechanical energy to thermal energy.

Extensions & Scaffolding

• Challenge: Ask students to design a low-energy braking system for a toy car using only friction materials they select, then test it and compare energy outputs.
• Scaffolding: Provide a partially completed energy bar chart template for the ski jump problem so students focus on filling in losses due to air resistance.
• Deeper: Invite students to research how engineers reduce energy loss in roller coasters by analyzing real coaster track designs and citing specific non-conservative forces at work.

Key Vocabulary

Non-conservative force	A force for which the work done in moving an object between two points is independent of the path taken. Examples include friction and air resistance.
Mechanical energy	The sum of kinetic and potential energy in an object or system. It is conserved only when non-conservative forces do no work.
Work done by non-conservative forces	The energy transferred into or out of a system by forces like friction, which results in a change in the system's total mechanical energy.
Thermal energy	Energy associated with the random motion of atoms and molecules in a substance, often generated when mechanical energy is dissipated by friction.