Lesson 7: Work: Connecting Forces and Energy

Introduction

Students explore how the presence of an external force can alter the energy of a given system by completing a simple lab experiment. In addition, students explore the concepts of mechanical work and an isolated/closed system.

Learning Outcomes

• Students will understand how an external force alters the energy of an object
• Students will gain a conceptual and mathematical understanding of mechanical work.
• Students will understand how doing work on a system affects that system’s energy (and what the absence of work means about a system).

Classroom Time Required

• Approximately 50 minutes

Materials Needed

• “Work and the Connection to Energy” handout
• spring scales
• wooden ramps approximately 4 feet long
• a method of changing the incline of each ramp (stacking textbooks works well, as do ring stands)
• rolling carts whose weight is consistent with the spring scale sensitivity

Pre-activities

• Collect Energy-Go-Round, follow-up activities. If you have extra time, allow students to share their findings with the class.

Activities

• Based upon the number of available labs stations, divide students into groups of 2,3, or 4. Try to avoid mixing students from one team with students from another.
• Distribute “Work and the Connection to Energy” handout and let students begin work. The lab should take most of the period.
• If time remains, allow student teams to continue calculating their hill heights. Detailed designs, with hill heights and loop radii are due by the beginning of Lesson 9.

Modifications/Author Comments

• When discussing the concept of work, it is useful to return to the money/bank analogy. Doing work on a system would be like making a deposit into a bank account; doing work on a system adds energy to that system. If the system does work on its surrounding, it would be losing energy, like withdrawing money from an account.
• An interesting conversation involves asking students whether it takes more work to lift a massive object up a vertical height of 2m, or push it up a ramp (with low-friction) that covers a vertical height of 2m. Most will agree that it is “harder” to lift the object vertically, but talk with students about the fact that a person does the same amount of work against gravity in both cases (the final GPE of the object is same, meaning you did the same amount of work in both cases). So what does a ramp do? It lowers the amount of required force by increasing the distance over which that force is applied.