Physics HL · Chapter 3: Work, Energy and Power
How to Think Through Work and Energy
Set up a system-based approach so that signs, force directions, and energy pathways stay consistent.
Estimated time: 14 minutes
From Force Stories to Energy Stories
In earlier mechanics, we focused on forces and acceleration. This chapter adds another lens: energy transfer. Both describe the same motion, but energy methods are often cleaner for multi-step processes, especially when speed changes and path geometry are involved.
The key shift is to track what enters and leaves a chosen system. Instead of asking only "what is the net force right now?", we also ask "what energy transfer has occurred over this interval?". That perspective makes it easier to connect local force interactions to global motion outcomes.
Sign Convention Still Matters
Energy itself is scalar, but work by a specific force can be positive, negative, or zero depending on relative direction. A forward pull on a moving body transfers energy into the body. A resistive force against motion transfers energy out of the body’s kinetic store and into thermal pathways.
When solving, name the system and list the relevant interactions first. For example, if the system is "block + Earth", gravitational potential changes are internal to the system. If the system is only "block", gravity appears as external work. Both methods can work if you stay consistent.
Chapter Flow
We begin with the definition of work for constant and varying forces, then link net work to changes in kinetic energy. Next we build gravitational and elastic potential energy models, then apply mechanical-energy conservation with and without dissipation. Finally we quantify rate of transfer using power and efficiency, and summarize multi-path transfers with Sankey-style accounting.