Module 6—Work and Energy
Lesson Summary
In this lesson you explored the following questions:
- Is mechanical energy conserved in an isolated (frictionless) and non-isolated (friction) environment?
- What effect does the presence of friction have when converting potential energy into kinetic energy?
- What effect does work have on mechanical energy?
The total mechanical energy (sum of the potential and kinetic energy) is conserved in an isolated environment. Any change in either potential or kinetic energy is associated with an equal but opposite change in the other energy. As an object slides down an incline, potential energy is converted into kinetic energy until all of the energy has been transferred from one form to the other. The forces acting in an isolated system are called conservative forces.
Mechanical energy is not conserved when friction is present, making the system non-isolated. In this case, some of the potential energy is lost due to the work done by friction rather than being converted to kinetic energy. Therefore, the mechanical energy decreases over time. By reducing friction, the loss in mechanical energy can be minimized, resulting in a greater conversion of potential energy into kinetic energy and a greater speed at the bottom of an incline. Forces acting on a non-isolated system from outside the system or from friction are called non-conservative forces.
In a frictionless (isolated) environment, mechanical energy is conserved. Therefore, Ep at the top equals Ek at the bottom. In a friction (non-isolated) environment, Ep at the top equals Ek at the bottom plus work done by friction.
Lesson Glossary
conservative force: a force such as gravity, acting in an isolated system where the total work done is independent of the path an object is moved through
law of conservation of energy: a principle that states that within an isolated system, energy cannot be created or destroyed, but it can be converted from one form to another
mechanical energy: the sum of potential energy and kinetic energy
non-conservative force: a force acting on a non-isolated system from outside the system or from friction; a force where the total work done depends on the path an object is moved through