Text Reading: Giancoli, Physics - Principles with Applications, Chapter 6: Sections 1-4
- 6.1: Work is one form of energy. We can define work as the result of a force moving a body through a given distance: W = F · d. Because both F and d are vectors that do not necessarily point in the same direction, we have to go a bit further and explain what we mean by the product of F and d in this case.
- 6.2: We can plot force and distance on a cartesian coordinate system with the magnitude of the component of F parallel to the distance along the y axis and distance itself along the x axis. If we have a constant force, the workdone by the force is equal to the area of the rectangle described by F|| × d. But what if F is not constant? The plot then has some function or squiggle of F with a different amount at each position for d. The result is the same though: the work done is the area "under" the squiggly F line between dinitial and dfinal. We have to use integral calculus to determine this ....so you don't have to know how to do non-constant forces for this course!
- 6.3: Kinetic energy is the energy something has by virtue of being in motion. An object at rest has no kinetic energy. The amount of KE is equal to ½mv2 . If we look at the units for kinetic energy (kg · m2/sec2) and for work done (Newtons * meters = kg · m/sec2 = kg ·m2/sec2), we see that they are the same. Because of the conservation of energy, work done on a moving object changes its KE an equal amount.
- 6.4: Potential energy is the "stored energy". Because of the principle of the conservation of energy, energy we expend to do work to put an object into position in a force field must "go" somewhere -- it becomes the potential energy of the object. If restraints on the object are removed so that the force field can act, the object will accelerate, and the kinetic energy expressed as the motion it achieves will be equal to a drop in potential energy. We can see the same exchange between potential energy and kinetic energy in the compression and expansion of a spring.
- Work Defintion:
- Kinetic Energy
- Potential energy for mass m near the surface of the Earth:
- Elastic potential energy for a spring:
Read the following weblecture before chat: Work and Energy
This simulation allows you to explore the exchange between potential and kinetic energy.
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