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Work - Mechanical Energy Relationships:
If non-conservative forces do net work upon an object, then the total mechanical energy of that object is changed. The sum of the kinetic and potential energies will change as work is done upon the object. If non-conservative forces do NOT do net work, then the total mechanical energy will be conserved.
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The potential energy is the energy stored in an object due to its vertical position above (or below) the ground or some zero level. The amount of kinetic energy (PE) possessed by an object depends upon its mass (m) and its height (h). The formula for calculating the potential energy is
PE = m • g • h
where g is the gravitational field strength (9.8 N/kg on Earth).
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The object is said to be in free-fall. The only force acting AND doing work upon the object is gravity. Gravity is a conservative force and so the total amount of mechanical energy is conserved. It is the same at every point during its fall as it is initially. See Know the Law section.
Potential energy is the stored energy of position. In the case of the falling object, its height (i.e., vertical position above the ground) is decreasing. In fact, it decreases to one-fourth its original value. The object starts at rest, so there is no kinetic energy in the initial state. The initial potential energy is the same as the total amount of energy. Once the object has fallen to one-fourth its original height, its potential energy is one fourth its original value. See Formula Frenzy section.
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