Energy Analysis 1 - Questions 2 Help

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Under certain circumstances, objects will conserve their total amount of mechanical energy over the course of their motion. The sum of their potential energy and kinetic energy will remain constant. As the object moves, there is a transformation of potential energy into kinetic energy (and back) while the total amount of these two forms remains the same.

There are multiple versions of this question that vary from one another in terms of their numerical values. Here is one of the countless number of possible questions.

Version 1:

A 1.40-kg disk starts at rest at the top of a 5.40-m high track (A). Its height at B is 3.60 m and its height at C is 2.20 m. Assuming mechanical energy conservation, determine the potential energy (PE) and kinetic energy (KE) values at locations A, B C, and D.

Help for Master Difficulty Level

The Master Difficulty Level involves a 4-location energy analysis. There are three big ideas you need to grasp to successfully complete this Difficulty Level.

Total Mechanical Energy
Each problem in this Concept Builder includes the phrase "assuming that mechanical energy is conserved." The Mechanical Energy is the sum of the potential energy (PE) and kinetic energy (KE). To say that this energy is conserved is to say that the total amount of it (KE + PE) is not undergoing change. It is the same at each location along the path. So if you know the value for the total mechanical energy (TME) at the initial location, then you know its value at every location. The significance of this idea is that the value of KE can be calculated if the value of TME and PE is known.

Potential Energy
The potential energy (sometimes called the gravitational potential energy) is the energy an object possesses due to its vertical position or height. Its value can be calculated from knowledge of the object mass and object height:

PE = m•g•h

where m is the mass (in kg), g is the gravitational field strength (9.8 N/kg or the more approximate 10 N/kg), and h is the height of the object. Anywhere that the mass and height is known, the PE can be calculated for that location. Furthermore, the PE calculation at the original location (A) allows one to determine the total mechanical energy at this location. Since the object is at rest, the total mechanical energy (TME) possessed by the object at location A is equal to the PE.

Kinetic Energy
The kinetic energy is sometimes referred to as the energy of motion. If an object is moving, then it has kinetic energy. If it is not moving, then it does not have kinetic energy. At each location, there should be enough kinetic energy to make up the difference between the total mechanical energy and the potential energy (PE).

Try this link to The Physics Classroom Tutorial for conducting an energy analysis:

Analysis of Situations in Which Mechanical Energy is Conserved