ChemPhys 173/273

Unit 11: Work, Energy and Power

Problem Set B

Overview:

Problem Set B targets your ability to understanding of the relationship between work and energy. In Physics, an object possessing mechanical energy has the ability to do work. There are two forms of mechanical energy which an object can possess - potential energy and kinetic energy. These forms are defined and discussed below:

• Potential energy is the stored energy of position. In this problem set, we will be most concerned with the stored energy due to the vertical position of an object within Earth's gravitational field. Such energy is known as the gravitational potential energy (PEgrav) and is calculated using the equation
PEgrav = m•g•h
where m is the mass of the object (with standard units of kilograms), g is the acceleration of gravity (9.8 m/s/s) and h is the height of the object (with standard units of meters) above some arbitraily defined zero level (such as the ground or the top of a lab table in a physics room).

• Kinetic energy is defined as the energy of motion possessed by an object. An object must be moving to possess kinetic energy. The amount of kinetic energy (KE) possessed by a moving obejct is dependent upon mass and speed. The equation for kinetic energy is
KE = 0.5 • m • v2
where m is the mass of the object (with standard units of kilograms) and v is the speed of the object (with standard units of m/s).

Like work, the standard metric unit of energy is the Joule (abbreviated J). The fact that both work and energy share the same unit is an obvious indicator that there must be some relationship between them. In fact, there is a relationship and that relationship is the focus of the remainder of the unit. In this problem set, the main connection between work and energy that is being probed is the idea that for horizontal motions, the net work done on an object is equal to the kinetic energy change of the object.

W = KE

(for horizontal motion)

When work is done upon an object to displace it in a horizontal direction, then the work done can be related to the change in kinetic energy of the object. In expanded form, the above equation can be written as

F • d • cos () = 0.5 • m • vf2 - 0.5 • m • vi2

To be successful on this problem set, you will have to master the use of the kinetic energy, potential energy, and work = energy change equations. Additionally, you will need to be able to:

• convert units from non-standard form to standard metric form.
• calculate a change in a quantity as the difference between the final value and the intial value.
• recognize that the total mechanical energy of an object is simply the sum of the two forms - kinetic energy plus potential energy.
• recognize that when an object loses mechanical energy, it shows up in other non-mechanical forms (such as the heating of the surroundings).
• determine the parallel component of the gravity force as m•g•sin(theta)

The following pages from The Physics Classroom tutorial may serve to be useful in assisting you in the understanding of the concepts and mathematics associated with these problems.

Work | Sample Work Calculations | Potential Energy | Kinetic Energy

View Sample Problem Set.

 Problem Description Audio Link 1 A problem requiring the relating of the loss of kinetic energy to the amount of heat which is generated. 2 Comparison of the KE of an object at two different speeds. 3 Calculation of the work done on an object which is braking to a stop. 4 Calculation of the final speed of an object from knowledge of the amount of work done upon it. 5 Calculation of the gravitational potential energy of an object. 6 Calculation of the gravitational potential energy of an object. 7 Calculation of the force acting upon an object if given KE information and a displacement 8 Calculation of the final speed of an object from knowledge of the amount of work done upon it. 9 Calculation of the work done on an object as it moves down an inclined plane; must determine the parallel component of gravity. 10 Calculation of the distance moved by an object if given force and KE information. 11 Complex problem requiring the use of trigonometry to determine the potential energy of an object. 12 Calculation of the total mechanical energy of an object from m, v, and h information. 13 Calculation of the total mechanical energy of an object from m, v, and h information. 14 Calculation of the change in potential energy of an object from knowledge of the mass and the initial and final heights.

Audio Help for Problem: 1 || 2 || 3 || 4 || 5 || 6 || 7 || 8 || 9 || 10 || 11 || 12 || 13 || 14 ||

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