Physics 163

Unit 5: Work, Energy and Power

Problem Set C

Overview:

Problem Set C targets your ability to use the work and energy equation and the expressions for work, potential energy and kinetic energy in order to analyze physical situations. The power of the work and energy concepts is that it allows a person to make predictions about how fast (speed) and how far (distance) an object moves if given some initial known parameters about the object.

In Problem Set B, the goal was to learn to use the work-energy equation to analyze a physical situation. The equation which was used with great regularity was

KEi + PEi + Wnc = KEf + PEf

Problem Set C is an extension of Problem Set B in that the same equation becomes used with regularity. However, there is an extra dimension to Set C which makes it one degree more complex than Set B. In Set B, actual values of KE, PE, and Wnc were frequently given and the problem commonly involved finding an unknown energy or work value. In Set C, values of mass, speed, height, force and displacement are commonly stated and work and energy values will have to be calculated using the following formulas:

 KE = 0.5 • m • v2 PE = m • g • h W = F • d cos

Detailed information about the use of these three equations is provided at The Physics Classroom as assessed by using the links above.

If the expressions for kinetic energy, potential energy and work are substituted into the work-energy equation shown above, then the equation can be re-written as follows.

0.5 • m • vi2 + m • g • h i + F • d • cos = 0.5 • m • vf2 + m • g • h f

This new form of the work-energy equation will be the dominant equation used in Set C.

Analyzing a Physical Situation:

A good deal of time and effort is spent in class analyzing physical situations in terms of work and energy. A conceptual tool which is commonly used in such analyses is the work-energy bar chart. A work-energy bar chart illustrates in a conceptual manner the forms of energy possessed by an object and how those forms change as the object moves from its initial state to its final state. The construction of a work-energy bar chart requires that a student understand the concepts of work and energy and the variables which effect these quantities. This conceptual understanding is crucial to the completion of Problem Set C. Examples and detailed discussions of work-energy bar charts are provided online at The Phyics Classroom.

You will be successful in Set C if you practice the following habits:

1. Read the problem carefully and construct a diagram to assist in visualizing the physical situation.
2. Extract and label explicitly stated numerical information from the problem description, giving attention to units and making proper conversions when necessary.
3. Rely on a strong conceptual understanding to make correct inferences about terms in the work-energy equation which can be neglected because they are either 0 or unchanging.
4. Identify the unknown quantity to be solved for.
5. Make proper substitutions of known numerical values into the work-energy equation.
6. Perform proper algebraic manipulations of the work-energy equation to solve for the unknown quantity.

View Sample Problem Set.

 Problem Description Audio Link 1 Determination of braking force on a car skidding to a stop 2 Determination of final speed for a skiier descending a hill in the presence of resistance forces 3 Determination of final speed of a volleyball spiked from an elevated position 4 Determination of final speed of a baseball moving through the air in the presence of air resistance 5 Determination of final speed of a pendulum ball 6 Determination of final speed of a root beer mug pushed along a countertop 7 Determination of final speed of a ski jumper after moving through the air in the presence of air resistance 8 Determination of the force applied to a baseball to bring it to a stop 9 Determination of the height of a pile of snow to which a force is applied to launch it from a shovel 10 Determination of final speed of a roller coaster car moving through a loop in the presence of air resistance 11 Determination of final speed of a cart which rolls along an incline in the absence of resistance forces 12 Determination of the final height of a Hot Wheels car which moves along an incline in the absence of resistance forces 13 Determination of the final height of a basebal which moves through the air in the absence of resistance forces 14 Determination of the stopping distance of a roller coaster car which is brought to rest by a braking force

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

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