With energy the approach is usually a little different. When forces and accelerations are used, you usually freeze the action at a particular instant in time, draw a free-body diagram, set up force equations, figure out accelerations, etc. © Texas Education Agency (TEA).Energy gives us one more tool to use to analyze physical situations. We recommend using aĪuthors: Paul Peter Urone, Roger Hinrichs Use the information below to generate a citation. Then you must include on every digital page view the following attribution: If you are redistributing all or part of this book in a digital format, Then you must include on every physical page the following attribution: If you are redistributing all or part of this book in a print format, Changes were made to the original material, including updates to art, structure, and other content updates. Want to cite, share, or modify this book? This book uses theĪnd you must attribute Texas Education Agency (TEA). If W = f d W = f d and work can be expressed in J, then P = W t = f d t P = W t = f d t so power can be expressed in units of N ⋅ m s N ⋅ m sĪlso explain that we buy electricity in kilowatt-hours because, when power is multiplied by time, the time units cancel, which leaves work or energy. Explain relationships between the units for force, work, and power. End the section by clearing up any misconceptions about the distinctions between force, work, and power. Stress that power is a rate and that rate means "per unit of time." In the metric system this unit is usually seconds. Ask them to provide more examples until they understand the difference between the scientific term work and a task that is simply difficult but not literally work (in the scientific sense). Ask the students to use the mechanical energy equations to explain why each of these is or is not work. Make it clear why holding something off the ground or carrying something over a level surface is not work in the scientific sense. Use the equations for mechanical energy and work to show what is work and what is not. Review the units of work, energy, force, and distance. Review the concept that work changes the energy of an object or system. Power is calculated by dividing the work done by the time it took to do the work. In this case, rate means per unit of time. Recall that a rate can be used to describe a quantity, such as work, over a period of time. Let’s take a look at how to calculate the time it takes to do work. Taking a half hour on the ascent will surely irritate riders and decrease ticket sales. For example, in roller coaster design, the amount of time it takes to lift a roller coaster car to the top of the first hill is an important consideration. In applications that involve work, we are often interested in how fast the work is done. Have the students distinguish between and understand the two ways of increasing the energy of an object (1) applying a horizontal force to increase KE and (2) applying a vertical force to increase PE. Repeat the information on kinetic and potential energy discussed earlier in the section. The force we exert to lift the rock is equal to its weight, w, which is equal to its mass, m, multiplied by acceleration due to gravity, g. If we drop the rock, the force of gravity increases the rock’s kinetic energy as the rock moves downward until it hits the ground. If we apply force to lift a rock off the ground, we increase the rock’s potential energy, PE. Let’s examine how doing work on an object changes the object’s energy. A roller coaster car at the top of a hill has gravitational potential energy. Gravitational potential energy is the stored energy an object has as a result of its position above Earth’s surface (or another object in space). Potential energy, sometimes called stored energy, comes in several forms.Kinetic energy is also called energy of motion.In this chapter we will be concerned with mechanical energy, which comes in two forms: kinetic energy and potential energy. Energy can take a variety of different forms, and one form of energy can transform to another. In fact, energy can be defined as the ability to do work. You (or an object) also expend energy to do work. When you do work to move an object, you change the object’s energy. Force is measured in newtons and distance in meters, so joules are equivalent to newton-meters ( N ⋅ m ) ( N ⋅ m ) Work is measured in joules and W = f d W = f d. Explain that, when this theorem is applied to an object that is initially at rest and then accelerates, the 1 2 m v 1 2 1 2 m v 1 2 term equals zero.
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