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# person b does twice the work of person a, and in one-half of the time . how does the power output of person b compare to person a?

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## Physics 1 Chapter 7 concepts Flashcards

Start studying Physics 1 Chapter 7 concepts. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

## Physics 1 Chapter 7 concepts

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A graph depicts force versus position. What represents the work done by the force over the given displacement?

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The work done is equal to the area under the curve.

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If the net work done on an object is zero, what can you determine about the object's kinetic energy?

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The object's kinetic energy remains the same.

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### Terms in this set (8)

A graph depicts force versus position. What represents the work done by the force over the given displacement?

The work done is equal to the area under the curve.

If the net work done on an object is zero, what can you determine about the object's kinetic energy?

The object's kinetic energy remains the same.

If the net work done on an object is positive, what can you conclude about the object's motion? Assume the object is in the absence of any other forces or friction.

The object is speeding up.

A person applies a 50 N force on a crate, causing it to move horizontally at a constant speed through a distance of 10 m. What is the net work done on the crate?

The net work done on the crate is zero joules.

A ball is thrown straight upward. How does the sign of the work done by gravity while the ball is traveling upward compare with the sign of the work done by gravity while the ball is traveling downward?

Work done by gravity is negative while the ball is traveling upward and positive while the ball is traveling downward.

An object weighing 20 N moves horizontally toward the right a distance of 5.0 m. What is the work done on the object by the force of gravity?

The work done on the object by the force of gravity is zero joules.

Person B does twice the work of person A, and in one-half of the time . How does the power output of person B compare to person A?

Person B has four times the power output of person A.

Which of the following statements is/are true?

A person is limited in the total work he or she can do by the rate at which energy can be transformed.

Power is the rate at which energy is transformed.

Power is the rate at which work is done.

The SI unit of power is the watt.

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Source : quizlet.com

## Work, Energy, and Power in Humans

### LEARNING OBJECTIVES

By the end of this section, you will be able to:

Explain the human body’s consumption of energy when at rest vs. when engaged in activities that do useful work.

Calculate the conversion of chemical energy in food into useful work.

## Energy Conversion in Humans

Figure 1. Energy consumed by humans is converted to work, thermal energy, and stored fat. By far the largest fraction goes to thermal energy, although the fraction varies depending on the type of physical activity.

Our own bodies, like all living organisms, are energy conversion machines. Conservation of energy implies that the chemical energy stored in food is converted into work, thermal energy, and/or stored as chemical energy in fatty tissue. (See Figure 1.) The fraction going into each form depends both on how much we eat and on our level of physical activity. If we eat more than is needed to do work and stay warm, the remainder goes into body fat.

## Power Consumed at Rest

The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate. The total energy conversion rate of a person at rest is called the basal metabolic rate (BMR) and is divided among various systems in the body, as shown in Table 1. The largest fraction goes to the liver and spleen, with the brain coming next. Of course, during vigorous exercise, the energy consumption of the skeletal muscles and heart increase markedly. About 75% of the calories burned in a day go into these basic functions. The BMR is a function of age, gender, total body weight, and amount of muscle mass (which burns more calories than body fat). Athletes have a greater BMR due to this last factor.

Table 1. Basal Metabolic Rates (BMR)

Organ Power consumed at rest (W) Oxygen consumption (mL/min) Percent of BMR

Liver & spleen 23 67 27

Brain 16 47 19

Skeletal muscle 15 45 18

Kidney 9 26 10 Heart 6 17 7 Other 16 48 19

Totals 85 W 250 mL/min 100%

Energy consumption is directly proportional to oxygen consumption because the digestive process is basically one of oxidizing food. We can measure the energy people use during various activities by measuring their oxygen use. (See Figure 2.) Approximately 20 kJ of energy are produced for each liter of oxygen consumed, independent of the type of food. Table 2 shows energy and oxygen consumption rates (power expended) for a variety of activities.

## Power of Doing Useful Work

Work done by a person is sometimes called useful work, which is work done on the outside world, such as lifting weights. Useful work requires a force exerted through a distance on the outside world, and so it excludes internal work, such as that done by the heart when pumping blood. Useful work does include that done in climbing stairs or accelerating to a full run, because these are accomplished by exerting forces on the outside world. Forces exerted by the body are nonconservative, so that they can change the mechanical energy (KE + PE) of the system worked upon, and this is often the goal. A baseball player throwing a ball, for example, increases both the ball’s kinetic and potential energy.

If a person needs more energy than they consume, such as when doing vigorous work, the body must draw upon the chemical energy stored in fat. So exercise can be helpful in losing fat. However, the amount of exercise needed to produce a loss in fat, or to burn off extra calories consumed that day, can be large, as Example 1 illustrates.

### EXAMPLE 1. CALCULATING WEIGHT LOSS FROM EXERCISING

If a person who normally requires an average of 12,000 kJ (3000 kcal) of food energy per day consumes 13,000 kJ per day, he will steadily gain weight. How much bicycling per day is required to work off this extra 1000 kJ?

### Solution

Table 2 states that 400 W are used when cycling at a moderate speed. The time required to work off 1000 kJ at this rate is then

Time = energy ( energy time ) = 1000 kJ 400 W = 2500 s = 42 min

Time=energy(energytime)=1000 kJ400 W=2500 s=42 min

### Discussion

If this person uses more energy than he or she consumes, the person’s body will obtain the needed energy by metabolizing body fat. If the person uses 13,000 kJ but consumes only 12,000 kJ, then the amount of fat loss will be

Fat loss = ( 1000 kJ ) ( 1.0 g fat 39 kJ ) = 26 g

Fat loss=(1000 kJ)(1.0 g fat39 kJ)=26 g

,

assuming the energy content of fat to be 39 kJ/g.

Figure 2. A pulse oxymeter is an apparatus that measures the amount of oxygen in blood. Oxymeters can be used to determine a person’s metabolic rate, which is the rate at which food energy is converted to another form. Such measurements can indicate the level of athletic conditioning as well as certain medical problems. (credit: UusiAjaja, Wikimedia Commons)

Source : courses.lumenlearning.com

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James 11 month ago

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