Table of Contents Work and Energy Section 1 • Work and Machines Section 2 • Describing Energy Section 3 • Conservation of Energy

What is work? To many people, the word work means something they do to earn money. The word work also means exerting a force with your muscles.

What is work? Someone might say they have done work when they push as hard as they can against a wall that doesn't move. However, in science the word work is used in a different way.

What is work? Remember that a force is a push or a pull. Work requires both force and motion. Work is force applied through a distance. If you push against the desk and nothing moves, then you haven't done any work.

Doing work There are two conditions that have to be satisfied for work to be done on an object. One is that the applied force must make the object move, and the other is that the movement must be in the same direction as the applied force.

Doing work For example, when you lift a stack of books, your arms apply a force upward and the books move upward. Because the force and distance are in the same direction, your arms have done work on the books.

Force and Direction of Motion When you carry books while walking, you might think that your arms are doing work. However, in this case, the force exerted by your arms does no work on the books.

Force and Direction of Motion The force exerted by your arms on the books is upward, but the books are moving horizontally. The force you exert is at right angles to the direction the books are moving.

Calculating Work The amount of work done depends on the amount of force exerted and the distance over which the force is applied. When a force is exerted and an object moves in the direction of the force, the amount of work done can be calculated as follows.

Calculating Work In this equation, force is measured in newtons and distance is measured in meters. Work, like energy, is measured in joules. One joule is about the amount of work required to lift a baseball a vertical distance of 0.7 m.

When is work done? Suppose you give a book a push and it slides along a table for a distance of 1 m before it comes to a stop. Even though the book moved 1 m, you do work on the book only while your hand is in contact with it.

Machines A machine is a device that change the force or increase the motion from work. Machines can be simple. Some, like knives, scissors, and doorknobs, are used everyday to make doing work easier. Photodisc/PunchStock

Making Work Easier Some machines, such as bicycles, increase speed. Some machines, such as an axe, change the direction of force. Some machines, such as a car jack, increase force.

Types of Simple Machines A simple machine is a machine that does work with only one movement of the machine. There are six types of simple machines: lever, pulley, wheel and axle, inclined plane, screw and wedge. The pulley and the wheel and axle are modified levers. The screw and the wedge are modified inclined planes.

Efficiency Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine. Every machine is less than 100% efficient.

Calculating Efficiency To calculate the efficiency of a machine, the output work is divided by the input work. Efficiency is usually expressed as a percentage by this equation:

Increasing Efficiency Machines can be made more efficient by reducing friction. This usually is done by adding a lubricant, such as oil or grease, to surfaces that rub together. A lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily.

Mechanical Advantage Two forces are involved when a machine is used to do work. The force that is applied to the machine is called the input force. Fin stands for the effort force. The force applied by the machine is called the output force, symbolized by Fout.

Mechanical Advantage The ratio of the output force to the input force is the mechanical advantage of a machine. The mechanical advantage of a machine can be calculated from the following equation.

Mechanical Advantage Window blinds are a machine that changes the direction of an input force. A downward pull on the cord is changed to an upward force on the blinds.

Mechanical Advantage The input and output forces are equal, so the MA is 1.

Section Check Question 1 __________ is force applied through a distance. A. Conversion B. Energization C. Power D. Work

Section Check Answer The answer is D. In order for work to be done, a force must be applied through a distance.

Section Check Question 2 The amount of work done depends on what two things? Answer The amount of work done depends on the amount of force exerted and the distance over which the force is applied.

Section Check Question 3 What do a knife, a doorknob, and a car jack have in common? Answer These are all machines, because they are devices that make doing work easier.

Section Check Question 4 What is the effect of increasing a machine’s efficiency? Answer Increasing efficiency increases the ratio of output work to input work.

What is energy? Wherever you are sitting as you read this, changes are taking place—lightbulbs are heating the air around them, the wind might be rustling leaves, or sunlight might be glaring off a nearby window. Every change that occurs—large or small—involves energy.

Change Requires Energy When something is able to change its environment or itself, it has energy. Energy is the ability to cause change. Anything that causes change must have energy. You use energy to arrange your hair to look the way you want it to. You also use energy when you walk down the halls of your school between classes or eat your lunch.

Work Transfer Energy Energy can also be described as the ability to do work. Therefore, energy can be measured with the same units as work. Energy, like work, can be measured in joules.

Systems It is useful to think of systems when describing energy. A system is anything that you can imagine a boundary around. A system can be a single object, such as a baseball, or a group of objects, such as the solar system.

Different Forms of Energy Energy has several different forms. Electrical, chemical, radiant, and thermal are examples. Is the chemical energy from food the same as the energy that comes from the Sun or the energy from gasoline? Radiant energy from the Sun travels a vast distance through space to Earth, warming the planet and providing energy that enables green plants to grow.

An Energy Analogy If you have $100, you could store it in a variety of forms—cash in your wallet, a bank account, travelers’ checks, or gold or silver coins. You could convert that money into different forms.

An Energy Analogy You could deposit your cash into a bank account or trade the cash for gold. Regardless of its form, money is money. The same is true for energy. Energy from the Sun that warms you and energy from the food that you eat are only different forms of the same thing.

Kinetic Energy An object in motion does have energy. Kinetic energy is the energy a moving object has because of its motion. The kinetic energy of a moving object depends on the object’s mass and its speed.

Potential Energy Even motionless objects can have energy. This energy is stored energy. A hanging apple in a tree has stored energy. Pixtal/age fotostock

Potential Energy Stored energy due to the interactions between objects is potential energy. If the apple stays in the tree, the energy will remain stored. Pixtal/age fotostock

Potential Energy If the apple falls, that stored energy is converted to kinetic energy. Pixtal/age fotostock

Elastic Potential Energy If you stretch a rubber band and let it go, it sails across the room. As it flies through the air, it has kinetic energy due to its motion. Where did this kinetic energy come from?

Elastic Potential Energy The stretched rubber band had energy stored as elastic potential energy. Elastic potential energy is energy stored by something that can stretch or compress, such as a rubber band or spring.

Chemical Potential Energy Gasoline, food, and other substances have chemical potential energy. Energy stored due to chemical bonds is chemical potential energy.

Chemical Potential Energy Energy is stored due to the bonds that hold the atoms together and is released when the gas is burned. In this chemical reaction, chemical potential energy is released.

Gravitational Potential Energy Together, an object near Earth and Earth itself have gravitational potential energy. Gravitational potential energy (GPE) is energy due to gravitational forces between objects.

Gravitational Potential Energy Gravitational potential energy can be calculated from the following equation. Near Earth’s surface, gravity is 9.8 N/kg. Like all forms of energy, gravitational potential energy can be measured in joules.

Changing GPE According to the equation for gravitational potential energy, the GPE of an Earth system can be increased by increasing the object’s height. Gravitational potential energy also increases if the mass of the object increases.

Section Check Question 1 Energy is the ability to cause __________. A. change B. heat C. motion D. work

Section Check Answer The answer is A. Energy is the ability to cause change and has several different forms.

Section Check Question 2 What are four different forms of energy? Answer Answers will vary. Different forms of energy include electrical, chemical, radiant, thermal, nuclear, mechanical, potential, and gravitational.

Section Check Question 3 The kinetic energy due to an object’s motion depends only on __________. A. the object’s mass and speed B. the object’s mass C. the object’s speed D. the acceleration of the object

Section Check Answer The answer is A. Kinetic energy depends on both the mass and speed of the moving object.