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DESCRIBING BASIC TECHNIQUES AND DERIVATIONS. OF INTRODUCTION.
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PHYSICS FOR GRADE 11TH AND 12TH (MODE-1)Energy, Work, and Power
On November 20, 1998, the first piece of the International Space Station lifted off aboard
a Russian proton rocket. This launch marked the dawn of a new era in both space science and
international co-operation. The collaborative expertise of nearly 100 000 people from 16
different countries is making this immense project possible. In all, 46 separate launches will be
required to haul all of the components to this construction site orbiting 400 km above Earth’s
surface.
Hauling these delicate objects into orbit is a technological feat in itself, requiring millions of
tonnes of rocket fuel. Once in orbit, the pieces must be assembled with precision. Much of the
delicate work is performed by a new-generation Cana arm called the “Special Purpose
Dexterous Manipulator”. Larger jobs require the 17 m Cana arm. Both “arms” are part of the
Canadian Mobile Services System that is permanently attached to the Space Station.
Once the station is assembled, what will keep the robotic arms functioning, the air filters
operating, and the inhabitants cozy and warm? The answers to these questions lie in our ability
to understand and then manipulate energy. You will begin to appreciate the real significance
and accomplishment that the International Space Station represents as you learn to see the
world as a physicist does. Understanding energy is the first step to understand- ing the
technological world in which you live.
Work and Mechanical Energy
Football flies through the air. The kicker’s foot has just done work on the football, causing it to
seemingly defy gravity as
it soars high above the field. The kick returner anxiously wait as the ball falls faster and faster
toward his arms. The opposing team bears down on him as rapidly as the ball descends.
Catching the ball, he runs about six yards. Then you hear the clash of helmets as the opposing
tackles bring him to the ground. Although you cannot actually see the energy that has been
transferred to the ball and among the players, you most certainly can see and even hear its
effects. By simply using your own five senses, you can witness the effects of energy and energy
transformations.
Although it may not be obvious, every object you see has some form of energy. When you
observe people walking, curtains blowing in the breeze, a jet plane in the distance, or hear the
quiet humming of a computer fan, you are detecting evidence of energy transformations. In this
chapter, you will learn to understand and describe, both conceptually and mathematically,
some important types of energy transformations
,Work and Energy
Each morning, people throughout world perform the same basic activities as they prepare for
the day ahead. The ritual might begin by swatting the alarm clock, turning on a light, and
heading for a warm shower. Following a quick breakfast of food taken from the refrigerator,
they hurry on their way, travelling by family car, bus, subway, train, bicycle, or on foot. This
ritual, repeated the world over, demands energy. Electrical energy sounds the alarm, lights the
hallways, heats the water, and refrigerates and then cooks the food. Fossil fuels provide the
energy for the engines that propel our vehicles. Energy is involved in everything that happens
and, in fact, is the reason that everything can happen.
Types of Energy
Physicists classify energy into two fundamental types — kinetic energy (the energy of motion)
and potential energy (energy that is stored). The many different forms of energy, such as light
energy, electrical energy, and sound energy that you will study in this and other units, all fit into
one of these two categories. In this chapter, you will focus on one form of energy called
mechanical energy.
The mechanical energy of an object is a combination of kinetic energy and potential energy. For
example, the football in the photo- graph on page 194 has kinetic energy because it is moving.
It also has potential energy because it is high in the air. The force of gravity acts on the ball,
causing it to fall. As it falls, its speed increases and it gains kinetic energy. The best way to begin
to understand energy is to study the relationship between energy and work.
Defining Work
If you have ever helped someone to move, you will understand that lifting heavy boxes or
sliding furniture along a rough floor or carpet requires a lot of energy and is hard work. You may
also feel that solving difficult physics problems requires energy and is also hard work. These
two activities require very different types of work and are examples of how, in science, we need
to be very precise about the terminology we use.
In physics, a force does work on an object if it causes the object to move. Work is always done
on an object and results in a change in the object. Work is not energy itself, but rather it is a
transfer of mechanical energy. A pitcher does work on a softball when she throws it. A bicycle
rider does work on the pedals, which then cause the bicycle to move along the road. You do
work on your physics textbook each day when you lift it into your locker. Each of these
examples demonstrates the two essential elements of work as defined in physics. There is
always a force acting on an object, causing the object to move a certain distance.
You know from experience that it takes more work to move a heavy table than to move a light
chair. It also takes more work to move the table to a friend’s house than to move it to the other
side of the room. In fact, the amount of work depends directly on the magnitude of the force
and the displacement of the object along the line of the force.
,The derived unit of work, or newton meter (N · m), is called a joule (J). One joule of work is
accomplished by exerting exactly one newton of force on an object, causing it to move exactly
.
The definition for work applies to an individual force, not the net force, acting on an object. As
shown in Figure 5.3, two forces are acting on the box. Both forces — the applied force and the
force of friction — are doing work. You can calculate the work done by the applied force or the.
When Work Done Is Zero
Physicists define work very precisely. Work done on an object is calculated by multiplying the
force times the displacement of the object when the two vectors are parallel. This very precise
definition of work can be illustrated by considering three cases where intuition suggests that
work has been done, but in reality, it has not.
, Case 1: Applying a Force That Does Not Cause Motion
Consider the energy that you could expend trying to move a house. Although you are pushing
on the house with a great deal of force, it does not move. Therefore, the work done on the
house, according to the equation for work, is zero Figure. In this case, your muscles feel as
though they did work; however, they did no work on the house. The work equation describes
work done by a force that moves the object on which the force is
applied. Recall that work is a transfer of energy to an object. In this example, the condition of
the house has not changed; therefore, no work could have been done on the house.
Case 2: Uniform Motion in the Absence of a Force
Recall from Chapter 4 that Newton’s first law of motion predicts that an object in motion will
continue in motion unless acted on by an external force. A hockey puck sliding on a frictionless
sur- face at constant speed is moving and yet the work done is still zero (see Figure 5.5). Work
was done to start the puck moving, but because the surface is frictionless, a force is not
required to keep it moving; therefore, no work is done on the puck to keep it moving.
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