Chapter 10.1: Work and Power Energy Transformations and the Work-Energy Theorem
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Chapter 10.1: Work and
Power
Energy Transformations and the
Work-Energy Theorem
Energy
The meanings of work and energy are different in
Physics than in the rest of the world.
Energy is described as the ability to cause change
in an object or its environment.
Energy can take many forms including thermal,
chemical, mechanical (kinetic), nuclear, electric,
and potential.
Energy of Motion
If the position of a moving object changes
over time, this indicates the object has
energy.
Energy of motion is called kinetic energy.
Using Newton’s 2nd Law of motion F=ma
and recalling that v2 = v02 + 2ad, we can
see that ½ mv2 – ½ mv02 = Fd to describe
the motion of an object.
Energy of motion, K, is ½ mv2
Work
Mass and velocity are properties of a system
and can be described by its kinetic energy.
When an agent of the environment (force)
causes a change in the system (d), WORK is
done and the energy of the system is changed.
Work = Force x distance (W = F x d)
In order for work to be done, the object must
move in the direction of the force applied.
Work – Energy Theorem
When work is done on an object, there is a
change in Kinetic Energy.
and
Since the relationship between work and energy
was found by James Prescott Joule, the unit of
energy is called a joule.
If a 2-kg object moves at 1m/s it has a KE of
1kg-m2/s2 or 1 J.
WK FdKK 01
Energy Transfer
The work done ON an object by environment, changes the amount of energy the object has. Work is positive, energy increases.
The work done BY an object on the environment, changes the amount of energy the object has. Work is negative, energy decreases.
Energy transfer between a system and the environment can go both ways.
Environment System
(work)
Calculating Work
Work = Force x distance, for constant
forces in the direction of the motion.
What if the force is perpendicular to the
motion like planets around the sun?
If orbits are circular… the perpendicular
force doesn’t change speed, just direction.
So, velocity is constant and KE is constant
and since K1-K0 = W, then W = 0.
If F and d are at right angles, W = 0.
Units: Work and Energy
Since work done = change in energy and
work is measured in Joules, J,… then energy
is also measured in Joules, J.
1J = 1N x 1m
If you lift an apple (1N) a distance of 1m, you
do 1J of work and give the apple 1J of
increased energy.
Working with anglesA force exerted in the direction of an object’s
motion does work W = F x d.
A force exerted at right angles to an object’s
motion does NO work.
Any force at an angle is replaced by
components of the force Fy and Fx
Use the law of cosines to solve for the desired
motion component, Fx = F cosθ.
If displacement is in x-direction, work done is
only in x-direction. W = Fcosθ d
What about other agents?In mowing the lawn, earth’s gravity acts
downward and the ground exerts a normal force
upward.
Friction exerts a horizontal force opposite the
motion.
The up and down forces having θ = 90º, making
cosθ = 0 and no work is done.
The friction force acts at 180º to the motion,
making cosθ = -1 so friction is negative and
decreases the energy of the mower.
Changing forces over time
Not all forces are constant.
A force vs displacement graph shows work
done.
Area under the curve shows work done on
a F-d graph EVEN if the force changes…like
a spring on an object.
“Watt” about Power?The work done by a person lifting a twinkie up a
set of stairs is the SAME regardless of how fast
they do it.
POWER is the RATE of doing WORK.
Power is measured in Watts. 1W= 1J of energy
transferred in one second.
A watt is relatively small. Most power is
measured in kilowatts (kW). 1kW = 1000W
t
WP
Your turn to Practice
Check the example problems on pgs 226,
228, & 230 for reference if you have
trouble.
Please do Practice Problems #1,2,3,4 pg
227
Please do Practice Probs # 7 & 8 pg 229
Please do Ch 10 Review pg 241 #s 1-5,&8
Please do Ch 10 Rev pg 242 #s
19,21,22,27,28,& 34.
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