Transcript Energy, Work, and Power

Unit 3
ENERGY, WORK, AND POWER
TYPES OF ENERGY
Energy is the capacity for an object to do work
 For example, when a car moves, the engine
performs work to get the car going.
 There are many different types of energy,
including: electrical, kinetic, gravitational
potential, and elastic potential to name a few.
 A more complete list can be found on p. 124

ENERGY TRANSFORMATION
An energy transformation occurs whenever
energy changes from one form into another.
 Examples of this would be a ball being held
above the ground (gravitational potential) and
then being released to fall to the ground
(kinetic).

WORK
WORK
This is the energy transferred to an object
 The object must move a distance as a result of
the force applied
 Does it matter what direction the object
moves??

HOW TO CALCULATE WORK
Work requires a force
 Work requires a distance
 This leads us to say: WαF and WαΔd
 This gives us: W = F Δd
 The units are Newton Meters (Nm) or, more
commonly, Joules (J)

EXAMPLES

A 600 N force is applied by a person to a
dresser that moves 2 m. Find the work done if
the force and the displacement are
 Parallel
 At
right angles
 Oppositely directed

A horse pulls a barge along a canal with a rope
in which the tension is 1000N. The rope is at
an angle of 10° with the towpath and the
direction of the barge
 How
much work is done by the horse in pulling the
barge 100m?
 What is the net force on the barge?
REMEMBER!!!!

For there to be work,
Force and Distance
must be in the same
plane.
POSITIVE AND NEGATIVE WORK
Any force applied in the same plane causes
work to be done
 If the force makes the object increase in speed,
then it is positive work
 If the force makes the object slow its speed,
then it is negative work. These forces are called
Dissipative Forces
 All friction is negative work.

GRAVITY
When we lift something up, we do work, why is
this?
 When we look at this type of work, we still must
look at the force we are working with

 Fg

= mg
This lead to the following
W
= Fgd
 W = mgd
EXAMPLE

A bag of groceries of mass 8.1 kg is raised
vertically without acceleration from the floor to
a counter top, over a distance of 92 cm.
Determine
 The
force needed to raise the bag without
acceleration.
 The work done on the bag against the force of
gravity
MECHANICAL ENERGY
MECHANICAL ENERGY

There are 2 types of mechanical energy
 Gravitational
Potential Energy
 Kinetic Energy

Gravitational Potential Energy
 This
is energy that can be used to do work at a
lower level

Kinetic Energy
 This
is the energy of motion
DETERMINING POTENTIAL ENERGY
To hit a nail with a hammer, what must you do?
 By lifting the hammer, Δh, you also need to
apply a force.
 The height is measured from a starting point or
equilibrium position.
 The force is found by lifting the mass against
gravity

 Ep
= FΔh
 Ep = mg(ℎ2 -ℎ1 )
EXAMPLE

Assume that a 59 kg
pole vaulter must raise
their center of mass
from 1.1 m off the
ground to 4.6 m off the
ground. What is the
jumper’s gravitational
potential energy at the
top of the bar relative to
where the jumper
started to jump?



Ep = mgΔh
Ep = (59)(9.81)(4.6-1.1)
Ep = 2.0 x 103 J
APPLICATIONS OF MECHANICAL ENERGY
Grain Auger
 Pile Drivers
 Hydro Dams
 We use this in Red Lake everyday

DETERMINING KINETIC ENERGY
If you are interested in how the formula is
generated, see p. 134
 Kinetic energy is the energy of motion, so what
do we need?
 Ek = ½ mv2

EXAMPLE

Determine the amount
of kinetic energy of a 48
g dart travelling at a
speed of 3.4 m/s.



Ek = ½ mv2
Ek = ½ (.048)(3.4)2
Ek = 0.28 J
LAW OF CONSERVATION OF ENERGY
ENERGY CONSERVATION
We know that there are many types of energy
transformations
 When energy changes forms, energy is
conserved
 What does this mean?
 Energy is never lost, it just changes form

EXAMPLE

An object which weighs 10 N is dropped from
rest from a height of 4m above the ground.
When it has free-fallen 1 meter, its total
mechanical energy with respect to the ground
is_______

An archer needs to exert 275 N of force to pull
her bow string back 0.500m. If the mass of the
arrow is 3.00 𝑥10−2 𝑘𝑔, what is the final speed
of the arrow?

A skier glides down a
frictionless hill of 100
meters, the ascends
another hill, of height
90 meters, as shown in
the figure below. What
is the speed of the skier
when it reaches the top
of the second hill?
100m
90 m
EFFICIENCY
A comparison of the amount of energy put into
a system compared to the amount of energy
output in a system
 %E = Eout/Ein

ENERGY INPUT AND OUTPUT
Energy input is the amount of energy that is
being supplied by the person doing the work
 Energy output is the amount of energy that
would be created if it could be vertically
released

POWER
Power is the rate of doing work or transferring
energy
 This is a scalar quantity
 P = W/ΔT
 P = ΔE/ΔT
