Transcript Chapter 8 Notes

Chapter 8 Notes
Applied Physics
Energy Unit
Introduction
Energy
______________
is the most
underlying concept for all sciences.
It was not recognized as a factor
until around ______________,
even
1850
though it had been there all along.
Though it is not usually directly
measured, the ______________
are
effects
detectible. The unit for energy is the
______________
(J), named after a
Joule
British scientist, James Prescott
Joule (1818-1889).
Work (8.1)
Push a cart over a long distance and you get
tired. This is because you exerted
______________
in the process. This particular
force
energy
spending of ______________
can be called
doing ______________.
In order to do this, you
work
must exert a ______________
over a
force
distance
______________.
Thus, the equation for work
is as follows:
W  Fd
Distance (m)
Work (J)
Force (N)
Work Example
Example 1: How much work is required to
push a box a distance of 7m across the
floor if you need to exert 14N of force?
W  Fd
W  14 N  7m
W  98J
Power (8.2)
A log can be sawed in two ways, by using a
hand saw or by using a chainsaw. In each case
the ______________
amount of work has been
same
done. However, the chainsaw used more
power
Power
______________.
______________
is the rate
at which work is done. In other words, it takes
more
______________
power to do a job faster.
Watts
Power is measured in ______________
(W),
named after another British Scientist. The
equation for power is as follows:
Work(J)
W
P
Power(W)
t
Time(s)
Sample Power Problem
Example 2: How much power does it take
for a truck expending 660J of work to drag
a stump in 6s?
W
P
t
660 J
P
6s
P  110W
Sample Power Problem
Example 3: How much power does it take
a bulldozer to push a concrete slab if
5000N of force is exerted over a distance
of 20m in 40s?
W
W  Fd
P
W   5000 N  20m
W  100, 000 J
t
100, 000 J
P
40 s
P  2500W
Mechanical Energy (8.3)
When ______________
has been acquired
energy
work
enabling a system to do ______________,
this
is called ______________
______________.
mechanical
energy
For our purposes, mechanical energy will be
broken into two categories: ______________
potential
energy and ______________
energy. These
kinetic
two important energies relate to
______________
and ______________
motion
position
respectively.
Potential Energy (8.4)
______________
energy is the type of mechanical
Potential
energy that relates to position, more particularly, how
______________
an object is (above the ground). An
high
object on the ground will have a potential energy of
zero
______________.
An object’s potential energy in
Joules
______________
(J) can be calculated using the
equation shown below.
GPE  mgh
Height (m)
Potential Energy (J)
Mass (kg)
Gravity (10m/s2)
Sample Potential Energy Problem
Example 4: What is the potential energy of
a medicine ball (m = 4kg) placed on a high
shelf that is 2m high.
GPE  mgh
GPE   4kg  10 sm2
GPE  80J

  2m 
Kinetic Energy (8.5)
Kinetic
______________
energy is the type of
mechanical energy that relates to motion. An
object at rest will have a kinetic energy of
zero
______________.
The kinetic energy of an
object in ______________
(J) can be calculated
Joules
using the equation shown below.
KE  mv
1
2
Kinetic Energy (J)
2
Velocity (m/s)
Mass (kg)
Kinetic Energy Sample Problems
Example 5: What is the kinetic energy of a
1kg rabbit running at 6.0 ms ?
KE  12 mv 2
m 2
1
KE  2 1.0kg   6.0 s 
KE  18J
Example 6: What is the kinetic energy of
the rabbit if it doubles its speed to 12.0 ms ?
KE  12 mv 2
KE 
1
2
1.0kg  12.0 
KE  72J
m 2
s
*Notice that a doubling
quadruples
of speed ___________
kinetic energy.
Conservation of Energy (8.6)
Just, as momentum is conserved in a closed isolated
system, so is ______________.
Thus, the total energy
energy
remains ______________.
However, energy may
constant
change ______________
between kinetic and
form
______________.
The energy conservation equation is
potential
shown below.
KE1  GPE1  Wo  KE2  GPE2
The Wo term refers to any ______________
done on the
work
system(+) or by the system(-).
An example of positive work would be that done by an
______________
or ______________.
Here energy is
engine
muscle
______________
to the system.
added
An example of negative work would be that done by
______________.
friction
removed
Here energy is ______________
from the system.
Sample Energy Conservation Problem
Example 7 (No Work Present): A ball (m = 0.2kg) is dropped from a height of 5m and
allowed to fall the full distance. Assume no friction losses.
a. What is the kinetic energy of the ball the instant it was released?
KE  12 mv 2
KE 
1
2
 0.2kg   0 
m 2
s
KE  0J
b. What is the potential energy of the ball the instant it was released?
GPE  mgh

GPE   0.2kg  10 sm2
 5m
GPE  10J
c. What is the potential energy of the ball the instant before contacting the ground?
GPE  mgh

GPE   0.2kg  10 sm2
  0m
GPE  0 J (At Ground)
d. What is the kinetic energy of the ball the instant before contacting the ground?
0
0
0
KE1  GPE1  Wo  KE2  GPE2
0  10 J  0  KE2  0
KE2  10 J
Sample Energy Conservation Problem
Example 8 (Work Present): A dog drags a sled (m = 10kg) from level ground (h1 =
0) to the top of a small hill (h2 = 13m). The dog was moving at a velocity of at the
bottom of the hill, but slowed down to a velocity of upon reaching the top.
a. What is the kinetic energy of the sled at the bottom of the hill?
KE  mv
1
2
2
KE 
1
2
10kg   2 
KE  20J
m 2
s
b. What is the potential energy of the at the bottom of the hill?
GPE  mgh

GPE  10kg  10 sm2
  0m 
GPE  0 J (At Hill Bottom)
c. What is the kinetic energy of the sled at the top of the hill?
KE  mv
1
2
2
KE 
1
2
10kg  1 
m 2
s
KE  5J
d. What is the potential energy of the sled at the top of the hill?
GPE  mgh


GPE  10kg  10 sm2 13m 
GPE  1300J
Sample Energy Conservation Problem (cont)
Example 8 (Work Present): A dog drags a sled (m = 10kg) from level
ground (h1 = 0) to the top of a small hill (h2 = 13m). The dog was moving
at a velocity of at the bottom of the hill, but slowed down to a velocity of
upon reaching the top.
e. What is the work done by the dog to
get the sled to the top of the hill?
0
KE1  GPE1  Wo  KE2  GPE2
20 J  0  WO  5J  1300 J
WO  1285J
Machines (8.7)
A ______________
can be defined as any device that
machine
either ______________
forces, changes the
multiples
______________
of forces, or both. Though machines
direction
may do these things, they do not ______________
create
energy because there is always a ______________
tradeoff
force
distance
between ______________
and ______________.
(Remember W = Fd)
The amount of times that a machine multiplies force is
mechanical
advantage
called the ______________
______________.
Mechanical
Advantage
FO
MA 
FI
Output Force (N)
Input Force (N)
Mechanical Advantage Example
Example 9: A wedge (another machine) provides
a force of 400N when only 50N is applied. What
is its mechanical advantage?
FO
MA 
FI
400 N
MA 
50 N
MA  8
Levers
The simplest machine we discuss is the
______________.
It always consists of three parts: the
lever
______________
(pivot point), the ______________
fulcrum
effort
(input force), and the ______________
(output force).
load
Because there are three parts to a lever, there are
______________
configurations.
three
Type I (1st Class): Fulcrum in Center
Examples: see saw pliers
scissors
Type II (2nd Class): Load in Center
Examples: wheel barrow nutcracker
Type III (3rd Class): Effort in Center
Examples: baseball bat
L
E
F
L
F
E
L
F
E
Pullies
Machines also include the ______________,
pulley
a grooved
wheel connected to a mounting. Pulleys can be used to
multiply
redirect
______________
or ______________
depending on
how they are placed. Mechanical advantage can be
found by simply counting the number of supporting
______________.
strings
See the examples below.
3
MA  ___
2
MA  ___
2
MA  ___
1
MA  ___
Efficiency (8.8)
friction
Due to the presence of ______________,
machines to
not convert all of the input work (WI) into useful output
work (WO). Because of this, it is important to find out
efficiency
how efficient a machine is. The ______________
of a
machine is the ______________
of the work output (WO)
ratio
to the work input (WI). This is then converted into a
percentage for convenience.
Work Output (J)
Efficiency
WO
eff 
 100  ___ %
WI
Work Input (J)
Efficiency Example 1
Example 10 (Work Given): A pulley system is set up to lift
a box. The work done by the pulley on the box (WO) was
120J. The work done on the pulley system by the
person (WI) was 130J. What is the efficiency of the
pulley system?
WO
eff 
100
WI
120 J
eff 
 100
130 J
eff  92.3%
Efficiency Example 2
Example 11 (Work To Be Found): A pulley system
connected to a tree is set up to pull a truck out of the
mud. The pullers pull on the rope with a force of 340N a
distance of 15m. The pulley system pulls on the truck
with a force of 900N and moves 5m.
WI  FI d I
WI   340 N 15m
WI  5100 J
WO  FO dO
WO   900 N  5m
WO  4500 J
WO
4500 J
eff  88.2%
eff 
 100
eff 
100
5100 J
WI