Download ch10-Energy [Repaired]

12/12/2011
Brick Problem
Energy
How would you
do this
problem?
Phy 121
Types of Energy
• Kinetic Energy
K=
An object is moving
• Gravitational Potential Energy
An object is at some elevation
• Spring Potential Energy
A spring is compressed or
stretched.
Work
1 2
mv
2
U g = mgh
U g = mgy
Us =
An 18.0 kg load of bricks is
pulled at constant speed by
a rope inclined at 20.0°
above the horizontal. It
moves 20.0 m on a
horizontal surface. The
coefficient of kinetic friction
between the sledge and
surface is 0.500. What is
the Tension in the rope?
1 2
kx
2
Calculating Work Done
• Find Work by Tension force of 20 N at 30º when
the sled moves to the right 10 m.
Tll
• Work causes the
energy of a system to
change.
Work = Fll ⋅ displacement
• Definition:
Component of F parallel
to the displacement
times the
displacement
Parallel
Calculating Work Done
• Find Work by Weight force of 300 N when the
sled moves to the right 10 m.
Wll=0
W = (T cos 30°) • ∆x
W = (20 N) cos 30°(10 m)
Work = (Wll ) • ∆x
W = 17.3 N(10 m)
W = 173 Nm
Work = (0 N )(10 m )
W = 173 J
Work = (mg ll ) • ∆x
Work = 0 J
Figure 5.6, p.92
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12/12/2011
Springs
∆x
Fspring
Energy-Bar Charts
∆x and F
are
opposite
direction
• Identify Initial and Final Situations (+ 0 -)
• Is there work done?
Fspring = −kx
Us =
1 2
kx
2
∆x
– Draw Coordinate, ∆x & Force vectors
– (+ 0 -)?
Energy
Type
Initial
+, 0, -
Between
Final
+, 0, -
K
U grav
∆x
NA
U spring
Work
Energy-Bar Charts
Ug
Usp
W
K
Ug
NA
Brick Problem
• Physics Principles
• Identify initial & final
situations
• Identify types of energy
• Is work done?
• Draw Energy-bar Chart
• Write Work-Energy
Equation
• List other information
• Solve
• Evaluate
• Draw Energy Bar Charts
• Write the Work-Energy Equation
K
NA
Usp
An 18.0 kg load of bricks is
pulled at constant speed by
a rope inclined at 20.0°
above the horizontal. It
moves 20.0 m on a
horizontal surface. The
coefficient of kinetic friction
between the sledge and
surface is 0.500. What is the
Tension in the rope?
Brick Problem
Brick Problem
An 18.0 kg load of bricks is pulled at constant speed by
a rope inclined at 20.0°above the horizontal . It
moves 20.0 m on a horizontal surface. The coefficient
of kinetic friction between the sledge and surface is
0.500. What is the Tension in the rope?
An 18.0 kg load of bricks is pulled at constant speed by
a rope inclined at 20.0°above the horizontal . It
moves 20.0 m on a horizontal surface. The coefficient
of kinetic
between the sledge and surface is
0.500. What is the Tension in the rope?
Wbyrope = Fbyrope _ parallel • ∆x
Wbyfriction = Fbyfriction _ parallel • ∆x
Wbyrope = Tx • ∆x
Wbyrope = + • +
Ty
+
Tx
Wbyfriction = f x • ∆x
Wbyfriction = − • +
+
2
12/12/2011
Brick Problem
Brick Problem
An 18.0 kg load of bricks is pulled at
by a rope
inclined at 20.0°above the horizontal. It moves 20. 0 m on
. The coefficient of kinetic friction between the sledge and
surface is 0.500. What is the Tension in the rope?
An 18.0 kg load of bricks is pulled at
by a rope
inclined at 20.0°above the horizontal. It moves 20. 0 m on
. The coefficient of kinetic friction between the sledge and
surface is 0.500. What is the Tension in the rope?
K
K
Ug
Usp
Wby
Wby
friction rope
K
Ug
Ug
Usp
Usp
Wby
Wby
friction rope
K
Ug
Usp
K i − W friction + Wrope = K f
Brick Problem
Brick Problem
An 18.0 kg load of bricks is pulled
at
by a rope
inclined at 20.0°above the
horizontal. It moves 20.0 m on
.
• Evaluate by checking
the FBD
– So what size are the
forces to make
Constant Speed?
– Does this make sense
with your answers to
the question?
An 18.0 kg load of bricks is pulled at
by a rope
inclined at 20.0°above the horizontal. It moves 20. 0 m on
. The coefficient of kinetic friction between the sledge and
surface is 0.500. What is the Tension in the rope?
K i − W friction + Wrope = K f
N
1
2
Ty
+
− f∆x + Tx ∆x = 0
Tx
Tx ∆x = f∆x
W
Brick Problem
An 18.0 kg load of bricks is pulled at
by a rope
inclined at 20.0°above the horizontal. It moves 20. 0 m on
. The coefficient of kinetic friction between the sledge and
surface is 0.500. What is the Tension in the rope?
∑F
y
Brick Problem
• Evaluate: How many
ways can we do this?
– Compare to a known
value
= ma
n −W = 0
n =W
f = µn
mv 2 − f∆x + Tx ∆x = 12 mv 2
• How do you find known
values?
– Check signs with MD
– Check forces with FBD
– Unit Analysis
• Did you really check?
An 18.0 kg load of bricks is
pulled at constant speed by
a rope inclined at 20.0°
above the horizontal. It
moves 20.0 m on a
horizontal surface. The
coefficient of kinetic friction
between the sledge and
surface is 0.500. What is the
Tension in the rope?
T=94N
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12/12/2011
Physics Principles
Motion
• MDs
• Kinematic
Equations
Energy
Forces
• Work
• Work Energy
Equation
• FBDs
• Newton’s
Laws
Skier Problem
• Physics Principle:
Forces
• FBD
• Newton’s 2nd Law
• Solve
• Evaluate
A skier starts from rest at the
top of a hill that is inclined
10.5°with respect to the
horizontal. The hillside is 200
m long, and the coefficient of
friction between snow and skis
is 0.075 0. At the bottom of the
hill, the snow is level and the
coefficient of friction is
unchanged. How far does the
skier glide along the horizontal
portion of the snow before
coming to rest?
Is It TRUE?
•Alfa Romeo Spider Road Test Data
•W=2645 lb
•Power rated at 115 hp
•Ref: Road & Truck , March 1984
t (s)
0
3.6
11.7
18.3
21.6
v(mph)
0
30
60
74
80
Skier Problem
• Physics Principle: Energy A skier starts from rest at the
top of a hill that is inclined
• Identify initial & final
10.5°with respect to the
situations
• Identify types of energy horizontal. The hillside is 200
m long, and the coefficient of
• Is work done?
friction between snow and skis
• Draw Energy-bar Chart is 0.075 0. At the bottom of the
• Write Work-Energy
hill, the snow is level and the
Equation
coefficient of friction is
• List other information
unchanged. How far does the
• Solve
skier glide along the horizontal
• Evaluate
portion of the snow before
coming to rest?
Power
• Rate of Change of
Position
• Rate of Change of
Velocity
• Rate of Use or
Transfer of Energy
v=
∆x
∆t
a=
∆v
∆t
P=
∆U
∆t
Challenges
• Find the spring constant for the low setting
on the projectile motion cannon.
• Estimate the spring constant for one of your
car’s suspension springs.
• Look up what you pay for your electricity. How
much is 8 hours of running up and down stairs
worth? Want to change jobs?
• How much energy could be generated when 1
kg of water falls over a 100 m water fall.
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