Upload
elinor-bond
View
224
Download
1
Tags:
Embed Size (px)
Citation preview
Chapter 6
Conservation of Energy
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 2
Conservation of Energy
• Work by a Constant Force
• Kinetic Energy
• Potential Energy
• Work by a Variable Force
• Springs and Hooke’s Law
• Conservation of Energy
• Power
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 3
The Law of Conservation of Energy
The total energy of the Universe is unchanged by any physical process.
The three kinds of energy are:
kinetic energy, potential energy, and rest energy.
Energy may be converted from one form to another or transferred between bodies.
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 4
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 5
Work by a Constant Force
Work is an energy transfer by the application of a force.
For work to be done there must be a nonzero displacement.
The unit of work and energy is the joule (J).
1 J = 1 Nm = 1 kg m2/s2.
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 6
Only the force in the direction of the displacement that does work.
An FBD for the box at left:
x
y
Fw
N
The work done by the force F is:
xFrFW xxF cos
rx
Frx
Work - Example
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 7
The work done by the normal force N is: 0NW
The normal force is perpendicular to the displacement.
The work done by gravity (w) is: 0gW
The force of gravity is perpendicular to the displacement.
Work - Example
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 8
The net work done on the box is:
xF
xF
WWWW gNF
cos
00cos
net
Work - Example
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 9
In general, the work done by a force F is defined as
cosrFW
where F is the magnitude of the force, r is the magnitude of the object’s displacement, and is the angle between F and r (drawn tail-to-tail).
Work Done
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 10
Example: A ball is tossed straight up. What is the work done by the force of gravity on the ball as it rises?
FBD for rising ball:
x
y
w
r
ymg
ywWg
180cos
Work - Example
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 11
A box of mass m is towed up a frictionless incline at constant speed. The applied force F is parallel to the incline.
Question: What is the net work done on the box?
F
w
NFx
y
0cos
0sin
wNF
wFF
y
x
Apply Newton’s 2nd Law:
An FBD for
the box:
Inclined Plane-V Constant
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 12
The magnitude of F is: sinmgF
If the box travels along the ramp a distance of x the work by the force F is
sin0cos xmgxFWF
The work by gravity is
sin90cos xmgxwWg
Example continued:
Inclined Plane-V Constant
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 13
Example continued:
The work by the normal force is:
090cos xNWN
The net work done on the box is:
0
0sinsin
net
xmgxmg
WWWW NgF
Inclined Plane-V Constant
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 14
Example: What is the net work done on the box in the previous example if the box is not pulled at constant speed?
sin
sin
wmaF
mawFFx
Proceeding as before:
xFxma
xmgxmgma
WWWW NgF
net
net
0sinsin
Inclined Plane-Acceleration
New Term
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 15
Kinetic Energy
2
2
1mvK is an object’s translational
kinetic energy.
This is the energy an object has because of its state of motion.
It can be shown that, in general
Net Work = Change in K
.net KW
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 16
Example: The extinction of the dinosaurs and the majority of species on Earth in the Cretaceous Period (65 Myr ago) is thought to have been caused by an asteroid striking the Earth near the Yucatan Peninsula. The
resulting ejecta caused widespread global climate change.
If the mass of the asteroid was 1016 kg (diameter in the range of 4-9 miles) and had a speed of 30.0 km/sec, what was the asteroid’s kinetic energy?
J 105.4
m/s 1030kg 102
1
2
1
24
23162
mvK
This is equivalent to ~109 Megatons of TNT.
Kinetic Energy
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 17
Gravitational Potential Energy Part 1- Close to Earth’s Surface
Potential energy is an energy of position.
There are potential energies associated with different forces. Forces that have a potential energy associated
with them are called conservative forces.
Not all forces are conservative, i.e. Friction.
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 18
Gravitational Potential Energy
Every conservative force has an associated potential function.
F is equal in magnitude and opposite in direction of mg. The object is raised a distance Δx doing work against the gravitational field. The change in U is positive
0
f i
f i
W = F Δx = mgΔxcos(0 )
W = ΔU = U - U = mgΔx(+1)
ΔU = U - U = mgΔx
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 19
The change in gravitational potential energy (only near the surface of the Earth) is
ymgU g
where y is the change in the object’s vertical position with respect to some reference point.
You are free to choose to location of this reference point where ever it is convenient.
Gravitational Potential Energy
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 20
The table is 1.0 m tall and the mass of the box is 1.0 kg.
Ques: What is the change in gravitational potential energy of the box if it is placed on the table?
J 8.9m 0m 0.1m/s 8.9kg 0.1 2
ifg yymgymgU
First: Choose the reference level at the floor. U = 0 here.
GPE Problem
U = 0
U = 0
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 21
Example continued:
J 8.9m 0.1 m0.0m/s 8.9kg 1 2
ifg yymgymgU
Now take the reference level (U = 0) to be on top of the table so that yi = 1.0 m and yf = 0.0 m.
The results do not depend on the location of U = 0.
GPE Problem
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 22
Mechanical energy is UKE
The total mechanical energy of a system is conserved whenever nonconservative forces do no work.
That is Ei = Ef or K = U.
Total Mechanical Energy
Then if K increases U decreases and vice versa
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 23
A cart starts from position 4 with v = 0.0 m/s to the left. Find the speed of the cart at positions 1, 2, and 3. Ignore friction.
4 3
4 4 3 3
2 24 3 3
3 4 3
E = E
U + K = U + K
1 1mgy + m(0) = mgy + mv
2 2
v = 2g y - y = 14.0 m/s
Mechanical Energy Problem
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 24
4 2
4 4 2 2
2 24 2 2
2 4 2
E = E
U + K = U + K
1 1mgy + m(0) = mgy + mv
2 2
v = 2g y - y = 9.1 m/s
4 1
4 4 1 1
2 24 1 1
1 4 1
E = E
U + K = U + K
1 1mgy + m(0) = mgy + mv
2 2
v = 2g y - y = 19.8 m/s
Or use E3=E2
Or use E3=E1
E2=E1
Mechanical Energy Problem
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 25
A roller coaster car is about to roll down a track. Ignore friction and air resistance.
40 m 20
m y=0
m = 988 kg
(a) At what speed does the car reach the top of the loop?
m/s 8.192
2
10 2
fif
ffi
ffii
fi
yygv
mvmgymgy
KUKU
EE
Roller Coaster Problem
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 26
(b) What is the force exerted on the car by the track at the top of the loop?
N w
y
x
N 109.2 42
2
2
mgr
vmN
r
vmwN
r
vmmawNF ry
Example continued:
FBD for the car:Apply Newton’s Second Law:
Roller Coaster Problem
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 27
(c) From what minimum height above the bottom of the track can the car be released so that it does not lose contact with the track at the top of the loop?
Example continued:
2min2
10 mvmgymgy
KUKU
EE
fi
ffii
fi
Using conservation of mechanical energy:
g
vyy fi 2
2minSolve for the starting height
Roller Coaster Problem
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 28
Example continued:
v = vmin when N = 0. This means that
grv
r
vmmgw
r
vmwN
min
2min
2
What is vmin?
m 0.255.22
22
2min r
g
grr
g
vyy fi
The initial height must be
Roller Coaster Problem
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 29
What do you do when there are nonconservative forces? For example, if friction is present
fricWEEE if
The work done by friction.
Nonconservative Forces
In the presence of non-conservative forces Total Mechanical Energy is not conserved.
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 30
Gravitational Potential Energy Part 2 - Away from Earth’s Surface
The general expression for gravitational potential energy is:
0 where
21
rUr
MGMrU
It is convenient in this case to assign the zero of PE to a point an infinite distance away.
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 31
Gravitational Potential Energy
0 where
21
rUr
MGMrU
The potential energy can be viewed as the work required to bring a second mass from infinity to its present position r. Since we can a conservative force that has a potential we don’t have to do the force x distance calculation.
Work = ΔU = U r - U r =
where U r = = 0
Work = ΔU = U r
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 32
Example:
What is the gravitational potential energy of a body of mass m on the surface of the Earth?
e
ee R
mGM
r
MGMRrU 21
Gravitational Potential Energy
The negative sign is useful in describing bound states, i.e. A moon bound to an planet, as having a negative energy. A positve energy state would represent a free state, i.e. an unbound moon that had somehow bee freed from its planet..
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 33
A planet of mass m has an elliptical orbit around the Sun. The elliptical nature of the orbit means that the distance between the planet and Sun varies as the planet follows its orbital path. Take the planet to move counterclockwise from its initial location.
QUES: How does the speed of a planet vary as it orbits the Sun once?
The mechanical energy of the planet-sun system is:
constant2
1 sun2 r
GmMmvE
rB A
Planetary Motion
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 34
constant2
1 sun2 r
GmMmvE
Starting from point “B” (where the planet moves the slowest), as the planet moves in its orbit r begins to decrease. As it decreases the planet moves faster.
At point “A” the planet reaches its fastest speed. As the planet moves past point A in its orbit, r begins to increase and the planet
moves slower.
rB A
Planetary Motion
At point “B” the planet is the farthest from the Sun. At point “A” the planet is at its closest approach to the sun.
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 35
Work by a Variable Force
Work can be calculated by finding the area underneath a plot of the applied force in the direction of the displacement versus the displacement.
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 36
x (m)
Fx (N)F3
F2
F1
x3x2x1
The work done by F1 is 0111 xFW
Example: What is the work done by the variable force shown below?
The net work is then W1+W2+W3.
The work done by F2 is 1222 xxFW
The work done by F3 is 2333 xxFW
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 37
By hanging masses on a spring we find that stretch applied force. This is Hooke’s law.
For an ideal spring: Fx = kx
Fx is the magnitude of the force exerted by the free end of the spring, x is the measured stretch of the spring, and k is the spring constant (a constant of proportionality; its units are N/m).
A larger value of k implies a stiffer spring.
Spring Force
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 38
(a) A force of 5.0 N applied to the end of a spring cause the spring to stretch 3.5 cm from its relaxed length.
Ques: How far does a force of 7.0 N cause the same spring to stretch?
For springs Fx. This allows us to write .2
1
2
1
x
x
F
F
Solving for x2: cm. 9.4cm 5.3N 5.0
N 0.71
1
22
x
F
Fx
Spring Force
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 39
Example continued:
N/cm. 43.1cm 3.5
N 0.5
1
1 x
Fk
(b) What is the spring constant of this spring?
N/cm. 43.1cm 4.9
N 0.7
2
2 x
Fk
Or
Spring Force
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 40
An ideal spring has k = 20.0 N/m. What is the amount of work done (by
an external agent) to stretch the spring 0.40 m from its relaxed length?
J 6.1m 4.0N/m 0.202
1
2
1
2
1
curveunder Area
22111
kxxkx
W
Spring Force
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 41
The work done in stretching or compressing a spring transfers energy to the spring.
Below is the equation of the spring potential energy.
2
2
1kxU s
Elastic Potential Energy
The spring is considered the system
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 42
A box of mass 0.25 kg slides along a horizontal, frictionless surface with a speed of 3.0 m/s. The box encounters a spring with k = 200 N/m.
Ques: How far is the spring compressed when the box is brought to rest?
m 11.0
02
1
2
10 22
vk
mx
kxmv
KUKU
EE
ffii
fi
Elastic Potential Energy
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 43
Power
cosFvP
Average Power
The unit of power is the watt. 1 watt = 1 J/s = 1 W.
1 Horsepower (hp) = 550 ft-lbs/s = 745.7 W
Instantaneous Power
Power is the rate of energy transfer.
av
av avg
Energy Work F Δx ΔxP = = = = F
Time Time Δt Δt
P = F v
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 44
A race car with a mass of 500.0 kg completes a quarter-mile (402 m) race in a time of 4.2 s starting from rest. The car’s final speed is 125 m/s. (Neglect friction and air resistance.)
Ques: What is the engine’s average power output?
watts103.921
5
2
av
t
mv
t
K
t
KU
t
EP
f
Power - Car Example
MFMcGraw-PHY 1401 Ch06b - Energy - Revised: 6/25/10 45
Summary
• Conservation of Energy
• Calculation of Work Done by a Constant or Variable Force
• Kinetic Energy
• Potential Energy (gravitational, elastic)
• Power