Chapter 7: Introduction to Work, Energy, and Energy Resources Flashcards
Define basal metabolic rate
the total energy conversion rate of a person at rest
Define chemical energy
the energy in a substance stored in the bonds between atoms and molecules that can be released in a chemical reaction
Define the conservation of mechanical energy.
the rule that the sum of the kinetic energies and potential energies remains constant if only conservative forces act on and within a system
Define conservative force
a force that does the same work for any given initial and final configuration, regardless of the path followed
Define efficiency
a measure of the effectiveness of the input of energy to do work; useful energy or work divided by the total input of energy
Define electrical energy
the energy carried by a flow of charge
Define energy
the ability to do work
Define fossil fuels
Oil, natural gas, coal, etc.
Define friction
the force between surfaces that opposes one sliding on the other; friction changes mechanical energy into thermal energy
Define gravitational potential energy
the energy an object has due to its position in a gravitational field
Define horsepower
An older non-SI unit of power, with 1hp = 746 W
Define kilowatt-hour
unit used primarily for electrical energy provided by electric utility companies (KWh)
Define kinetic energy:
the energy an object has by reason of its motion, equal to1/2mv^2for the translational (i.e., non-rotational) motion of an object of massm moving at speedv
Define law of conservation of energy
the general law that total energy is constant in any process; energy may change in form or be transferred from one system to another, but the total remains the same
Define mechanical energy
the sum of kinetic energy and potential energy
Define metabolic rate
the rate at which the body uses food energy to sustain life and to do different activities
Define net work
work done by the net force, or vector sum of all the forces, acting on an object
Define nonconservative force
a force whose work depends on the path followed between the given initial and final configurations
Define nuclear energy
energy released by changes within atomic nuclei, such as the fusion of two light nuclei or the fission of a heavy nucleus
Define potential energy
energy due to position, shape, or configuration
Define the potential energy of a spring.
the stored energy of a spring as a function of its displacement; when Hooke’s law applies, it is given by the expression1/2kx^2wherex is the distance the spring is compressed or extended andk is the spring constant
Define power
The rate at which work is done
Define radiant energy
the energy carried by electromagnetic waves
Define thermal energy
the energy within an object due to the random motion of its atoms and molecules that accounts for the object’s temperature
Define useful work
Work done on an external system
Define Watt
SI unit of power, with1 W=1 J/s
Define work
the transfer of energy by a force that causes an object to be displaced; the product of the component of the force in the direction of the displacement and the magnitude of the displacement
Define work-energy theorem
the result, based on Newton’s laws, that the net work done on an object is equal to its change in kinetic energy
What is the formula for work?
W = Fdcosθ
W = work
F = force
d = displacement
cosθ = cos of the angle between f and d
What is the equation for kinetic energy?
KE = 1/2 m v^2
What is the work-energy theorem equation?
Wnet = 1/2mv^2 - 1/2mv_o^2
Wnet = net work
m = mass
v = velocity
v_o = initial velocity
What is the equation for the change in gravitational potential energy?
ΔPEg=mgh
ΔPEg = change in gravitational of potential energy
m = mass
g = gravity
h = height
How do KE (kinetic energy) and PE (potential energy) relate to each other?
ΔKE= −ΔPEg
What is the formula for the potential energy of a spring?
PEs = 1/2 kx^2
PEs = potential energy of spring
k = spring constant
x = displacement
What is the formula for mechanical energy?
KE + PE
or
KEi = PEi = KEf + PEf
What does the law of conservation of energy state?
the total energy is constant in any process. Energy may change in form or be transferred from one system to another, but the total remains the same.
When all forms of energy are considered, how is the conservation of energy written in equation form?
KEi+PEi+Wnc+OEi=KEf+PEf+OEf
KEi = inital kinetic energy
PEi = inital potential energy
Wnc = word done by non-conservative energy
OEi = initial other forms of energy
vice versa
How to calculate efficiency?
Eff = Wout / Ein
Eff = efficiency
Wout = useful work output
Ein = energy consumed
What is the equation for power?
P = W/t
P = power
W = work
t = time
What is power’s unit?
Watt (W)
Magnus has reached the finals of a strength competition. IN the first round, he has to pull a city bus as far as he can. One end of a rope is attached to the bus and the other is tied around Magnus’s waist. If a force gauge placed halfway down the rope reads out a constant 2100 Newtons while Magnus pulls the bus a distance of 1.05 meters, how much work does the tension force do on Magnus? The rope is perfectly horizontal during the pull.
-2205 J
Explanation:
I. Assess what you have: force and distance
II. Assess what you need: work
III. Find the right equation: W = +/- Fd
IV. plug in the numbers: W = -(2100N)(1.05m)
a. negative because it’s frictional
V. Solve: W = -2205 J
a. J = N/m
A ball of mass 0.600kg is carefully balanced on a shelf that is 2.30 m above the ground. What is its gravitational potential energy?
13.5 J
Explanation:
PE = mgh
PE = (0.600kg)(9.81m/s^2)(2.3m)
PE = 13.5J
A boy swings a ball on a string at constant speed in a circle that has a circumference equal to 6m
What is the work done on the ball by the 10 N tension force in the strong during one revolution of the ball?
0 J
Explanation: Centripetal force is always perpendicular to velocity. Perpendicular would give us cos90 which is equal to 0
A ball of mass 0.600 kg fell from a shelf 2.3 m off the ground with a potential energy of 13.5J (before falling). How fast will the ball be moving just before it strikes the ground?
6.71m/s
Explanation:
KE = 1/2mv^2
Get velocity to one side
v = sqrt 2KE / m
Plug in numbers and solve
v = sqrt 2 * 13.5J / 0.600
V = 6.71m/s
What is the work-energy theorem regarding kinetic energy?
Wnet = KE - KEo
