Ch. 2: Work and Energy Flashcards
defn: energy
a system’s ability to do work or (more broadly) to make something happen
defn: kinetic energy
the energy of motion
objects that have mass and that are moving with some speed will have an associated amount of kinetic energy
what is the SI unit for all forms of energy?
joule (J)
defn: potential energy
energy that is associated with a given object’s position in space or other intrinsic qualities of the system
it has the “potential” to do work
what are the two types of potential energy that are dissipated as movement?
- gravitational potential energy
- elastic potential energy
defn: gravitational potentail energy
depends on an object’s position with respect to some level identified as the datum (“ground” or the zero potential energy position which is usually chosen for convenience)
what state is a spring in when it is at equilibrium?
relaxed! and it has a characteristic length
defn: elastic potential energy
the energy that a spring has when it is stretched or compressed from its equilibrium length
defn: spring constant (k)
a measure of the stiffness of the spring
defn: total mechanical energy
the sum of an object’s potential and kinetic energies
defn: first law of thermodynamics
accounts for the conservation of mechanical energy
energy is never created nor destroyed, merely transferred from one form to another
does the first law of thermodynamics imply that total mechanical energy will be constant? why or why not?
no! not necessarily
the total mechanical energy equation does not account for other forms of energy (like thermal transferred as a result of friction)
if frictional forces are present, some of the mechanical energy will be transformed into thermal energy and will be “lost” (dissipated) from the system and not accounted for by the equation
if frictional forces are absent (or other nonconservative forces), the sum will be constant
defn: conservative forces
those that are path independent and that do not dissipate energy
do conservative forces have potential energies associated with them?
yes!
what are the two most common conservative forces on the MCAT?
- gravitational
- electrostatic
what are the two methods for determining if a force is conservative?
- If the change in energy of a system around any round-trip is zero
- a system that is experiencing only conservative forces will be “given back” an amount of usable energy equal to the amount that had been “taken away” from it in the course of a closed path
- if the change in energy is equal despite taking any path between two points
is total mechanical energy conserved when nonconservative forces are present?
no!
what are three nonconservative forces?
- friction
- air resistance
- viscous drag
what is the work done by nonconservative forces equal to?
the amount of energy “lost” from the system
(not actually lost, just transformed into another form of energy that is not accounted for in the mechanical energy equation)
are nonconservative forces path dependent or path independent? what does this imply?
path dependent
the longer the distance traveled the larger the amount of energy dissipated
what is the SI unit for work?
the joule (j)
defn: work
not actually a form of energy itself, but a process by which energy is transferred from one system to another
what are the two methods of energy transfer?
- work
- heat
how is energy transferred through work?
energy is transferred through the process of work when something exerts forces on or against something else
what does work being a dot product (a function of the cosine of the angle between the force and displacement vector) imply?
only forces (or components of forces) parallel or antiparallel to the displacement vector will do work (transfer energy)
what are the two main processes by which work is done?
- application of force through some distance
- a combination of pressure and volume changes
what happens to a piston system (piston, volume) when a gas expands? when a gas is compressed?
when has work be done in this situation?
GAS EXPANDS –> gas pushes up against the piston, exerting a force that causes the piston to move up and the volume of the system to increase
GAS COMPRESSED –> piston pushes down on the gas, exerting a force that decreases the volume of the system
work has been done when the volume of the system has changed due to an applied pressure
what is the x-axis and y-axis of P-V graphs?
what kind of processes do these help us represent?
x-axis: volume
y-axis: pressure
gas expansion and compression processes
what does the area enclosed by the corresponding pressure-volume curve tell us?
the work done on or by a system undergoing a thermodynamic process
describe work when a gas expands? when a gas is compressed?
GAS EXPANDS = work was done by the gas and is positive
GAS COMPRESSED = work was done on the gas and is negative
defn + work + aka: isovolumetric process
aka: isochoric process
volume stays constant as pressure changes –> no work is done because. there is no area to calculate on the P-V graph
defn: isobaric process
pressure remains constant as volume changes
defn + SI unit + ex: power
the rate at which energy is transferred from one system to another
SI unit: watt (W)
example: many devices (toasters, light bulbs, etc) are quantified by the rate at which they transform electrical potential energy into other forms (thermal, light, sound, kinetic)
func + defn: work-energy theorem
func: offers a direct relationship between the work done by all the forces acting on an object and the change in kinetic energy of the object
defn: the net work done by forces acting on an object will result in an equal change in the object’s kinetic energy
why is the work-energy theorem so helpful?
it allows us to calculate work without knowing the magnitude of the forces or the displacement through which forces act
defn (3): mechanical advantage
a measure of the increase in force accomplished by using a tool
for any given quantity of work, any device that allows for work to be accomplished through a smaller applied force is thus said to provide mechanical advantage
the ratio of magnitudes of the force exerted on an object by a simple machine to the force actually applied on the simple machine
unit: mechanical advantage
trick question!
dimensionless, because its a ratio
what are the 6 simple machines?
- inclined plane
- wedge (two merged inclined planes)
- wheel and axle
- lever
- pulley
- screw (rotating inclined plane)
what is the associated cost that comes with reducing the force needed to accomplish a given amount of work?
the distance through which the smaller force must be applied in order to do the work must be increased
defn: load
the weight held by a pulley system
defn: effort
the force required to lift a weight in a pulley system
defn: load distance
the height in the air that the object needs to be lifted by the pulley system
defn: effort distance
the length of rope that one must pull through to move the weight through the pulley
equal to twice the load distance
what does efficiency apply to in the context of mechanical advantage?
conserving energy output to input
defn + corollary: efficiency of a machine
a measure of the amount of useful work generated by the machine for a given amount of work put into the system
also: the percentage of the work put into the system that becomes unusable is due to nonconservative or external forces
does adding more pulleys increase mechanical advantage or have no impact?
increases mechanical advantage! thus reducing the effort
