Ch. 2: Work and Energy

Question 1

defn: energy

Answer 1

a system’s ability to do work or (more broadly) to make something happen

Question 2

defn: kinetic energy

Answer 2

the energy of motion

objects that have mass and that are moving with some speed will have an associated amount of kinetic energy

Question 3

what is the SI unit for all forms of energy?

Answer 3

joule (J)

Question 4

defn: potential energy

Answer 4

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

Question 5

what are the two types of potential energy that are dissipated as movement?

Answer 5

1. gravitational potential energy
2. elastic potential energy

Question 6

defn: gravitational potentail energy

Answer 6

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)

Question 7

what state is a spring in when it is at equilibrium?

Answer 7

relaxed! and it has a characteristic length

Question 8

defn: elastic potential energy

Answer 8

the energy that a spring has when it is stretched or compressed from its equilibrium length

Question 9

defn: spring constant (k)

Answer 9

a measure of the stiffness of the spring

Question 10

defn: total mechanical energy

Answer 10

the sum of an object’s potential and kinetic energies

Question 11

defn: first law of thermodynamics

Answer 11

accounts for the conservation of mechanical energy

energy is never created nor destroyed, merely transferred from one form to another

Question 12

does the first law of thermodynamics imply that total mechanical energy will be constant? why or why not?

Answer 12

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

Question 13

defn: conservative forces

Answer 13

those that are path independent and that do not dissipate energy

Question 14

do conservative forces have potential energies associated with them?

Answer 14

yes!

Question 15

what are the two most common conservative forces on the MCAT?

Answer 15

1. gravitational
2. electrostatic

Question 16

what are the two methods for determining if a force is conservative?

Answer 16

1. 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

2. if the change in energy is equal despite taking any path between two points

Question 17

is total mechanical energy conserved when nonconservative forces are present?

Answer 17

no!

Question 18

what are three nonconservative forces?

Answer 18

1. friction
2. air resistance
3. viscous drag

Question 19

what is the work done by nonconservative forces equal to?

Answer 19

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)

Question 20

are nonconservative forces path dependent or path independent? what does this imply?

Answer 20

path dependent

the longer the distance traveled the larger the amount of energy dissipated

Question 21

what is the SI unit for work?

Answer 21

the joule (j)

Question 22

defn: work

Answer 22

not actually a form of energy itself, but a process by which energy is transferred from one system to another

Question 23

what are the two methods of energy transfer?

Answer 23

1. work
2. heat

Question 24

how is energy transferred through work?

Answer 24

energy is transferred through the process of work when something exerts forces on or against something else

Question 25

what does work being a dot product (a function of the cosine of the angle between the force and displacement vector) imply?

Answer 25

only forces (or components of forces) parallel or antiparallel to the displacement vector will do work (transfer energy)

Question 26

what are the two main processes by which work is done?

Answer 26

1. application of force through some distance
2. a combination of pressure and volume changes

Question 27

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?

Answer 27

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

Question 28

what is the x-axis and y-axis of P-V graphs?

what kind of processes do these help us represent?

Answer 28

x-axis: volume

y-axis: pressure

gas expansion and compression processes

Question 29

what does the area enclosed by the corresponding pressure-volume curve tell us?

Answer 29

the work done on or by a system undergoing a thermodynamic process

Question 30

describe work when a gas expands? when a gas is compressed?

Answer 30

GAS EXPANDS = work was done by the gas and is positive

GAS COMPRESSED = work was done on the gas and is negative

Question 31

defn + work + aka: isovolumetric process

Answer 31

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

Question 32

defn: isobaric process

Answer 32

pressure remains constant as volume changes

Question 33

defn + SI unit + ex: power

Answer 33

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)

Question 34

func + defn: work-energy theorem

Answer 34

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

Question 35

why is the work-energy theorem so helpful?

Answer 35

it allows us to calculate work without knowing the magnitude of the forces or the displacement through which forces act

Question 36

defn (3): mechanical advantage

Answer 36

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

Question 37

unit: mechanical advantage

Answer 37

trick question!

dimensionless, because its a ratio

Question 38

what are the 6 simple machines?

Answer 38

1. inclined plane
2. wedge (two merged inclined planes)
3. wheel and axle
4. lever
5. pulley
6. screw (rotating inclined plane)

Question 39

what is the associated cost that comes with reducing the force needed to accomplish a given amount of work?

Answer 39

the distance through which the smaller force must be applied in order to do the work must be increased

Question 40

defn: load

Answer 40

the weight held by a pulley system

Question 41

defn: effort

Answer 41

the force required to lift a weight in a pulley system

Question 42

defn: load distance

Answer 42

the height in the air that the object needs to be lifted by the pulley system

Question 43

defn: effort distance

Answer 43

the length of rope that one must pull through to move the weight through the pulley

equal to twice the load distance

Question 44

what does efficiency apply to in the context of mechanical advantage?

Answer 44

conserving energy output to input

Question 45

defn + corollary: efficiency of a machine

Answer 45

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

Question 46

does adding more pulleys increase mechanical advantage or have no impact?

Answer 46

increases mechanical advantage! thus reducing the effort