Ch 2 - Work and Energy

Question 1

What is energy?

Answer 1

• the property of a system that enables it to do something or make something happen, including the capacity to do work
• SI unit for all forms of energy: joules (J)

Question 2

What is kinetic energy?

Answer 2

• energy associated with the movement of objects
• depends on mass and speed squared (not velocity)
  KE = 1/2mv^2

Question 3

What is potential energy?

Answer 3

• energy stored within a system

- exists in gravitational, elastic, electrical, and chemical forms

Question 4

Gravitational potential energy?

Answer 4

• related to the mass of an object and its height above a zero point (datum)
  U=mgh

Question 5

Elastic potential energy?

Answer 5

• related to the spring constant (a measure of the stiffness of a spring) and the degree of stretch or compression of a spring squared
  U=1/2kx^2
  k= spring constant; x = magnitude of displacement

Question 6

Electrical potential energy?

Answer 6

exists between charged particles

Question 7

Chemical potential energy?

Answer 7

the energy stored in the bonds of compounds

Question 8

Mechanical energy?

Answer 8

the sum of its kinetic and potential energies

E = U + K

Question 9

What are conservative forces and how do they relate to mechanical force?

Answer 9

• path independent and do not dissipate the mechanical energy of a system
• if only conservative forces, the total mechanical energy is conserved
• ex. gravity and electrostatic forces (created by springs)
  deltaE = delatU + deltaK = 0

Question 10

What are non conservative forces and how do they relate to mechanical forces?

Answer 10

• path dependent and cause dissipation of mechanical energy from a system
• while total energy is conserved, some mechanical energy is lost as thermal or chemical energy
• ex. friction, air resistance, and viscous drag
  deltaE = delatU + deltaK = Wnonconservative (work done by non conservative forces)

Question 11

What is work and how may it be expressed?

Answer 11

• a process by which energy is transferred from one system to another (no energy, but measure of energy transfer)
• may be expressed as the dot product of force and displacement, or the force and distance traveled with the cosine of the angle between the 2
  W = F . d = Fdcos(angle)
• may also be expressed as the area under a pressure-volume curve W = PdeltaV

Question 12

What is power?

Answer 12

• the rate at which work is done or energy is transferred
• SI unit: watt (W)
  P = W/t = deltaE/t

Question 13

What does the work-energy theorem state?

Answer 13

• that when work is done on or by a system, the system’s kinetic energy will change by the same amount
• the work done or by a system can be transferred to other forms of energy as well
  Wnet=deltaK=Kf-Ki

Question 14

What is Mechanical advantage and how does it make accomplishing a given amount of work easier?

Answer 14

• the factor by which a simple machine multiplies the input force to accomplish work = Fout/Fin
• the input force necessary to accomplish the work is reduced; the distance through which the reduced input force must be applied, however, is increased by the same factor (assuming 100% efficiency)

Question 15

What are the 6 simple machines and what do they provide?

Answer 15

provide the benefit of mechanical advantage

• inclined plane
• wedge
• wheel and axle
• lever
• pulley
• screw

Question 16

What is the load and load distance?

Answer 16

the load is the output force of a simple machine which acts over a given load distance to determine the work output of the simple machine
= mgh–> Wout = load x load distance

Question 17

What is the effort and the effort distance?

Answer 17

the effort is the input force of a simple machine which acts over a given effort distance to determine the work input of the simple machine

Question 18

What is efficiency?

Answer 18

the ratio of the machine’s work output to work input when non conservative forces are taken into account
= Wout/Win = (load x load distance)/(effort x effort distance)

Question 19

How does work correlate to gas expansion and compression?

Answer 19

• gas expands = work done by the gas (+)

- gas compresses = work done on the gas (-)

Question 20

Isochoric v isobaric process?

Answer 20

• isochoric (isovolumetric) = constant volume, no work done

- isobaric = constant pressure

Question 21

What happens to work and energy in uniform circular motion?

Answer 21

• displacement and force vectors are always perpendicular, so no work is done
• potential energy is constant