Chapter 3Energy
Work
• An applied force acting through a distance parallel to the force
• Units of work (and energy) = joule (J)
• Zero // distance, no work• Displacement
perpendicular to applied force, no work
• An applied force acting through a distance parallel to the force
• Units of work (and energy) = joule (J)
• Zero // distance, no work• Displacement
perpendicular to applied force, no work
Fig 3.3 Work Against Gravity
Power • The rate at which work
is done• Units: watts (W) ;
horsepower (hp)• Example: Walking
versus running upstairs• The “power bill” - you
actually pay for “work”(kWh), which is related to energy
• The rate at which work is done
• Units: watts (W) ; horsepower (hp)
• Example: Walking versus running upstairs
• The “power bill” - you actually pay for “work”(kWh), which is related to energy
Units of Power
• Horsepower (1 hp = 550 ft·lb/s)
• Watts (N·m/s or J/s)
• 1 hp = 746 W
• Horsepower (1 hp = 550 ft·lb/s)
• Watts (N·m/s or J/s)
• 1 hp = 746 W
Fig 3.5
Convert 9.8 kW to hp
Motion, Position and Energy
• Work and energy are related
• Energy = ability to do work
• When work is done on something, a change occurs in its energy level
• Work and energy are related
• Energy = ability to do work
• When work is done on something, a change occurs in its energy level
Next: • Relationship
between work and energy associated with position
• Relationship between work and energy of motion
Next: • Relationship
between work and energy associated with position
• Relationship between work and energy of motion
Potential Energy (PE)• Energy associated with
“position”– “Potential” to then do work
• Gravitational Potential Energy (GPE)– Measuring h: need a
reference position (or reference height)
• Work can “change” PE • Potential Energy can
“change” into Kinetic Energy
• Energy associated with “position”– “Potential” to then do work
• Gravitational Potential Energy (GPE)– Measuring h: need a
reference position (or reference height)
• Work can “change” PE • Potential Energy can
“change” into Kinetic Energy
PE = mgh
Kinetic Energy (KE)
• Energy associated with “motion”
• Results from work or change in potential energy
• Speed squared!
• Double the speed,KE increases by 4
• Energy associated with “motion”
• Results from work or change in potential energy
• Speed squared!
• Double the speed,KE increases by 4
KE = 12mv2
Energy Flow
Energy is used to do work onan object. Work against five main groups of resistance:• Inertia• Gravity• Friction• Shape• Usually combinations of
four aboveWhat is the result?
Energy is used to do work onan object. Work against five main groups of resistance:• Inertia• Gravity• Friction• Shape• Usually combinations of
four aboveWhat is the result?
Energy Conversion
• Any form of energy can be “converted” into another form
• Energy flows from one form to another in natural processes
• Example - pendulum
• Any form of energy can be “converted” into another form
• Energy flows from one form to another in natural processes
• Example - pendulum E pendulum fixed = KE + PE
Energy Conservation
• Energy is never created or destroyed
• Energy can be converted from one form to another but the total energy remains constant
• When energy is transformed– Working or heating is always
involved
• 2 good examples of “Energy Conservation”: – A rock in “free-fall” – Swinging pendulum bob
• Energy is never created or destroyed
• Energy can be converted from one form to another but the total energy remains constant
• When energy is transformed– Working or heating is always
involved
• 2 good examples of “Energy Conservation”: – A rock in “free-fall” – Swinging pendulum bob
Energy Forms
Fig 3.17
Energy Forms
Mechanical energy• Kinetic plus potential
energy (external)
Chemical energy• Energy involved in
chemical reactions
Radiant energy• Electromagnetic energy• Visible light = small part
Electrical energy• Charges, currents, etc.
Nuclear energy• Energy involving the
nucleus and nuclear reactions
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