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