Mechanical Work and Energy

Everyday examples of energy sources

-wind turbines

-nuclear power plant

-solar panel fields

-hydroelectric power plant

Energy

is the ability to do work

-Joules (J)

Work (W)

is the transfer of energy by a force causing displacement

-Joules (J)

Work W (scalar quantity) 

-done on a system by an agent  exerting a constant force on the system is the product of magnitude F or force, magnitude d of displacment of point of application of force, and costheta

Chemical energy

-fuels such as gasoline

§which is potential energy arising from their molecular structure. Chemical energy can be converted into thermal energy by reactions like oxidation.

Electrical energy

§is a common form that is converted to many other forms and does work in a wide range of practical situations.

Thermal energy

-atoms and molecules inside al objects are in random motion.

the internal kinetic energy from these random motions

-because it is related to the temperature of the object

Radiant energy

-light is just one kind of electromagnetic radition, which also includes radio, infrared, ultraviolet, x-rays, and gamma rays. all bodies with thermal energy can radiate energy in electromagnetic waves

Nuclear energy

§comes from reactions and processes that convert measurable amounts of mass into energy. Nuclear energy is transformed into radiant energy in the Sun, into thermal energy in the boilers of nuclear power plants.

mechanical work

These and all other forms of energy can be transformed into one another and, to a certain degree, can be converted into

Kinetic energy

is the energy associated with an object or system’s relative motion

Potential Energy

is the stored energy associated with an object or system’s position, shape, or configuration

Conservative force

force always directed toward equilibrium position in an attempt to restore itself to its original position

Gravitational potential energy (GPE)

is the stored energy associated with an object or system’s relative position to the earth

• related to height and gravity

A spring has elastic potential energy(EPE)

which is the stored energy associated with a spring’s length relative to the rest position

-related to spring constant (k) and length compressed/stretched

Non-conservative forces

cause change in the mechanical energy of a system

Non-conservative force exmaples

-applied force

-kinetic friction

-drag force

Non-conservative forces do work on the system

by adding or removing energy from the system

if a non-conservative force does positive work

on a system, energy is added from the environment to the system

if a non-conservative force does negative work

on a system, energy is removed from the system and added to the enviornment

A block is pushed so that it moves up a ramp at constant speed, from point A near the bottom of the ramp, to point B near the top of the ramp.

-the work done on the block by the hand is

greater than zero

A block is pushed so that it moves up a ramp at constant speed, from point A near the bottom of the ramp, to point B near the top of the ramp.

the work done on the block by the normal force is

zero

A block is pushed so that it moves up a ramp at constant speed, from point A near the bottom of the ramp, to point B near the top of the ramp.

-the work done on the block by the friction is

less than zero

A block is pushed so that it moves up a ramp at constant speed, from point A near the bottom of the ramp, to point B near the top of the ramp.

-the work done on the block by the weight is

less than zero

Conservation of energy in humans

-energy consumed by humans is converted to work, thermal energy, and stored fat.

by far the largest fraction goes to thermal energy, although the fraction varies depending on the type of physical activity

Human energy consumption is

proportional to oxygen consumption

When non conservative forces are absent or no work done by external forces, then

the total mechanical energy of the system is conserved

Can an object be in motion of its energy is conserved

yes

Power (work per second)

can be useful when considering energy consumption.

•We can compare the use of an incandescent lightbulb (60 W) to that of the human heart for someone at rest (8 W on average). 

•Example: Electric power companies report energy used in kW∙h (kilowatt-hours).

•We can also think about the efficiency of an appliance (for example).