Physics & Math -- Work and Energy

Energy

the property of a system that enables it to do something or to make something happen, including the capacity to do work

SI unit for all forms of energy

joules (J)

Kinetic energy

energy associated with the movement of objects

Kinetic energy depends on

mass and speed squared (not velocity)

Potential energy

energy stored within a system

Gravitational potential energy

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

Elastic potential energy

the potential energy of an object that is stretched or compressed; related to the spring constant and the degree of stretch or compression of a spring squared

Spring constant

a parameter that is a measure of a spring's resistance to being compressed or stretched (a measure of the stiffness of a spring)

Chemical potential energy

stored in the bonds of compounds

Mechanical energy

sum of the potential energy and kinetic energy in a system

Kinetic energy equation

K = 1/2 mv^2

Gravitational potential energy equation

U = mgh

Elastic potential energy equation

U = 1/2 kx^2

Conservative forces

are path independent and do not dissipate the mechanical energy of a system

If only conservative forces act on an object,

then the total mechanical energy is conserved

Examples of conservative forces

Gravity and electrostatic forces

Which forces are nearly conservative?

elastic forces

Nonconservative forces

are path dependent and cause dissipation of mechanical energy from a system

With nonconservative forces, ________ is conserved while some ____________ is lost as thermal or chemical energy

total energy; mechanical energy

Examples of nonconservative forces

friction, air resistance, viscous drag

Work

a process by which energy is transferred from one system to another; force x distance

Work equation

W = Fd = Fd cosx

Work may also be expressed as

the area under a pressure-volume (P-V) curve

Power

the rate at which work is done or energy is transferred

SI unit for power

Watt (W)

Work-energy theorem

when net work is done on or by a system, the system's kinetic energy will change by the same amount

Power equation

P = W/t = ΔE/t

Mechanical advantage

the factor by which a simple machine multiplies the input force to accomplish work

The six simple machines are:

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

Mechanical advantage makes it easier to accomplish a given amount of work because

the input force necessary to accomplish the work is reduced

Load

output force of a simple machine

Load distance

A quantity multiplied by the distance that the quantity will travel, to create an overall expression of the work required by that action.

Effort distance

The distance over which the effort force acts in a simple machine.

Efficiency

ratio of the machine's work output to work input when nonconservative forces are taken into account