Work

Work is the product of the magnitude of the displacement and the component of the force acting in the direction of the displacement

Work

Most of the time F is in the direction of d so θ = 0° and cos 0° = 1 so…

Work is done by a force acting on a body!

FdW max

Work

Symbol: W Unit : J, joule 1 J = 1 Nm

If force and displacement are in the… Same directions, + W Opposite directions, - W Perpendicular directions, W = 0

Work

Work generally falls into 2 categories:

1. When you force something to move against the influence of an opposing force

– (push-ups & gravitational force, walking & friction, bow & elastic force)

2. When you change the speed of something

– A net force is present

Work - graphing

Work - graphing

Work and Energy

Energy is the ability to do work

OR

Work causes changes in energy

OR

Work is a transfer of energy

Work and Energy

EW

Power

Power is the rate at which work is done or the rate at which energy is transformed.

aveave Fvt

Fd

t

E

t

WP

Power

Symbol: P Unit: W, Watt 1W = 1J/s

Power and Work

Example:One liter of gas can do a specific amount of total work.

However, the POWER produced when we burn the gas can be any amount depending on the rate of burn

If…

1L is burned over 30 min or 1800 sec, like a car ~50 units of power is produced

1L is burned in 1 sec, like a supersonic jet ~90,000 units of power is produced

Energy - Types

Mechanical Energy: Energy due to position in a field force or energy due to movement

Non-mechanical Energy: Energy that does not fall into the above category

Energy – Flow Chart

Energy

Mechanical Non-mechanical

Light Sound HeatKinetic Potential

Linear

Rotational

Gravitational

Elastic

Electric

Magnetic

Mechanical Energy - Types

1. Kinetic Energy, KE: Energy of a moving object

Linear, KE → center of mass moving (this chapter)

Rotational, KR → object rotating around center of mass (not covered)

Mechanical Energy - Types

2. Potential Energy, PE: Energy due to position in a field force

Gravitational, PEg (this chapter)

Elastic, PEs (this chapter)

Electric, PEE (2nd Semester)

Magnetic, PEB (Not covered)

*You must choose a zero point for these*

Kinetic Energy Equation

2

2

1mvKE

Potential Energy Equations

mghPEg

2

2

1kxPEs

Work – Energy Principle or work done by a net force or net work done on an object

KEdFW netnet

for why board theSee

Energy Conservation

The total energy is neither increased nor decreased in any process.

Energy can, however, be transformed from one type to another AND transferred from one body to another, BUT, the total amount of energy in the process remains CONSTANT!

Conservative and Nonconservative Forces Conservative Force: A force such that the work

done on an object by the force does not depend on the path taken, rather it depends only on the initial and final positions (gravitational, elastic, electric)

Nonconservative Force: A force such that the work done on the object by the force does depend on the path taken (friction, air resistance, rocket propulsion). A lot of times these forces generate heat or sound which are non-mechanical energies.

Work – Energy Principle Redefined

So if energy is conserved we can write it this way using mechanical and non-mechanical energies

NCWPEKE

Work – Energy Principle & Mechanical Energy Conservation If we ignore nonconservative forces (friction and

the such), the implication is that no non-mechanical energies are present (heat, sound, light, etc) therefore…

0 PEKE

Mechanical Energy Conservation

21 EE

Mechanical Energy Conservation with energy lost

friction

lost

WEE

EEE

21

21

Homework and Reading

Chapter 5:

Work, Energy, and Power

 

Read:

p 159 - 191

Problems:

p 162, 1 - 4; p 163, 1 - 6; p 166, 1 - 5;

p 168, 1 - 4; p 172, 1 - 3; p 177, 1 - 5;

p 178 1 - 3; p 181, 1 - 5, 1 - 2; p 185, 19 - 25; p 186, 26 - 36; p 187 42, 44, 48; p 189, 52