Download How is Work and Power Related? Chapter 5 Work and Power

How is Work and Power Related?
Chapter 5 Work and Power
Goals: define and calculate work,
energy, kinetic energy, potential
energy, power and use the
concept of conservation of energy
What is Work? Section 5.1
Work is a transfer or conversion of
energy
transfer occurs when a force is
applied in the direction of the force
force = mass x acceleration (g)
conversion occurs from one form of
energy to another
mechanical, thermal, electrical,
radiant, nuclear and chemical
W=Fxd
W = F x d cos θ
Simple Machines demonstrate the
concept and calculations of Work
Simple Machine- device that makes work
easier
multiply force OR distance
Increasing force or distance are at the
expense of the other variable
Energy is conserved in an ideal situation
no friction
Work in would equal Work out in another
words F x d (in) = F x d (out)
Work in is done on the machine and Work
out is done by the machine
R
---2m-----f--------4m------E
MA = 2
R= 40N---------f------------------- E =
20N
MA = 2
MA = Fr / Fe
or
MA = Xe / Xr
Levers include 1st, 2nd, 3rd, classes, wheels
and axels and pulleys
Pulley- grooved wheel and a rope MA=
the number of strands supporting
resistance
Inclined Planes include ramps,
wedges and screws
Gears- toothed wheels MA = ratio of
teeth of each gear increase force or
speed of rotation
Efficiency- ratio of work out to work
in
eff = W out / W in
What is Energy? 5.2
Energy – ability to cause a change
to do work work = transfer of energy
6 forms of energy and two states of
energy
Kinetic – motion
W=Fxd
= m x a x ΔX a ΔX = vf² - vi² /
2
= m x vf² - vi² / 2
= ½ m v²
increase mass vs increase velocity v²
Work – Energy Theorem
transfer of energy by a force
through a distance
transfers Kinetic Energy
Work = ΔKE
= KE f – KE i
Units
KE = ½ mv²
= kg m²/s²
=Nm
=J
What is Potential Energy?
Potential Energy – state of energy where a
change is stored
position relative to a measured height
gravitational potential energy
PE = m g h
weight = m g
= kg m/s² m
=N
=Nm
=J
Elastic potential energy – energy
stored in an object stretched or
compressed
Spring
ex. spring scale
PE elastic = ½ k x²
k = spring constant
x = distance stretched
How is Energy Conserved ?
Conserved – energy is not gained or lost
within a system
only converted or transferred
Mechanical Energy is the sum of the KE
and the PE in a system
the exception is friction that converts
some ME into heat that is nonmechanical
ME i = ME f
KE + PE = KEtotal
½ mv² + mgh = ½ mv² + mgh
ex. Falling rock PE to PE+KE to KE
What is the Rate at which work is
done?
Power – the rate at which work is done or energy
converted or transferred
P = ΔW / t
=Fxd/t
=Fxv
mgh / t
or ½ mv² / t
units
P=W/t
=J/s
= watts
1000 w = kw
1 hp = 746 w
YOU have the POWER
To get the WORK done
And
Accelerate on this next Exam