Work, Power,Energy Notes (2)

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Notes on WORK
What do you think of when you hear the word “work”?
WORK
happens when a FORCE moves an object through a distance
W
F
d displacement
(Newtons)
W = F x d
(meters)
Work is measured in Newton meters or JOULES
work is a scalar quantity: F and d have to be parallel to each other
if F and d are perpendicular then that Force is not the cause of the work done
F
d
Forces exerted at an angle:
only the vector component parallel to the distance moved does work
F
Fx
W = F cos θ d
Since work is F x d and one Force we deal with is Fg (force of gravity) and Fg = mg
work could = (mg)d
Examples of Work Problems:
500N
8m
4m
To get the 500N block to the top takes
the same amount of WORK whether you
lift it straight up or push it up the ramp.
The FORCE to lift anything is its WEIGHT
Fg = m g
W = Force x distance
lifting Work = (500N) (4m) = 2000Nm or 2000 Joules
slide up ramp Work = F x distance up the ramp
2000J = F (8m)
F = 250 N
I doubled the distance so the Force is halved
An Inclined Plane is a Simple Machine.
Simple Machines allow us to do the same amount of work with less force (effort).
Simple Machines include: inclined planes, levers, screws, wedge, pulley, wheel & axle
Notes on POWER
POWER = rate that work is done
P =
work
time
Joules
= Watt
sec
a 100 Watt light bulb puts out 100 Joules of NRG per sec
1 horsepower = 746 Watts
1 kW = 1000 W
☺
always coming back to past lessons
P = work
time
remember that work = F d
also P = F d
t
and
Example Power Problems:
so
P = F d
t
or
P = F x velocity
the Force might be Fg which = mg so P = mg d
t
Energy Notes
Energy = ability to do work
Forms of Energy: solar, electrical, mechanical, thermal, chemical, nuclear, hydroelec,
light, sound, wind, potential, kinetic, electromagnetic, etc
Chemistry – we focused on thermal, chemical, and nuclear
Physics – 1st sem focus on mechanical, kinetic, potential
2nd sem study electrical, magnetic, thermal, sound, light
Mechanical Energy:
nrg which is possessed by an object due to its motion or its stored nrg
mechanical nrg = kinetic + potential
(as a car rolls down a hill it loses potential energy and gains kinetic energy)
Kinetic Energy – nrg of a moving object
KE = ½ x mass x velocity2
KE & mass are directly related (if m is doubled, KE doubles)
KE & v2 are exponentially related (if v doubles, KE quadruples;
if v triples, KE x 9)
ex: In 1994 Leroy Burrell set a world record in the 100m dash by running it in 9.85 sec.
What was his mass if his kinetic energy was 3400J ? (assume constant speed)
Potential Energy - nrg of position, shape, or form
position example: an object at the top of a hill or cliff or table that has the
potential to fall from a height
shape example: a spring has (stored energy) potential to snap back into shape
form example: a rubber band, a snap bracelet, a bow to shoot an arrow
gravitational potential energy = potential (stored) energy due to a location
relative to a reference level. Assume reference is ground or floor unless
otherwise stated.
GPE = mass x acceler due to gravity x height above or below reference
kg
9.8 m/s2
meters
GPE = m g h
elastic potential energy = potential energy of an “elastic” object that is
stretched or compressed
the spring or rubber band or bow string has to be able to go back to its
original shape and size
relaxed length
stretched length
lr
ls
elastic potential energy = ½ x spring constant x how far it stretched
(stiffness)
(ls - lr)
N/m
meters
EPE = ½ k x2
EPE = 1 N m2
m
Example Potential Energy Problems:
=
Nm or Joules
Notes Conservation Of Energy
Law of Conservation of Energy = energy cannot be created nor destroyed, only changed form
In other words numerically, Total Energy will remain constant
Mechanical Energy = sum of kinetic and potential
ME = KE + GPE + EPE
GPE = 75J
KE = 0
Conservation of NRG
Etop = Ebottom
for cliff diver
GPEt + KEt = GPEb + KEb
GPE = 0
KE = 75 J
Pendulum
GPEmax
KE=0
GPEmax
KE=0
loses GPE
gains KE
halfway
GPE = KE
V= 0 KE = 0
GPEmax = 100J
Roller Coaster
starts high so we
have lots of PE
GPE = mgh
losing GPE gaining KE
b/c h is lower
V increasing
if GPE = 60J
then KE = 40J
GPE = 0 J
KE = 100 J
Example Conservation of Energy Problem
GPE = 50J
KE = 50J
Work-Energy Theorem
If you do Work on an object, you change its (kinetic and potential) Energy
If I lift books from desk
Did I do Work?
Was there a force applied in direction of object moved ?
Did I change the GPE of the book? KE?
Work = ∆ E
if work = change in Kinetic Energy
F d = KEf - KEi
F d = ½ m v2f - ½ m v2i
if work = change in Gravitational Potential Energy
F d = mghf - mghi
F d = m g ∆h
if work = change in Elastic Potential Energy
F d = m k x 2f - m k x 2i
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