F Chapter 3 - Energy Changes in motion and position -involves forces leads to changes in energy ENERGY - ability to do work position change, motion change: study, mow lawn, wind mill MECHANICAL WORK - transformation of forces into energy force must move object-displacement
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Chapter 3 - Energy
Changes in motion and position
leads to changes in energy
ENERGY - ability to do work
position change, motion change:
study, mow lawn, wind mill
MECHANICAL WORK -
transformation of forces into energy
force must move object-displacement
} no work
W = Fdd
Force in direction of motion
Fd = 1 N
d = 1m
W = Fdd = (1 N)(1 m) = 1 N m (kg m2/s2)
= 1 Joule = 1J
EXAMPLE : weight lifting
Work to lift a 100 kg barball a distance of 1 m
( @ constant speed)
Lift w/ no a
Fnet = 0 = weight-Fup
in direction of motion
m = 100 kg
W = Fdd = Fupd = (mg)h =
(100 kg)(10 m/s2)(1 m)
1000 kg m2/s2 = 1000 J
to change the position of the object
Note: no mechanical work done to hold barbell
d=0 when holding still above head
A 1000 kg car goes from 0 to 20 m/s in 5 seconds.
How much work is done (by the engine)?
CONNECTNewton’s 2nd law
F=ma engine accelerates car
W = Fdd = Fxd maxd
Work changes the motion of the car
WORK - forces used to give energy
to an object
chemical, electrical, heat
Power - rate at which work is done
how fast work is done
Powerful: can do work fast
Power (P) = (Work-Energy)/time = W/t
J/s = Watt = WSI unit for power
Light bulb: 75 W
How much (work) energy in 1 hour?
P = (W-E)/ t E = W = Pt
HORSEPOWER: compare machines to horses
F=150 lbs English unit hp
v= 2.5 mph 550 ft-lb/s
P = W/ t= (Fdd)/t = Fd (d/ t) = Fdv
1 hp = 1 horsepower = 746 W
Other Power units:
megawatt = MW = 106 W
KINETIC ENERGY - energy associated with
the motion of an object
Applied forces cause objects to move - accelerates
motional energy – mass and velocity
velocity influences more
K=1/2 mv2 work increases velocity
W = DKE change in kinetic
= 1/2 mv2-1/2 mvo2
Work to bring an object to rest:
W=1/2 mv2just the KE
Car example: W= 200,000 J
W= KE = ½ mv2
What is the velocity?
another way of looking at the problem!
POTENTIAL ENERGY - energy associated with the position of an object
potential for doing work
drop rock – falls –
gains KE (motional)
Transforms force of gravity into KE of motion
- does work when released
Definition: PE = -W work to achieve
Gravitational Potential Energy –
work required to raise the object to a
PEgrav = Fdd = (mg)h
zero reference – always be consistent in zero height
SPRINGS PE= ½ kx2
change position by compressing
zero – uncompressed
Electrical – work to move charge
Chemical – work to break bond
Energy associated with the mechanical
work on an object – motional and positional
sum of both
E = KE + PE(grav)
The mechanical energy is conserved
when friction absent
FRICTION – nonconservative
– heats environment
Law of CONSERVATION OF (MECHANICAL) ENERGY
-energy is neither created or destroyed
-assumption: no friction present
E = KE+PE =constantenergies transformed
E=KE+PE relates motion and position
Einitial = Efinal can solve for position
or motion (v)
EXAMPLE: ROLLER COASTER
A 100 kg rollercoaster moves along the track
shown starting from rest.
For each position marked, find the:
EXAMPLE: A 1.4 kg PSC book is
dropped out of a 20 m high building.
a) What is the PE at the top?
b) What is the KE at the bottom?
c) What is the velocity of the book at the bottom?
NOTE: PE at the top is transformed into
KE at the bottom!
E= (KE+PE)top = (KE+PE) bot
PEtop = KEbot
no energy lost – just transformed
Can work problems with friction and other forces
-leave this for more advanced course, but
if whole system considered
Etot = PEgrav + PEother + KE
GENERALIZED WORK-ENERGY THEOREM
Ef - Ei=Wncwork due to friction
NOW ALL ENERGY IS CONSERVED – everywhere
the energy goes is taken care of
-heat to environment
Mechanical energy is the energy of motion, or the energy of position, of many familiar objects. This boat has energy of motion.
Chemical energy is a form of potential energy that is released during a chemical reaction. Both (A) wood and (B) coal have chemical energy that has been stored through the process of photosynthesis. The pile of wood may provide fuel for a small fireplace for several days. The pile of coal might provide fuel for a power plant for a hundred days.
The electromagnetic spectrum includes many forms of radiant energy. Note that visible light occupies only a tiny part of the entire spectrum.
The blades of a steam turbine. In a power plant, chemical or nuclear energy is used to heat water to steam, which is directed against the turbine blades. The mechanical energy of the turbine turns an electric generator. Thus a power plant converts chemical or nuclear energy to mechanical energy, which is then converted to electrical energy.
This pendulum bob loses potential energy (PE) and gains an equal amount of kinetic energy (KE) as it falls through as distance h. The process reverses as the bob moves up the other side of its swing.