Transcript energy - Eastside Physics

ENERGY
• Different forms; Chemical, Electrical, Heat,
Electromagnetic, Nuclear and Mechanical
• Energy can be transformed from one type to
another but the total amount never changes
• If one form of energy decreases then
another must increase
• Mechanical Energy = the sum of kinetic and
potential energies ( motion & position)
WORK
• Work is done if an object is moved through a
displacement while a force is being applied to
it
• If either the force or displacement is doubled
so is the work
• W=F delta x
• Force and displacement are in the same
direction
• Joule= newton .meter = kg.m2/s2
Facts about Work
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Work is a scalar quantity (no direction)
Work does not depend on time
Units are joule or foot-pound
When force is not in the same direction as
displacement only the component parallel to
displacement does work on the object
• W= (F cos a) x where a is the angle to the
horizontal
Frictional Work
• Frictional Work is the work done due to
friction during motion. Energy dissipates
(lost)
• W fric = -fk x = -uk n.x = -uk mg.x
• W net = W applied - W fric
Kinetic Energy
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K.E is the energy of movement
K.E. = ½ mv2
As W = F .S = m.a.s ; s= displacement
From v2 = vo2 + 2as; then as =(v2 – vo2 )/2
Then W = m(v2 – vo2 )/2
Therefore Work = K.E. final – K.E. initial
Potential Energy
• P.E is the energy an object has due to its
position in space (units Joules)
• Gravitational P.E. = m. g. y
where y = vertical height and g = gravity
Gravitational P.E is the energy of an object
based on its position in space
Work = P.E initial – P.E final
Conservative Forces
• The work done on an object moving between two
points, is independent of the path taken.
It depends only on the initial and final positions.
Gravitational force is conservative
• If the work done by a force on an object moving
through a closed path is zero
• Potential energy functions can be defined only for
conservative forces.
Non-conservative Forces
• The work done on an object is dependent on
the path taken . Ex. Work due to friction
will differ for different paths.
a
Path 1 W= F.S
b
Path 2 W= F.(22/7d)/2
If the force leads to the dissipation of mechanical energy
Conservation
• A physical quantity is conserved when the
value of the quantity remains constant.
The form of the quantity may change but the
final value is the same as the initial value.
For example the gravitational potential
energy of a falling object will change to
kinetic energy, but the total energy will
remain the same.
Mechanical Energy
• The total mechanical energy in any isolated
system remains constant if the objects
within the system interact only through
conservative forces.
• Total mechanical energy in a system is the
sum of the total kinetic and potential
energies in the system
• Therefore 1/2mvi2+mgyi=1/2mvf2+mgyf
Spring Potential Energy
• A spring neither stretched or compressed is
in equilibrium
• When compressed or stretched by a force
there is stored potential energy in the spring
• Stored spring energy is called elastic P.E.
• Hooke’s law F = k.x Where k is the spring
constant particular to a spring
• (heavy spring = a large k value)
Work done to store spring energy
• The force require is proportional to the
distance the spring is change from
equilibrium
• Therefore Fave.= Fo + Fx/2 = 0 + kx/2=1/2kx
• As work is F.x then W = 1/2kx2 =P.Es
• The potential energy is maximum when the
spring has reached its max. compression or
stretch P.Es is always positive
New Mechanical energy Formula
• Incorporating the value of spring potential
energy in the previous formula
• (KE + PEg + PEs)I = (KE + PEg + PEs)f
• Where g is for gravity and s is for spring
Non-conservative forces and work
• In the real world non-conservative forces
such as friction are present.
• Therefore; Wnc = (KEf – KEi) + (PEf- PEi)
• The work done by all non-conservative
forces equals the change in kinetic energy
plus the change in potential energy.
Power
• Power is the rate of energy transfer with
time
• P = W/t SI units (watt W = J/s ) kg.m2/s3
• Also P = F.x/t as x/t = v then P = F.v
Power therefore is a constant force times
the average speed
The component of force is in the direction
of the average velocity
Power cont.
• U.S. customary system unit of power is the
horsepower (hp) =746W
• Power consumption is referred to in
kilowatt-hours 1kWh = (103W) (3600s)
= 3.60 x 106 J
• A kilowatt-hour is a unit of energy not
power
• A 100 watt bulb consumes 3.6 x 105 J in 1h
Mechanical Advantage
• MA. Is the advantage granted to us by the
use of machines
• MA. = Fr/Fe where Fr is the resistance force
Fe is the effective force
• MA. Is usually greater than 1
Ideal Mechanical Advantage
• IMA. Is mechanical advantage based on a
distance. Ex. The rope one pulls in a pulley
system to move an object.
• IMA.= de/dr where de is the distance of effort
dr is the distance of resistance
• IMA. Is usually greater than 1
Efficiency
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e is the extent that a machine is efficient
e = Work out/Work in
e = Fr.dr/Fe.de
e =( MA./IMA ) .100