Download Work Power and Energy PPT

Work, Power, Energy
Work Concepts
• Work (W) is the product of the force
exerted on an object and distance the
object moves in the direction of the force.
– W is transfer of energy by mechanical means.
– W is done on an object only if it moves in the
direction of the force.
– Only the component of the force in the
direction of the motion does work.
– Force and displacement vectors must be
parallel for work to be done.
Work Formulas
Work = Force x displacement
F
W = F*d
d
1 joule (J) = 1 Newton * meter (N * m)
If force is exerted at an angle to direction of motion:
W = (Fcos) x d
Or W = Fdcos()
F

d
Power
• Power (P) is the rate at which work is done
or rate at which energy is transferred.
Measured in watts.
– Watt (W) is one joule of energy transferred in
one second.
Work W F * d
Power 


 F *v
time
t
t
With units of…
Joule (J)
Newton-meter (Nm)
1 Watt (W) = 1
1
sec (s)
sec
(s)
Energy
•
•
•
•
Often defined as the ability to do work.
Measured in Joules
Scalar quantity – direction does not matter
There are many forms of energy but we
will focus on Mechanical Energy for now.
• Energy must be conserved. This is the
fundamental law of the universe!
Mechanical Energy
Any energy associated with a moving object or an object
with the ability to start moving. Includes kinetic
energy and potential energy
• Kinetic Energy
– energy an object has because it is in motion.
– depends on mass and velocity
• Potential Energy-Energy stored in an object
• Could be stored due to its position (gravitational
potential) OR
• Could be stored due to its shape (elastic potential)
Mechanical Energy
Mechanical energy is the energy which is
possessed by an object due to its motion
or its stored energy of position, shape, or form
Mechanical Energy = K + U
Where K = kinetic energy
and U = potential energy
Kinetic Energy (K)
• The energy an object has because of its motion.
• Measured in Joules (J), which is equal to a Nm
• Changes in velocity have a greater effect than
changes in mass
K = ½ x mass x velocity²
K
1
2
mv
2
Potential Energy (U)
Potential energy is the energy stored in an object
because of its position, shape, or form
Gravitational Potential Energy (Ug)
• the stored energy an object has due to its position
(usually compared to its lowest point)
• depends on the mass of an object, the
gravitational field it is in, and the height
of the object.
• measured in Joules
Grav. Pot. Energy = mass * gravity * height
U g  mgh
Elastic Potential Energy (Uel)
• energy an elastic object stores when it is stretched or
compressed out of its original shape.
• applies most commonly to springs, rubberbands, and
bungee cords
• Depends on “stiffness” of elastic material and how much
it is stretched or compressed.
• “k” is used to represent the “spring constant” or the
“stiffness” of the material. It is measured in newtons per
meter or N/m. (Note: not the same k as Kinetic energy K!
Elastic potential energy = ½ * “spring constant” * (“stretch”)2
U el 
1
2
kx
2
Where “k” is spring constant and “x” is the
distance of stretch OR compression.
These concepts explained by
cartoons
Wile E Coyote and the
Roadrunner
Work-Energy Theorem
According to the Work-Energy theorem, the work
done on an object, by the net force acting on it, is
equal to the change in kinetic energy of the body
Work = change in Kinetic Energy
W  K  K f  K i
Fd 
1
mv f 
2
2
1
2
mvi
2
NOTE: Doing work on an object can also change the amount of potential
energy an object has by the amount of work done.
Example: 10 J of work is used to lift a box from the ground.
That box now has 10 J of potential energy.
Generally speaking…work equals the change in energy of an object!
Law of Conservation of Energy
also known as the “fundamental law of the universe”
Energy cannot be created or destroyed,
it can only change forms.
Since we are focusing on mechanical energy, the total
mechanical energy (all forms added together) must
remain constant at all times although it may change
from potential to kinetic and back again.
initial energy = final energy
(K+ Ug + Uel)initial = (K + Ug + Uel)final
Apply this equation to falling objects, rollercoasters, hot wheels, arrows shot
upward, … almost anything really…and it will help you find what you need!
Efficiency
• Ratio of work output to work input expressed in
percent.
• Ratio of final energy to initial energy expressed
as percent
• The percentage of work or energy put into a
system that stays with the system and is NOT
lost to heat.
efficiency = (Wout / Win) * 100
Or
efficiency = (Ef / Ei)*100