Transcript Work, Power and Energy

Power,
by definition, is
the time rate of doing work;
or the time rate transfer of energy.
P=W/t
Power is a scalar quantity.
The SI unit of power
is the Watt,
named in honor of
James Watt.
One Watt, W, of power
is the power achieved
when 1.0 J of work is done or
1.0 J of energy is transferred
in a time of 1.0 s.
Power example
• An 80 kg human walks up a flight of stairs
in .22 s that have an altitude gain of 3.75 m.
What is the power of the person?
Power Example
• First find the work= force*distance. F=mg
80*9.8=784N
• W=Fd
• 784N*3.75m=2940J
• P=W/t=2940/.22=13363.64W
Work,
by definition, is
the product of the force exerted on
an object and the distance the object
moves in the direction of the force.
W = F·d
Work is a scalar quantity.
The SI unit of work
is the Joule,
named in honor of
James Prescott Joule.
One Joule, J, of work
is the work done when
1.0 N of force is applied
through a distance of 1.0 m.
Work Example
• An intern pushes a 75 kg patient on a 15 kg
gurney, producing an accleration of .06m/s2.
How much work does the intern do by
pushing the patient and gurney through a
distance of 2.5 m? Assume there is no
friction.
• Find the F= ma 15+75=90 kg* .60 m/s2
• F= ma 90*.6= 54N
• W=Fd= 54*2.5=135J
Graphically, work is
the area under a
“Force vs. Displacement” graph.
displacement, m
If the force and displacement are not
in the exact same direction, then
work = Fd(cosq),
where q is the angle between the force
direction and displacement direction.
F =40 N
35
d = 3.0 m
Example 2:The work done in moving the block 3.0 m
to the right by the 40 N force at an angle

of 35
to the horizontal is ...
W = Fd(cos q) = (40N)(3.0 m)(cos 35) = 98 J
Law of Conservation of Energy
“Energy can be neither created nor destroyed.
It may only change forms.”
S all types of energy before the event
= S all types of energy after the event
Examples:
•A dropped object loses gravitational PE as it gains KE.
•A block slides across the floor and comes to a stop.
•A compressed spring shoots a ball into the air.
Energy
the ability (capacity) to do work
Energy comes in many forms:
mechanical, electrical , magnetic, solar,
thermal, chemical, etc...
The SI unit of energy is the Joule.
Energy, like work, is a scalar.
Kinetic Energy
energy of motion
All moving objects that
have mass have kinetic energy.
KE = 1/2
2
mv
m - mass of the object in kg
v - speed of the object in m/s
KE - the kinetic energy in J
Energy Example
• A 50 kg boy and his 100 kg father went
jogging. Both ran at a rate of 5 m/s. Who
had more kinetic energy? Show your work
and explain.
Example Problem - answer
• KE = ½mv2
Boy…
• KE = ½(50 kg)(5 m/s)2
• KE = 625 J
Dad…
• KE = ½(100 kg)(5 m/s)2
• KE = 1250 J
•
Dad had more Kinetic energy because his mass was
greater.
Work-Energy Theorem
the net work done on an object is
equal to its change in kinetic energy
Wnet KE
Potential Energy
• Energy stored in a motionless
object, giving it the potential to
cause change
Potential Energy
energy of position or condition
• Chemical Potential Energy - energy stored
in chemical bonds between atoms (Snickers
bar, food, even gasoline)
Potential Energy
energy of position or condition
gravitational potential energy
PEg = mgh
m - mass of object in kg
g - acceleration of gravity in m/s2
h - height of object, in m,
from some arbitrary reference point
PE – gravitational potential energy in J
Example Problem
• What is the potential energy of a 10 N book
that is placed on a shelf that is 2.5 meters
high?
Example Problem - answer
•
•
•
•
GPE = mgh
GPE = (10 N) (2.5m)
GPE = 25 J
Remember that weight = mg and
that the force provided is weight.
NOTE: you may want to change
your variable for weight to Fg.
Potential Energy
energy of position or condition
elastic potential energy
PEe =
2
½ kx
k – elastic constant in N/m
x - elongation or compression in m
PEe – elastic potential energy in J
Click here to investigate elastic constants.
Example
• A spring with a spring constant of 120 N/m
is compressed a distance of 2.3 cm. How
much potential energy is stored in the
spring?
Example
•
•
•
•
U= ½ kx2
U= ½ (120)*(.025)2
Remember we work in meters!
U= .032J