Energy comes in many forms:
mechanical, electrical , magnetic, solar,
thermal, chemical, etc...
Energy
the ability (capacity) to do work
*Energy, like work, is a scalar.
The SI unit of energy is the Joule.
British Engineering…Ft-lbs
Other common units…Calorie = 4.184 J and….BTU= 1,055 J
(British Thermal Units)
Kinetic comes from Greek word
KINETIKOS which means “motion”
Kinetic Energy
energy of motion
A measure of recoverable work done in changing the
state of motion of an object. (From rest to motion or
motion to rest or any change in velocity)
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
Work-Energy Theorem
the net work done on an object is
equal to its change in kinetic energy
Wnet KE
*There is NO NET work done in lifting an object!
Your positive work is “cancelled” by the negative
work done by gravity.
W = F·d
F = M·A
HW #5-3
Potential Energy
energy of position or condition
1. Stored Energy
2. A measure of recoverable work done in changing
the position or configuration of a body
1. Work done against gravity…lifting up object …work
done in lifting is now stored as potential
energy….recovered by gravity when released
2. Non- Recoverable…work against friction…turns to
heat that goes into surroundings and can’t be retrieved
Potential Energy
energy of position or condition
gravitational potential energy
GPE = Ug = 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
Potential Energy
energy of position or condition
elastic potential energy
Since work is energy and energy is work, all work done on a
spring is stored as energy in the spring….W = ½ k x2….
EPE=Ue = PEe =
2
1/2kx
k – elastic constant in N/m
x - elongation or compression in m
PEe – elastic potential energy in J
HW #5-4