Transcript 7.3 Energy

Chapter 7: Work and Energy
(Ewen et al. 2005)
•Define different forms of mechanical energy.
•Apply work-energy theorem to energy changes in mechanical
Energy is defined as the ability to do work.
 There are many forms of energy: mechanical,
electrical, thermal, fluid, chemical, atomic, and
The mechanical energy of a system is due to
 Position
 Motion
 Internal structure
There are two kinds of mechanical energy:
 Potential energy – stored energy due to its
internal characteristics or its position.
 Kinetic energy – energy due to the mass and
velocity of a moving object.
Internal potential energy – potential energy
due to the nature or condition of the substance.
 Examples: gasoline, a spring, a stretched rubber band.
Gravitational potential energy – potential
energy of a object due to its position relative to a
particular reference level.
 Example: A potted plant on the third floor window
with respect to the second floor or the ground.
Joule (J) in the metric system and foot-pound
(ft lb).
GPE  mgh
 GPE = gravitational potential energy
 m = mass
 g = 9.80 m/s2 and 32.2 ft/s2
 h = height above reference level
A wrecking ball of mass 2.00 E kg is located
4.00 m above a concrete platform whose top
is 2.oo m above the ground.
 A.) With respect to the platform, what is the
potential energy of the ball?
 B.) With respect to the ground?
KE 
 KE = kinetic energy
 m = mass of the moving object
 v = velocity of the moving object
The change in the kinetic energy (KE)of an
object is equal to the work done by or on the
W   KE  FS  KE
 KE i
A pile driver with a mass of
1.00 E4 kg strikes with a
velocity of 10.0 m/s
 A.) What is the kinetic energy of
the driver as it strikes the pile?
 B.) If the pile is driven 20.0 cm
into the ground, what force is
applied to the pile by the driver
as it strikes the pile? Assume all
KE of the driver is converted to
A 60.0 g bullet is fired from a gun with 3150 J
of KE. Find its velocity.