# Work, Energy, and Power by VRX8nK

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```									Work, Energy, and
Power
A.   Work
B.   Work: Practice Problems
C.   Potential Energy
D.   Potential Energy: Springs
E.   Kinetic Energy
F.   Mechanical Energy
G.   Power
A. Work
   A force acting upon an object to cause a
displacement

   F = force
   d = displacement
   theta is the angle between the force and
the displacement vector
A. Work
A. Work
   A force acts upward upon an object as it is
displaced rightward
   The force vector and the displacement
vector are at right angles to each other
   The angle between F and d is 90 degrees
   Calculate Work
   cos 90 = 0
   A vertical force can never
cause a horizontal displacement
A. Work
   Determining the angle
   It is the angle between the force
and the displacement vector
   A force is applied to a cart to pull it up an incline
at constant speed
   Several incline angles were used; yet, the force
was always applied parallel to the incline
   The displacement of the cart was also parallel to
the incline
   Since F and d are in the same direction, the angle
was 0 degrees
A. Work
   Measured in Joules (J)
   The Joule is the unit of work.
1 Joule = 1 Newton * 1 meter
1J = 1 N * m

   One Joule is equivalent to one Newton of
force causing a displacement of one meter
B. Work: Practice Problems
B. Work: Practice Problems

   A = 500 J

   B = 433 J

   C = 750 J
B. Work: Practice Problems
   A 10-N forces is applied to push a block
across a friction free surface for a
displacement of 5.0 m to the right
   Calculate Work.

   W = 50 J
B. Work: Practice Problems
   A 10-N frictional force slows a moving block
to a stop after a displacement of 5.0 m to
the right
   Calculate work.

   W = -50 J
C. Potential Energy
   the stored energy of position possessed by an
object

   2 Types
   Gravitational PE
   Elastic PE
C. Potential Energy

   Gravitational PE
   the energy stored in an object as the result of its
vertical position (height)
   dependent on two variables
   mass of the object
   height
C. Potential Energy

   Elastic PE
   the energy stored in elastic materials as the
result of their stretching or compressing
   Stored in rubber bands, bungee chords,
trampolines, springs, an arrow drawn into a bow
   the more stretch, the more stored energy
C. Potential Energy

   Elastic PE
   the amount of force is directly proportional to the
amount of stretch or compression (x)
   the constant of proportionality is known as the
spring constant (k).
C. Potential Energy

   Elastic PE
D. Potential Energy: Springs
   Hooke’s Law
   If a spring is not stretched or compressed, then
there is no elastic potential energy stored in it
   The spring is said to be at its equilibrium position
   The equilibrium position is the position that the
spring naturally assumes when there is no force
applied to it
E. Kinetic Energy
   The energy of motion
   An object which has motion - whether it be
vertical or horizontal motion - has kinetic
energy
E. Kinetic Energy
   Types of KE:
   vibrational (the energy due to vibrational
motion)
   rotational (the energy due to rotational
motion)
   translational (the energy due to motion from
one location to another)
E. Kinetic Energy
   Translational KE
   Depends upon two variables
   the mass (m) of the object
   the speed (v) of the object

   m = mass of object
   v = speed of object
E. Kinetic Energy
   Kinetic energy is a scalar quantity; it does
not have a direction
   The kinetic energy of an object is
completely described by magnitude alone
   KE is measured in Joules (J)
F. Mechanical Energy
   The energy which is possessed by an
object due to its motion or its stored energy
of position
   Can be PE or KE
   An object which
possesses mechanical
energy is able to do
work
F. Mechanical Energy
F. Mechanical Energy
   The mechanical energy of an object can be
the result of its motion and/or the result of
its stored energy of position
   The total amount of mechanical energy is
merely the sum of the potential energy and
the kinetic energy
   TME = PE + KE
   TME = PEgrav + PEspring + KE
G. Power
   The rate at which work is done
   The work/time ratio
   The standard metric
unit of power is the Watt
   Watt is equivalent to a Joule/second
G. Power
   Most machines are designed and built to do work
on objects
   All machines are typically described by a power
rating
   The power rating indicates the rate at which that
machine can do work upon other objects
   The power of a machine is the work/time ratio for
that particular machine
G. Power
G. Power
   Suppose that Ben elevates his 80-kg body
up the 2.0 meter stairwell in 1.8 seconds. If
this were the case, then calculate Ben's
power rating
G. Power
   Suppose that Ben elevates his 80-kg body
up the 2.0 meter stairwell in 1.8 seconds. If
this were the case, then calculate Ben's
power rating

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