Download Work Kinetic and Potential Energy

Work
What do you think?
• List five examples of things
you have done in the last
year that you would
consider work.
• Based on these examples,
how do you define work?
Work
• In physics, work is the magnitude
of the force (F) times the
magnitude of the displacement (d)
in the same direction as the force.
W = Fd
Units of Work
force
x
N
x
distance
1 N·m =
m
=
work
=
N·m
1 joule =
1J
You perform ≈1 J of work when you
lift a 100-gram apple from the floor
to your table top
Work
• Pushing this car is work
because F and d are in the
same direction.
• Why aren’t the following
tasks considered work?
– A student holds a heavy chair
at arm’s length for several
minutes.
– A student carries a bucket of
water along a horizontal path
while walking at a constant
velocity.
Work
• How would you calculate
the work in this case?
– What is the component of
F in the direction of d?
• F cos 
– If the angle is 90°, what is
the component of F in the
direction of d?
• F cos 90° = 0
– If the angle is 0°, what is
the component of F in the
direction of d?
• F cos 0° = F
Net Work Done By a Constant Net
Force
Wnet  Fnet  d  cos 
Work is a Scalar
F
F
F
F
• In which
direction (+
or ) is the
x-component
of F in each
case?
Classroom Practice Problem
• A 20.0 kg suitcase is
raised 3.0 m above a
platform. How much
work is done on the
suitcase?
• Answer: 590 J
Now what do you think?
• Based on the physics
definition, list five examples
of things you have done in
the last year that you would
consider work.
Kinetic and Potential Energy
What do you think?
• You have no doubt heard the term kinetic
energy.
– What is it?
– What factors affect the kinetic energy of an
object and in what way?
• You have no doubt heard the term
potential energy.
– What is it?
– What factors affect the potential energy of an
object and in what way?
Kinetic Energy
1
2
KE  mv
2
• What are the SI units for KE?
–kg•m2/s2 or N•m or J
Kinetic Energy
Wnet  F x  max
Since
then
or
v  v  2ax
2
f
2
i
Wnet  m(
Wnet
v v
2
f
2
i
2
)
1 2 1 2
 mv f  mvi
2
2
Work - Kinetic Energy Theorem
Wnet  KE  KEf  KEi
• KE is the work an object can do if the
speed changes.
• Wnet is positive if the speed
increases.
Classroom Practice Problems
• A 6.00 kg cat runs after a mouse at 10.0
m/s. What is the cat’s kinetic energy?
– Answer: 3.00 x 102 J or 300. J
• Suppose the above cat accelerated to a
speed of 12.0 m/s while chasing the
mouse. How much work was done on the
cat to produce this change in speed?
– Answer: 1.32 x 102 J or 132 J
Potential Energy
• Energy associated with an object’s
potential to move due to an interaction
with its environment
– A book held above the desk
– An arrow ready to be released from the bow
• Some types of PE:
– Gravitational
– Elastic
– Electromagnetic
– Chemical
Gravitational Potential Energy
PE g  mgh
• What are the SI units?
– kg•m2/s2 or N•m or J
• The height (h) depends on the “zero
level” chosen where PEg = 0.
• g = 9.80 m/s2 if h is positive
Elastic Potential Energy
• The energy available for use in deformed elastic
objects
– Rubber bands, springs in trampolines, pole-vault
poles, muscles
• For springs, the distance compressed or
stretched = x
Spring Constant (k)
Visual Concept
Click below to watch the Visual Concept.
• The spring constant (k) depends
on the stiffness of the spring.
–Stiffer springs have higher k values.
–Measured in N/m
• Force in newtons needed to stretch a
spring 1.0 meters
Elastic Potential Energy
PEelastic = kx2
k = the spring constant (N/m)
x = the distance the spring is stretched (m)
• What are the SI Units for
PEelastic?
–Joules (J) (N·m)
–The joule is the SI unit for any form
of energy
Classroom Practice Problems
• When a 2.00 kg mass is attached to a
vertical spring, the spring is stretched 10.0
cm such that the mass is 50.0 cm above
the table.
– What is the gravitational potential energy
associated with the mass relative to the table?
• Answer: 9.81 J
– What is the spring’s elastic potential energy if
the spring constant is 400.0 N/m?
• Answer: 2.00 J