Transcript Potential and Kinetic Energy

Potential
and Kinetic
Energy
How is all energy divided?
All Energy
Potential
Energy
Gravitation
Potential
Energy
Elastic
Potential
Energy
Kinetic
Energy
Chemical
Potential
Energy
What is Potential Energy?
o Energy that is stored
and waiting to be
used later
What is Gravitational Potential
Energy?
o Potential energy
due to an object’s
position
Don’t look down,
Rover!
Good boy!
o P.E. = mass x
height x gravity
What is Elastic Potential Energy?
o Potential energy due compression or
expansion of an elastic object.
Notice the ball compressing
and expanding
What is Chemical Potential Energy?
o Potential energy
stored within the
chemical bonds of
an object
What is Kinetic Energy?
o Energy an object has due to its motion
o K.E. = .5(mass x speed2)
Conservation of Energy
Not created or destroyed as long as the
system studied has boundaries, is
closed, energy changes to all sorts of
different forms, but total energy is
constant.
Conservation of Energy
Problem Solving Strategies
Carefully identify the system. Make
sure it is closed
Identify the initial and final states of the
system.
A) If there are no external forces acting
on the system, then the total energy of
the system is constant: Ebefore=Eafter
B) Ug + K = Ug + K
Energy practice
Large chunk of ice 15.0 kg mass, falls
from a roof 8.00 m above the ground.
Ignore air resistance
Find kinetic energy as it hits the
ground.
Find its speed just before it hits the
ground.
Practice
You lift a 2.00kg textbook from
the floor to a shelf 2.10 m above
the floor.
A. What is the book’s
gravitational potential energy
relative to the floor.
B. What is the gravitational
potential energy relative to your
head, assuming that you’re 1.65
m tall?
More practice
A bike and rider approach a hill at a speed
of 8.5 m/s. The mass of the bike and rider
together is 85kg.
A) Find the initial kinetic energy of the
system.
B) The rider coasts up the hill. If there is
no friction at what height will the bike
come to a rest?
A 655 N diver leaps into the water from a
height of 10.0 m. What is the divers speed
just before she hits the water? What is her
speed 2.00 m above the water?
Conservation of Energy:
White board time
Tarzan, mass 85 kg swings down on the
end of a 4 m vine from a tree limb 4.0 m
above the ground.
A) How fast is Tarzan moving when he
reaches the ground
B) Does your answer to a depend on
Tarzan’s mass
Prepared
A crane lifts a car 24 m in the air, where it
hangs motionless. The car has a mass of
1.3 x 103 kg.
How much work does it take to lift the car?
If the car were dropped, how fast would it
be moving when it hit the ground?
Prepared
Which would take more power
a) lifting a fish, mass 11 kg, 12 m in the air
in 10 seconds
Or
b) lifting a monkey (36 kg), 5 meters in the
air in 13 seconds.
Prepared
A crane applies 25 Watts of power to a
Barbie doll for 13 seconds. If the Barbie
starts on the ground and the crane lifts
straight up, how high is the Barbie doll
(mass=30. g) after the 13 seconds.
Prepared
A rubber chicken with a mass of 20 g is shot
out of a cannon with a velocity of 200
cm/s. If work to accelerate the chicken
from rest is done over 1 meter, how much
force was applied to the chicken?
In terms of energy, which would you rather
do:
a) Stop a speeding train, mass 1.3 x 104 kg
traveling 64 km/hr
Or
b) Jump from a building (h=50 m) and come
to an abrupt stop (Spiderman style).
Assume Spiderman’s mass is 70 kg.
Prepared
You move a box across a horizontal surface,
pulling on a rope that makes an angle of
54 degrees above the horizontal. The
tension in the rope is 230 N. How fast is
the 6-kg box moving after it was pulled 3
meters?
Prepared
A child drops a 1.2 kg ball from the top of a
5.0 m tall building. When the ball has
fallen a distance of 2.0 m, what is the
velocity of the ball.