Transcript energy & energy transformations

ENERGY & ENERGY
TRANSFORMATIONS
Energy (E)
• The capacity to do work
(more to come in a couple slides)
• All forms of energy can be classified into one of
two broad categories:
Kinetic Energy
(energy of motion)
Potential Energy
(stored energy)
Forms of Energy
• Use the internet to prepare a definition for the
following types of energy and classify them as
kinetic or potential:
Chemical
Gravitational
Kinetic
Sound
Elastic
Electrical
Nuclear
Thermal
Radiant
Energy Transformation Equations
• Energy cannot be created or destroyed, only
transformed from one type to another
• Energy transformation equations show how
energy changes throughout a process
• Energy transformation equation for a
microwave:
electrical  radiant  thermal
Arrows indicate a
transformation
Create Energy Transformation
Equations for: (simple)
• A child jumps on a trampoline
• A portable CD player operates with a battery
• An incandescent light bulb is switched on
• A rollercoaster climbs and descends the first hill
• A person playing an electric organ presses a key
and we hear the note played
Create Energy Transformation
Equations for: (complicated)
• A nuclear core heats up water to the boiling
point which turns a generator
• Spring with a mass attached is pulled down and
then released (moves up and down until it comes
to a rest on it’s own)
• A match is struck against a matchbox and ignites
Work (W)
• The energy transferred to an object by an
applied force over a measured distance
• WORK is ENERGY TRANSFERRED
You can think of energy as
…
•
•
The ability to do work
Work waiting to happen
The Work-Energy Equivalence
•In general, doing work gives an object energy
AND an object that has energy can do work.
•Both Energy and Work have the unit Joule (with a capital
“J” = N.m)
The following conditions must be
met in order for work to be done:
1.) A force must be exerted on an
object
2.) The object must be displaced
(moved) by the force
3.) At least part of the force must be
in the same direction as the
displacement
Examples
•A person lifting a box from
the floor to the table
•A person pushing a box
along the floor from one spot
to another
What is Work?
• In Physics, work has a very specific definition
• Is this work?
▫ Video
Work
• Energy transferred to an object over a
distance
• WORK IS ENERGY GAINED OR LOST
W=FΔd
Where:
W – Work (J)
F – Magnitude of Force (N)
d – distance (m)
W = F∆d
Important Notes:
• Work is scalar – directions are
ignored for F and d
• This equation is used for 1D
problems only
• Work is measured in N∙m = J
(Joule)
Example
A worker pulls a heavy cart with a
force of 40N [E] for 5.0m [E]. What
is the work done by the person?
W = F∆d
= (40N)(5.0m)
2
= 2.0 x 10 J
Positive Work
• When the force is in the same
direction as the displacement
• "+" work = force tends to increase the
energy of the object
Negative Work
• If the force is opposite to the direction
of the displacement (i.e. friction)
• "-" work = force tends to decrease the
energy of the object
Example
A 42 kg child runs and then slides 7.8m
along an iced over pond. The coefficient
of kinetic friction between the child's
boots and the ice is 0.0050.
a.) What is the force of kinetic friction
acting on the child?
b.) Calculate the work done by the kinetic
friction
Solution
a.) FK = μKFN
FN = Fg = mg
=(42kg)(9.8m/s2)
= 4.1 x 102N
FK = (0.0050)(4.1 x 102N)
= 2.1N
b.) W = F∆d
= (2.1N)(7.8m)
= 16 J
Work Done Against Gravity
• Lifting an object to a higher position means
you are working against gravity which is
exerting a downwards force on that object
• If the velocity of the object being lifted is
constant, than Fnet acting on the object is 0.
Therefore,
F A = Fg
Example
A 23kg box is lifted 1.2m from the floor to a desk
with no acceleration. Determine the work done on
the box.
FA = Fg = mg
= (23kg)(9.8m/s2)
= 225N
W = F∆d
= (225N)(1.2m)
= 2.7 x 102J
Who thinks they can do zero work?
• Hold a textbook at arms length
Zero Work (ZERO Energy Transferred)
• Exerting a force but 0
displacement
▫ a student holding another
student on their shoulders
• Force is 0 but displacement
occurs
▫ a puck on an air table (no
friction)
• Displacement is perpendicular to
the force
▫ a javelin thrower runs with the
javelin above their head
Fa
Challenge Question
Who will do more work?
A.) 2 people lifting a piano into the back of a truck
OR
B.) 2 people pushing the same piano up a ramp to
the back of the same truck
Homework!
• Create two questions with full solutions