Transcript 1-WorkAndEnergy

Work and Energy
http://youtu.be/xBnS23U_ao4
Energy (E)
• The ability to do work
• Energy is the fuel of an object. When work is
being done on the object, the amount of fuel that
the object has changes. This fuel can be
expressed in many different ways.
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
A Kind of Kinetic
• Kinetic Energy
▫ Energy of objects in motion
A Kind of Kinetic
• Thermal Energy
▫ is the energy that makes things hot. It is a form of
kinetic energy at the molecular level. Can you
think of how heat or steam is used to do work?
A Kind of Kinetic
• Sound Energy
▫ The vibration of sound waves traveling through a
medium produces sound energy. At your last rock
concert, could you feel the music?
A Kind of Potential
• Gravitational Potential Energy
▫ is the energy stored in an object due to its height
in an area where the force of gravity can act on it
to make it fall.
A Kind of Potential
• Elastic Potential Energy
▫ is the energy stored by the bending, stretching or
compressing of matter
A Kind of Potential
• Chemical Potential Energy
▫ is energy stored in the chemical bonds of matter
and can be released by way of a chemical reaction,
such as lighting a match. Can you think of an
example of chemical energy release in the human
body?
A Kind of Potential
• Radiant Energy
▫ is carried by electromagnetic waves, like light.
Radiant energy from the Sun can be transferred
into heat to melt our snow. It can also move
certain objects in a vacuum.
A Kind of Potential
• Nuclear Energy
▫ is stored in the nucleus of every atom. An
enormous amount of energy is released in a
nuclear fission or fusion reaction.
A Kind of Potential
• Electrical Energy
▫ is energy possessed by charged particles. The
charges can transfer energy as they move through
an electric circuit. We use electrical energy for
many, many things!
A Kind of Potential
7. Magnetic Potential Energy
•
is stored in the place between two magnets
What Kind of Energy?
Kettle – demonstrates two forms of energy. What are they?
Basketball – hold the basketball in front of you and then
drop it.
Theraband – stretch and release the theraband. What kind
of energy is being stored in the theraband?
Bicep Curls – perform some bicep curls and guess what
form of energy is being accessed and produced.
What Kind of Energy?
Lights – what kind of energy is being used to light the room?
Magnets – can you feel the forces of attraction and repulsion?
Solar Sail – read the article to find out what makes the solar sail
fly.
Stereo – put your hand near the speakers. Can you feel anything?
Nuclear Power – do we use this in Canada?
Rolling Cart – give the cart a push. What form of energy do you
see?
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-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)
W  Fapp  d
Conditions for Work
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 (BBT)
Work
• Energy transferred to an object over a distance
• WORK IS ENERGY GAINED OR LOST
What if we push the box 16m?
What if the mass of the
bucket was 40kg?
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?
FK = μKFN
FN = Fg = mg
=(42kg)(9.8m/s2)
= 4.1 x 102N
FK = (0.0050)(4.1 x 102N)
= 2.1N
Example Continued
b.) Calculate the work done by the kinetic friction
W = F∆d
= (-2.1N)(7.8m)
= -16 J
Who thinks they can do zero work?
Zero Work
•
∆𝑑 = 0
but a force is applied
▫ a student holding another student on their
shoulders
• F=0 but displacement occurs
▫ a puck on an air table (no friction)
•F
▫ 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
• Work and Energy problem set