Transcript Energy & Work

Energy & Work
Energy
• What is energy?
• The capacity to do work
• What is Work?
• The transfer of energy from one physical system
to another
Bottom line: it is the outward manifestation of energy which causes a change
in something (e.g. changes in motion or position).
• Energy is measured in Joules
• Joules = newton*meter = kg*m2/s2
• In English units we call them calories or
kilocalories
Types of energy
• Kinetic energy – energy of motion
• Potential energy – stored energy
Kinetic Energy
• Mechanical energy – energy inherent in
objects in motion
• Light energy – highly organized packets of
energy called photons
• Electrical energy – energy generated by
charged particles
• Heat energy – atomic and molecular
motion create heat
Potential Energy
• Positional energy – energy stored due to
position of object relative to some force
• Electrical energy – stored charged
particles
• Bond energy – energy stored in the
chemical bonds holding 2 or more atoms
together
Laws of thermodynamics
• laws of the movement of heat?...why
heat?
• Push your book of the desk!
– 1) Positional energy to simple kinetic energy
– 2) positional energy also converted to heat due to friction
– 3) book hits ground, the kinetic energy is converted to
heat energy in the book and the ground
– 4) pick it back up and bond energy is converted into
mechanical energy in my muscles
– 5) both bond and mechanical muscle energy are
converted to heat
– 6) the heat is eventually radiated into the atmosphere
Laws of thermodynamics
• 1st Law - Conservation of Energy
• Energy is neither created nor destroyed
• Energy within a system is only converted from one
form of energy to another
• Energy can be moved into or out of a system
• Total energy in the universe is a constant
Laws of thermodynamics
• 2nd Law – Entropy
• Energy naturally flows from more ordered forms of energy to
less ordered forms of energy
• Corollaries:
– The total disorder in any closed system always
increases.
– Every conversion of energy from one form to another
converts some energy into disordered energy in the
form of heat.
Back to Work
• If work is the transfer of energy and energy can
be neither created or destroyed…….the work
done within a closed system……..
»Decreases over time?
»Remains constant ?
How is Work measured?
• The force applied to an object times the distance the
object moves due to that force.
• W = F*d
(Recall, F=ma)
• Peculiarities:
– 1) Movement perpendicular to the direction of the
force does not result in work
– 2) Supporting an item does not result in work. If the
distance moved is zero, then there is no work.
Animal Movement
• Moving a body is doing work
• There are a variety of ways of dealing with
forces and distance that relate to the ease
of accomplishment of work
• Humans, being animals, have made a
habit of using these ways to accomplish
work more easily in the form of Simple
Machines
A machine is a device that helps
make work easier to perform
• Transferring a force from one place to
another
• Changing the direction of the force
• Increasing the magnitude of a force
• Increasing the distance or speed a force is
applied
4 Basic Simple Machines
• Levers
• Inclined planes
• Pulleys
• Wheel and axle
Incline Plane
Horizontal distance
Vertical
distance
How much force is required to lift a
50kg weight 3 meter off of the ground?
F=ma
F = 50 kg (9.8 m/s2)
= 490N
3m
50kg
How much work vs. gravity needs to be
done to move a 50 kg weight up 3
meters?
Work = F*d
= 490N*3m
= 1470 Joules
50kg
3m
5m
How much force do you need to apply
to move the same weight up the ramp?
Work = F*d
F = W/d
= 1470J/5m
= 294 N
3m
5m
50kg
How much easier is it to do the work (1470J)
with the inclined plane than without?
Mechanical advantage = force without incline / force with incline
Force w/o
Force w
490 N
=
294 N
=
1.66
For inclined planes you get the same number by:
Length of plane
Height of plane
3m
5m
5m
=
3m
= 1.66
Mechanical Advantage
• force produced by the machine divided by
force applied to the machine
–F out / F in
Levers
Resistance
force
Effort
Force
de
Resistance
force
dr
Effort
Force
de
What is the mechanical advantage
of the lever system?
5 kg
15 kg
R
E
de
15 kg * 9.8 m / s2
F out
=
F in
5 kg * 9.8 m / s2
If the mechanical advantage is 3 :1
can you calculate dr and de?
15 kg
R
5 kg
E
de
If you need to
move the Fr 9
meters what is
the distance the
Fe will have to
move?
This is a different type of lever;
what is different from the previous
lever?
Fr
Fe
Where are dr and de?
Fe
dr
de
This is the last type of lever; what is
different?
Fe
Fr
Where are dr and de?
de
dr