Download Work - RSD 17

Work and Energy
Chapter 5
By Mr Leavings
What is Work
In science Work has a specific
meaning. If you push a box with a
force of one newton for a distance
of one meter, you have done
exactly one joule of work.
Units of Work: joules.
One joule = One newton-meter
What is Work
W=Fxd
What is Work
What is Work
Work is force times distance moved in
the direction of the force
Force B
Force C
Force A: Does 1
joule
Force B: Does < 1
joule
Force C: No Work
Force A
1 Meter
Work
When we apply Force in
machines we are doing Work.
For example , when a Block
and Tackle machine lifts
weight, force is applied to do
this.
As a result of the force, the
weight moves a distance.
Work done by Machines
For a machine to do
work it must have an
input force.
The work output of
a simple machine
CAN NEVER exceed
the work input.
Work done by Machines
Nature does not give
something for nothing.
The work input of
the block and tackle
is the same as the
work output. You get
Mechanical
Advantage by
trading force for
distance
Work and Efficiency
In a very Efficient machine, all (or
most) of the input work becomes
output work. In the block and
tackle example before the 10 joules
of input work was transformed to
10 joules of output work. This
machine was 100% efficient.
Work and Efficiency
Is 100% efficiency possible?
In a very efficient machine, all (or most) of
the work input becomes output.
In REAL machines, work output is always
less than work input. Other forces such as
friction use up some of the input work.
Work
Output
______________
Efficiency =
Work Input
Work and Efficiency
4 Joules
4 Joules
Input work = Output work
Then it is 100% efficient
4 Joules
Input Work does not
equal output work
3 Joules
1 Joule
75 % Efficiency
Work and Efficiency
65% of the energy
in gasoline is
converted to heat.
As far as moving
the car goes, this
heat energy is
“lost”.
The energy doesn’t
vanish, it just does
not appear as
useful output
work.
Work and Efficiency
The most efficient machine designed?
The bicycle is the
most efficient
machine ever
invented for turning
the work of humans
muscles into motion.
Its efficiency is
more than 95%!
Work and Power
What is power?
You and a friend are pulling boxes
across a floor. You drag one box with
a force of 100 Newtons for 10
meters, and it takes you 10 seconds.
Your friend then drags a similar box
for the same distance but it takes
him 60 seconds. Have you done the
same amount of work?
Work and Power
Power: The RATE at which work is
done.
Energy
The different Kinds of Energy:
6 kinds that we will talk about
1. Kinetic Energy (more on that Later)
2. Potential Energy (more on that Later)
3. Radiant Energy
4. Electrical Energy
5. Chemical Energy
6. Nuclear Energy
Energy
Mechanical energy is the energy possessed by
an object due to its motion or its position.
Potential energy and kinetic energy are both
forms of mechanical energy.
Energy
Radiant energy is energy that is carried by
electromagnetic waves.
Light is one form of radiant energy.
Energy
Electrical energy comes from electric
charge, which is one of the fundamental
properties of all matter.
Energy
Chemical energy is a form of energy stored in
molecules.
Batteries are storage devices for chemical
energy.
Energy
Nuclear energy is a form
of energy stored in the
nuclei of atoms.
In the Sun, nuclear
energy is transformed
to heat that eventually
escapes the sun as
radiant energy.
Potential Energy
Potential Energy comes from the
position of an object relative to earth.
Consider a marble that is lifted
off the table. Since the Earth’s gravity
pulls the marble down we must apply a
force to lift it up. Remember the
concept of WORK, we are applying a
force over a distance, which is stored
as the potential energy of the marble.
Potential Energy
From Newton’s second law we know that
the weight (Force due to weight) is equal to
the mass of the marble (m, in kilograms)
times the acceleration of gravity (g, equal
to 9.8 m/sec2). We also know that the work
is equal to force times distance.
Since the force is the weight of the marble
(mg) and the distance is how far up we lift
the marble (h), the work done equals weight
times height.
Potential Energy
PE (joules)
mass of object (kg)
EP = mgh
height object raised (m)
gravity (9.8 m/sec2)
Potential Energy
Potential Energy
Systems or objects with potential energy are
able to exert forces (exchange energy) as
they change.
Potential energy is energy due to position.
Kinetic Energy
Objects also store energy in motion. A
moving mass can exert forces, as you would
quickly observe if someone ran into you in the
hall.
Kinetic energy increases as the
square of the speed. This means the
faster you are going the harder it is
to stop!
Kinetic Energy
KE (joules)
mass of object (kg)
EK = ½ mv2
velocity (m/sec)
Going 60 mph a car has four times as much
kinetic energy as it does at 30mph! At 90 mph
you have 9 times the energy!
Solving Problems




A 2 kg rock is at the edge of a cliff
20 meters above a lake.
It becomes loose and falls toward
the water below.
Calculate its potential and kinetic
energy when it is at the top and
when it is halfway down.
Its speed is 14 m/s at the halfway
point.
Solving Problems
1. Looking for:

…initial EK, EP and EK, EP half way down.
2. Given:


mass = 2.0 kg; h = 20 m
v = 14 m/s (half way)
3. Relationships:



EP =mgh
EK = ½ mv2
Assume rock starts from rest.
Conservation of Energy
Law of Conservation of Energy: Nature
NEVER creates or destroys energy;
energy only gets converted from one
form to another.