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*
Mechanical Systems 2.2: The Science of Work
Textbook pages 287-292
*In the following 4
scenarios, decide if work
is being done.
*Write your answer and
explanation in the first
column.
Scenario 1:
A teacher applies a force to a
wall and becomes exhausted.
Scenario 2:
A book falls off a
table and free
falls to the
ground.
Scenario 3:
A waiter carries
a tray full of
meals above his
head by one
arm straight
across a
restaurant at a
constant speed.
Scenario 4:
A rocket
accelerates
through
space.
*In Physics….
*WORK is done on an object
when a FORCE acts on an
object to move it a certain
DISTANCE.
*W = F x d.
*3 things needed for work:
*1.
*2.
*3.
There must be a force on an object.
There must be a displacement.
The force must cause the
displacement.
*
The Joule is the unit of Work.
1 Joule = 1 Newton x 1 meter
1 J = 1 N*m
*
James was the son of a rich
beer brewer from England. He
was homeschooled until he was
15, when he was taught the
fundamentals of chem, physics
and mathematics by John
Dalton ((another famous British
chemist). James Joule was
able to test out his theories
about heat and energy
transfer by giving electrical
shocks to his brother and
playing with the equipment at
the brewery.
Scenario 1:
A teacher applies a force to a
wall and becomes exhausted.
This is NOT WORK. The wall
DOES NOT move.
Scenario 2:
A book falls off a
table and free
falls to the
ground.
This is WORK. A
force of gravity
acts on the book
to displace it
down (falling).
Scenario 3:
A waiter carries a
tray full of meals
above his head by
one arm straight
across a restaurant at
a constant speed.
This is NOT work. The waiter uses a force to
hold the tray up, but the tray moved horizontally
across the room. The force DOES NOT cause the
displacement of the tray.
Scenario 4:
A rocket
accelerates
through
space.
This is WORK. There is a force (the
expelled gases push on the rocket)
which causes it to be displaced
through space.
*
*A nurse pushes a
700 N patient in a
wheelchair 5.0 m
up an inclined
plane with a force
of 320 N.
Calculate how
much work the
nurse did.
W=Fxd
W = 320 N x 5.0 m
W = 1600 J
This is the INPUT WORK!
*
*The height of the ramp is 2.0 m.
*The weight of the patient and
wheelchair is 700 N.
W=Fxd
W = 700 N x 2.0 m
W = 1400 J
This is the OUTPUT WORK!
*
Workoutput
Efficiency =
x100%
Workinput
1400J
Efficiency =
x100%
1600J
Efficiency = 87.5%
The ramp was 87.5 % efficient because some
work was lost due to friction!
* Textbook page 292 #1-9
*
*
*Power is the measure of how FAST
the WORK is done.
Work
Power =
time
W
P=
t
*
The Watt is the unit of Power.
1 Watt = 1 Joule/second
1 W = 1 J/s
*
* A horse is pulling a cart with a force of 150 N by 10 m
every 20 seconds. What is the POWER of the horse?
* F=150 N
* d=10 m
* t= 20 s
W = F*d =150N *10m =1500J
W = 1500J = 75W
P=
20s
t
*
*Energy is the ability to cause a change
in motion and do WORK!
*Remember, to do WORK…
a FORCE must be applied to an object to
cause the object to MOVE in the SAME
direction as the FORCE.
*
*Energy
does not
have MASS
and does
NOT take
up SPACE!
*
1) KINETIC ENERGY is the energy of motion
Examples: cars in motion, people that are
moving, particles moving
2) POTENTIAL ENERGY is stored energy due to
an objects POSITION or CONDITION
Examples: a book held in your arms has
gravitational potential energy and an arrow in
a stretched bow has elastic potential energy.
*In our universe, energy cannot be created
or destroyed.
*Energy can be transformed from one form
to another, but the total amount of energy
in the universe is constant.
*
*Forms of energy include chemical,
electrical, thermal, light (radiant),
mechanical (kinetic & potential)
nuclear, sound
*Types of Energy