WORK
The transfer of energy to cause or
make an object move
WORK
If there is no movement, no work is done
Ex: Lifting a text book (work) vs pushing
on a brick wall (no movement = no work)
WORK AND ENERGY
When work is done, a transfer of
energy occurs
You become tired when you walk or
carry things up a flight of stairs
JOULE
The unit used to express work (J)
HOW DO YOU CALCULATE WORK?
Work = Force X Distance
Or
W=FxD
TRY IT
You apply a force of 10 N to a
shopping cart. You moved the cart
10 m. How much work did you do
on the shopping cart?
POWER
The rate at which work is done or
energy is transferred
WATT
The unit used to express power.
•
Symbol for Watt (W)
•
Usually written in italics
HOW DO YOU CALCULATE POWER?
Power = Work/Time
Or
P = W/T
TRY IT
It takes you 5 s to do 100 J of work on
a shopping cart to move it down the
sidewalk. What is your power
output?
MACHINES
A device that makes doing work
easier
Changes force (increases it)
DOES NOT change the amount of
work
WORK DONE BY MACHINES
Input force – force that is applied to
the machine = Fin
Output force – the force applied by
the machine = Fout
WORK INPUT
B/c of friction machines aren‘t 100% efficient
Work Input= force YOU exert on machine = Win
Input Distance = distance YOU are using
Ex: Rowing boat
Input force - how hard you pull oars
Input distance - how much oar handles move
WORK OUTPUT
Output Force = force exerted by machine
Output distance = distance machine uses
Work Output = what machine does/accomplishes =
Wout
Ex: Rowing boat
Output force - how hard oars push water
Output distance - distance oar ends move in water
Work Output - moving through water
WORK INPUT AND OUTPUT
CONSERVING ENERGY
When energy is used by a machine, some
of the energy is transferred as heat due
to friction
Wout is never greater than Win
Wout is always smaller than Win
MECHANICAL ADVANTAGE
Advantage of using a particular machine
Mechanical Advantage Equation:
Mechanical Advantage = output force (in newtons)
input force (in newtons)
OR
MA = Fout / Fin
TRY IT
Calculate the mechanical
advantage of a hammer if the
input force is 125N and the
output force is 2000N.
MECHANICAL EFFICIENCY
Measure of how much of the work put into a machine is
changed into useful work output by the machine
Calculating Efficiency:
Efficiency(%) = output work (in joules) x 100%
input work (in joules
OR
efficiency = Wout / Win x 100%
Higher the number, the more efficient
TRY IT
Find the efficiency of a machine
that does 800J of work if the
input work is 2400J.
TYPES OF MACHINES (SIMPLE MACHINES)
6 Simple Machines make other machines
1. Lever Family
1. Lever
2. Pulley
3. Wheel & axle
2. Inclined Plane Family
1. Simple inclined plane
2. Wedge
3. Screw
LEVERS
Have a rigid arm that turns around a
fulcrum
3 classes:
1. 1st class
2. 2nd class
3. 3rd class
1ST CLASS LEVER
Fulcrum in
middle of arm
Exs: scissors,
pliers,
hammer claw,
seesaw
2ND CLASS LEVER
Fulcrum at one
end and force
at other end
Load in middle
Exs:
wheelbarrow,
door,
nutcracker
(MA > 1)
3RD CLASS LEVER
Fulcrum at one
end, force in
middle, and
load on other
end
Exs: tweezers,
biceps
PULLEY
Fulcrum in
middle of
circle
Lever = rope
More pulleys –
easier work
A.) Fixed
TYPES:
wheel attached in a fixed position
MA = 1
B.) Movable
attached to the object being moved
MA = 2
C.) Block and Tackle
Combination of fixed and movable pulleys
MA = depends on the number of rope segments
WHEEL & AXLE
2 different sized
wheels
Axle is fulcrum,
wheel is lever
Exs: steering
wheel, screw
driver
Gears – toothed W
&A
MECH. ADV. OF A WHEEL AND AXLE
MA = Radius of Wheel/Radius of Axle
INCLINED PLANE
Spreads work
over long
distances
Easier to use a
long ramp
Exs: stairs,
ramps,
escalators
MECH. ADV. OF INCLINE PLANES
MA = Length/Height (L/H)
WEDGE
2 inclined planes
back to back
Holds together or
separates
objects
Exs: nails, axes
WEDGES
SCREW
Threads are
spiraled
incline plane
Exs: jar lid, spiral
staircase
SCREW
COMPOUND MACHINE
More than 1
simple
machine
together
Ex: scissors lever (handles)
and wedge
(blade)