Chapter 15
Work, Power, and Simple
Machines
15-1 Work
*force acting through a distance
- Work = force X distance
-W=FXd
- Units: newton-meter(N-m) or
Joule(J)
15-2 Power
*how fast work is done
-Power = work / time
-P = W / t or P = F X d / t
15-2 Power
-Units: N-m / s or J / sec
or watt (W)
-1J/sec = 1 watt
-1000 watts = 1 kilowatt(kW)
-Horsepower
*(hp) = to 750 watts (745.56)
15-3 Machines
*device that makes work easier
-effort and resistance
-make work easier by changing
size or dir of applied force
*effort force(Fe): force applied to
a machine
-effort and resistance
*work input(Wi): work done on a
machine
-Wi = Fe X de
Ex: crowbar
-effort and resistance
*resistance force(FR): force
applied by the machine
*Work output(WO): work done
by a machine
-effort and resistance
*resistance distance(dr): dist through
which the object moves
-Wo = FR X dR
-Machines do not multi work, they
can multi force
-Worko is never greater than worki
-mechanical advantage
*(MA) # of times a machine
multi the effort force
-MA = FR / FE
-mechanical advantage
-MA is not always greater than1,
sometimes = to 1,
-mechanical advantage
-MA of one changes the dir of the
effort force
-MA is less than 1, increases the
dist an object is moved or the
speed with which it is moved
-efficiency
*comparison of worko to worki
- Efficiency = WO / WI X 100
-High eff = worki is changed to
worko
-eff can never be greater than
100%
-role of friction
-less friction = higher its effic
-effic can be increased by
reducing friction
ex: oil, grease, wax,bearings
15-4 Simple Machines
-6 simp mach: incline plane, the
wedge, the screw, the lever, the
pulley, and the wheel and axle
-inclined plane
*slanted surface used to raise an
object(a ramp)
-smaller effort force, but moved
through a greater distance
-inclined plane
-MA = length of the plane
divided by its height
-length can never be shorter than
its height
-MA = never be < 1
-wedge and screws
*wedge: incline plane that moves
-large force exerted on a small
surface
-wedge and screws
-longer and thinner the less
effort force required
ex: sharpening
-wedge and screws
*screw: an incline plane wrapped
around a cylinder to form a
spiral
-multi an effort force by acting
through a long effort distance
-the closer the threads > the MA
-levers
*bar that is free to pivot about a
fixed point when effort force is
applied
ex: seesaw, shovel,
nutcracker, crowbar
*fulcrum: is the fixed point
-levers
-3 classes of levers
-based on the position of the
fulcrum, effort force, resist
force
-levers
1) 1st class: multi effort force and
also change dir
Ex: pliers, scissors, seesaws
-levers
2)
nd
2
class: resist force is
between the fulcr and the
effort force
-multi eff force but don’t
changed dir
Ex: wheelbarrow, door,
nutcracker
-levers
3)
rd
3
class: eff force > resistan
-doesn’t multi force
-multi dist of eff force
-levers
-MA: # of times the lever increases
the eff force
*effort arm: dist from the eff force to
the fulc
*resist arm: dist from resis force to
the fulc
-levers
-MA = eff arm length
resist arm length
-1st and 2nd levers mult the eff
force
rd
-3 levers multi the distance
-pulleys
-can change either dir or amount of
eff force
-fixed pulley: attached to a stationary
object
-can’t multi eff force
-change dir of eff force
-pulleys
-MA of a fixed pulley is 1
-pulleys
-movable pulley:
-can multi eff force
-can’t change dir of an eff
force
-MA>1
-pulleys
-can predict MA by counting
the number of supporting
sections of rope.
Complex Machines
*is a combination of 2 or more
simple machines.
**you can get no more work out
of a machine than you put in it!