Transcript Document 7887616

Work & Power
Simple & Compound Machines
Mechanical & Ideal Mechanical
Advantage
Efficiency
By: Deborah Wang
modified by: S. Ingle
Work Concepts
 Work (W) ~ product of the force exerted on
an object and distance the object moves in
the direction of the force.
– W is transfer of energy by mechanical means.
– W is done on an object only if it moves in the
direction of the force.
– Only the component of the force in the
direction of the motion does work.
Work Formulas
Work = Force x displacement
F
W = Fd
d
1 joule (J) = 1 Newton * meter (N * m)
if force is exerted at an angle to direction of motion,
W = (Fcos)d
or W
= Fd cos 
F

d
Power
 Power (P) ~ rate at which work is done or
rate at which energy is transferred.
Measured in watts.
– Watt (W) ~ one joule of energy transferred in
one second.
Power = Work / time
1 watt = 1 joule / second (J/s)
Simple Machines
 Machine ~ makes work easier either by
changing the magnitude or the direction of a
force, but can not increase the amount of
work done.
1.
2.
3.
4.
5.
6.
lever ~ bottle opener, seesaw, crowbar
pulley ~ rope on flagpole, block & tackle
wheel-an-axle ~ wheel barrow, bike pedals
inclined plane ~ wheelchair ramp
wedge ~ knife, axe, chisel
screw ~ bottle cap, nut, bolt
Mechanical Energy
 Effort force (Fe) ~ force you exert on a
machine.
 Resistance force (Fr)force exerted by the
machine.
Win= Fede
Work input
Wout= Frdr
Work output
Ideal Machines
 Ideal machines exist only in a frictionless,
air resistance-less world.
 No energy or work is lost to the system
through outside forces
 For ideal machines:
Work input = Work out put
so:
Fe de = Fr dr
Mechanical Advantage
 Mechanical Advantage (MA) ~ ratio of
resistance force to effort force.
MA = Fr / Fe
 Ideal Mechanical Advantage (IMA) ~
ratio of effort distance to resistance
distance.
IMA = de / dr
Efficiency
 Efficiency ~ ratio of work output to work
input expressed in percent.
efficiency = (Wout / Win) * 100%
or
efficiency = (MA / IMA) * 100%
Compound Machines
 Compound machine ~ consists of two or
more simple machines linked so that the
resistance force of one machine become the
effort force of the second.
– Ex. The pedal and gear of a bicycle.
 To calculate the efficiency of a compound
machine you simply multiply the
efficiencies of the simple machines that
make it together.