Work, Power, and Simple
Machines
Recall…..
• An object only starts moving
when a force acts on it.
Work
• Work is the product of the force on the object
and the distance that object moves.
• One thing to remember,
• There maybe force on something, but that
doesn’t mean that the object is doing work.
For force to work on an object, some of the force
must act in the same direction as the object
moves. If there is no movement, there is not work
being done.
Work
Work, if any, depends on the direction of
the force and the direction of the movement
Work= Force X Distance
units=Joule or J
Power
• Power is the rate at which the work gets done.
• To increase power, you can increase the
amount of work done in a given time, or you
can do a given amount of work in less time.
Power
Power=work/time unit for power is a watt
Watt=j/s
Horse Power
• James Watt was trying to find some kind of power to
compare his steam engine to and the most popular source of
power at the time was a horse. Therefore, he created the
horsepower to show just how powerful his method was.
1horse power=746watts
Work and Machines
• A machine is a device that changes a force.
• Ex: a car jack
Machines
Machines make work easier to do. They change
the size of a force needed, the direction of a
force, or the distance over which a force acts
Ways that Machines Work
• Increasing Force– Exerts a small amount of force over a larger
distance. Thus, cuts down on the amount of force
you need to exerts
Ways that Machines Work
• Increasing Distance
– Oars of a canoe show how a small distance
exerted covers a much larger distance
Ways that Machines Work
• Change Direction
– Some machines change direction of an applied
force
Work Input and Work Output
Because of friction, the work done by a machine is
always less than the work done on the machine
Work Input and Work Output
• The force you exert on a machine is called the input force
• The distance the input force acts through is the input
distance
• The work done by the input force acting through the input
distance is called the work input
Work Input=Input Force X Input Distance
Work Input and Work Output
• Force that is exerted by a machine is called
the output force
• The distance the output force is exerted
through is the output distance
Work output= output force X output distance
Mechanical Advantage
Mechanical advantage: of a machine is the
number of times that the machine increases an
input force
Mechanical Advantage
Two types of mechanical advantage
• Actual
• Ideal
Actual Mechanical Advantage
• Determined by measuring the actual forces
acting on a machine
• Equals the ratio of output force to the input force
Actual mechanical = output force
Advantage
input force
Ideal Mechanical Advantage
• The mechanical Advantage in the absence of
Friction
Because friction is always present, the actual
mechanical advantage of a machine is always
less than the idea mechanical advantage
Ideal mechanical=input distance
Advantage
output distance
Efficiency
• The percentage of the work input that
becomes work output is the efficiency of a
machine
Because there is always some friction, the
efficiency of any machine is always less than 100
percent
Efficiency
Efficiency=( work output/work input)X 100
Simple Machine
6 types of simple machines
• Lever
• Wheel and axle
• Inclined plane
• Wedge
• Screw
• pulley
Levers
• A rigid bar that is free to move around a fixed
point
• The fixed point that the bar rotates around is
the Fulcrum
Levers
• Input arm-The distance between the input
force and the fulcrum
• Output arm-The distance between the output
force and the fulcrum
Levers
3 types of levers
• The position of the fulcrum identifies the class
of the lever
– First class levers
– Second class levers
– Third class levers
First class levers
• In a first class lever, the fulcrum is ALWAYS
located between the input and output force
• Ex: seesaw
• Sissors
• Tongs
• Screwdriver/paint can
Second Class Lever
• The output force is ALWAYS located between
the fulcrum and the input force
» Ex: wheel barrel
Third Class Lever
• The input force is ALWAYS located between
the fulcrum and output force
Wheel and Axle
• A simple machine that consists of two disks or
cylinders, each one with a different radius
• Ex: steering wheel
• Screwdriver
» The two cylinders act as a fulcrum
»
Incline Plane
• A simple machine in which slanted surfaces
along which a force moves an object to a
different elevation
• Ex: wheel chair ramp
• Stairs
Wedges
• A v-shaped object whose sides are two incline
planes sloped toward each other
• Ex: axe
– Nail
– knife
Screw
• An incline plane wrapped around a cylinder
– Two screws of the same length, the one whose
threads are closer together moves forward less
for each turn of the screw.
– Ex:
– Screw
– Corkscrew
Pulley
• A simple machine that consists of a rope that
fits in a groove in a wheel.
• 3 types of pulley (figure 19 pg 432)
Compound Machines
• A combination of two or more simple machines
that operates together
• Ex: sissors, where the edges are like wedges and the
blades/handles function as levers