#28: The student will calculate the
Ideal Mechanical Advantage of a lever, an inclined plane, and a block and tackle
#29: The student will manipulate the following type of simple
machines in order to understand the directional nature of forces and the
multiplication of forces: inclined
plane, level, simple pulley, block and tackle, and wheel and axle.
The student will understand that no machine operates with perfect
efficiency (Workout is less than Workin ) and identify
friction as one reason for a decrease in efficiency.
used a machine today? You probably
know that a bicycle is a machine.
Pencil sharpeners and can openers are also machines.
A machine is a device that makes work easier.
machines are powered by engines or electric motor; others are people-powered.
A simple machine is a device that does work with only one movement.
There are six types of simple machines.
Can you name some in the kitchen, in sports, in construction?
you wanted to pry a lid off a wooden crate with a crowbar.
You would slip the end of the crowbar blade under the edge of the crate
lid and push down on the handle.
You would do work on the crowbar, and the crowbar would do work on the
lid. Two forces are involved when a
machine is used to do work. The
force applied to the machine is called the effort force (Fe).
The force applied by the machine to overcome resistance is called the
resistance force(Fr). In
the crate lid example, you apply the effort force to the crowbar handle.
The resistance force is the force the crowbar applies to the lid.
also two kinds pf work to be considered when a machine is used-the work done on
the machine and the work done by the machine. The work done on the machine is called work input(Win);
the work done by the machine is called work output(Wout).
that work is the product of force and distance: W = F x d.
input is the product of the effort force and the distance that force is exerted:
Win =Fe x
output is the product of the resistance force and the distance that force
moves: Wout =Fr
that energy is always conserved. So,
you can never get more work out of a machine than you put into it.
In other words, W out can never be greater than Win.
In fact, whenever a machine is used, some energy is changed to heat due
to friction. So, Wout is
always smaller than Win.
a perfect machine has never been built, it helps to imagine a frictionless
machine in which no energy is converted to heat. Such an ideal machine is one in which work input equals work
output. For an ideal machine,
de =Fr x
number of times a machine multiplies the effort force is the mechanical
advantage(MA) of the machine. To
calculate mechanical advantage, you divide the resistance force by the effort
MA = resistance force =
worker applies an effort force of 20N to pry open a window that has a resistance
force of 500N. What is the
mechanical advantage of the crowbar?
is known? Resistance force
Effort force =
is unknown? Mechanical advantage
= 500N= 25
the mechanical advantage needed to lift a 2000N rock, using a jack with a
mechanical advantage of 10.
= 2000N/10 =200N
A simple machine does work with only one ________________________.
The force applied to a machine is called the ________________________.
The force applied by a machine is called the _______________________.
The number of times a machine multiplies is the ____________________ of the
how simple machine can make work easier without violating the law of
conservation of energy.
carpenter uses a claw hammer to pull a nail from a board.
The nail has a resistance of 2500N.
The carpenter applies an effort force of 125N.
What is the mechanical advantage of the hammer?
Critically: Give an example of a simple machine you’ve used recently.
How did you apply effort force? How
did the machine apply resistance force?
7.2 The Simple Machines
is a bar that is free to pivot, or turn, about a fixed point.
The fixed point of a lever is called the fulcrum.
The part of the lever on which the effort force is applied is called the
effort arm. The part of the lever that exerts the resistance force is
called the resistance arm.
following equation, which assumes no friction, can be used to find the ideal
mechanical advantage(IMA) of any lever.
IMA = length
of effort arm =
length of resistance arm Lr
A worker uses an iron bar to raise a manhole cover weighing 65N.
the effort arm of the lever is 60cm long.
The resistance arm is 10cm long. What
is the ideal mechanical advantage of the bar?
Le = 60cm
resistance arm Lr
????ideal mechanical advantage
IMA = Le/Lr
Complete the practice problems on page 187
Refer to page 199.
Create your own Rube Goldberg device