Chapter 2.1.2
Work in Mechanical Systems
Principles of Technology I
Edinburg North High School
Notes 2.1.1
Work in Mechanical Systems
Objectives:
• Identify basic machines.
• For a given machine, identify the input and output
work.
• Explain how efficiency relates to input and output
work for a mechanical system.
• Calculate efficiency for non-ideal (real machines)
Machines
•
Machines can make work easier by:
•
•
•
changing the direction of the force used to do the work;
and multiplying the force used to do the work.
In an ideal machine, the work put into the machine
is equal to the work coming out of the machine.
Win  Wout
An Inclined Plane (or Ramp)
Mechanical Advantage
•
•
Mechanical Advantage (MA) is the number of
times the output force is multiplied by a machine.
If in an ideal machine
Win  Wout
•
then
Findin  Fout dout
•
By definition of MA
Fout d in
MA 

Fin d out
Six Types of Machine in Two
Families
Real Machines Are Not 100%
Efficient
•
•
Machines are designed to convert input work into
useful forms of energy or output work.
In real machines the output work is always less
than the input work.
•
•
Some input work has to overcome undesired loses such
as the negative work of friction.
Machines are often rated by efficiency or percent
efficiency.
•
Efficiency in real machines is always less than 1, or
100%
Wout
Efficiency
Win
Wout
Percent Efficiency
100%
Win
Sample Problem
•
A sailor uses a rope and an old, squeaky pulley to
raise a sail that weighs 140 N. He finds that he
must do 180 J of work on the rope in order to raise
the sail by 1 m (doing 140 J of work on the sail).
What is the efficiency of the pulley? Express your
answer as a percentage.
Example 2.3 Work and Efficiency for
a Block and Tackle, p. 89
•
The block and tackle shown in figure 2.3 (p. 88) is
used to lift an automobile engine. The engine
weighs 600 lb and is raised 0.9 ft. The operator
pulls with a force of 100 lb over a distance of 6 ft.
Calculate the efficiency of the block and tackle
system.