WELCOME TO PERIOD 10
Homework Exercise #9 is due today.
Results of Midterm 1 will be available at your
next class meeting: Tuesday or Wednesday.
PHYSICS 1103 – PERIOD 10
•How do pulley systems work?
•What is a block and tackle?
•What are ideal and actual mechanical
advantages?
•Remember to put away your phone.
No calls or texting during class.
Pulley systems
Pulley systems are an example of a simple
machine.
Fixed pulleys change the direction of the
force in.
Moveable pulleys spread the force of the load
over two or more rope segments.
A block and tackle is a system of two or more
pulleys with rope threaded between them.
Mechanical advantage
• Machines allow work to be done with less input force.
• The mechanical advantage of a machine is a measure of
how much a machine multiplies the input force.
Force In
= 50 lbs
Load = 100 lbs
This lever doubles the force input of 50 lbs to lift a
100 lb load. It has a mechanical advantage of 2.
Ideal mechanical advantage
Ideal mechanical advantage is an ideal case with no
energy wasted by frictional forces.
MA ideal
D in

Dout
Din = distance you move the machine
Dout = distance the load moves
Ideal mechanical advantage of pulleys
The ideal mechanical advantage of a pulley system
equals the number of directly attached rope segments
supporting the load. (The rope you pull to apply force
does not count.)
This pulley system has two attached ropes.
Its ideal mechanical advantage = 2
Actual mechanical advantage
Actual mechanical advantage takes into account the
energy wasted by frictional forces.
MA actual

Fout
Fin
Fin = the force you exert on the machine
Fout = the force exerted on the load by the
machine
Summary of mechanical advantage
Ideal mechanical advantage ignores any energy
wasted by friction.
D in
MA ideal 
Dout
Actual mechanical advantage takes into account the
energy wasted by friction.
MA actual

Fout
Fin
Work done with machines
Machines cannot reduce the amount of work needed
to perform a task.
However, machines make it possible to use less input
force applied over a longer distance.
Ignoring energy wasted by friction, the work put into
a machine equals the work done by the machine:
Work in = Work out
Because some energy is always wasted overcoming
frictional forces, the amount of work required using a
machine is greater than the amount of work required
without a machine.
Efficiency of machines
Efficiency
Wout

Win
Win = work put into the machine (joules or ft-lbs)
Wout = work done by the machine (joules or ft-lbs)
Since Work = Force x Distance,
Efficiency
Wout

Win

Fout Dout
Fin Din
Example
A machine requires 2,000 joules of energy to raise a 20
kilogram block a distance of 6.0 meters. What is the
machine’s efficiency?
First, find the work done to raise the block:
W = M g h = 20 kg x 9.8 m/s2 x 6.0 m = 1,176 J
Next, use the efficiency equation. The work in equals 2,000
joules.
Efficiency
Wout

Win
= 1,176 joules = 0.59 = 59%
2,000 joules
Efficiency as a ratio of mechanical advantages
Efficiency
Wout

Win

Fout Dout
Fin Din
The efficiency equation involves two familiar ratios:
MA actual

Fout
Fin
and
MA ideal 
D in
Dout
Substituting these ratios into the efficiency equations gives
Efficiency

MAactual
MAideal
Analyzing three pulley systems
Take measurements for the three pulley systems.
Use the equations below to fill in the table.
Show your calculations in the space below the table.
MA actual

Fout
Fin
Efficiency
MA ideal

MAactual
MAideal
D in

Dout
BEFORE THE NEXT CLASS…
Read textbook chapter 11
Complete Homework Exercise 10
Bring a blank Activity Sheet 11 to class.