Download Pulley Worksheet

POE
0204
Name: __________________________________________________ Period: _______________
Pulley Worksheet
A pulley is an adaptation of a wheel and axle. A single pulley simply reverses the direction
of a force. When two or more pulleys are connected together, they permit a heavy load to be
lifted with less force. The trade-off is that the end of the rope must move a greater distance
than the load.
Concepts:
Effort
Force applied
Direction change
Mechanical Advantage = Total number of strands supporting the load
The first case involves using the known Effort and the M.A. to find the Load that can be
moved:
The second case involves using the known Load and the M.A. to find the Effort needed:
POE
0204
Fixed Pulley
The Three Types of Pulleys
Movable Pulley
Block and Tackle
Fixed Pulley:
The pulley is physically attached or fixed to a strong member, which will not move. When a
fixed pulley is used the force needed to lift a weight does not change.
Movable Pulley:
A movable pulley splits the work in half. The effort needed to lift 180 pounds weight is 90
pounds. The mechanical advantage of a movable pulley is 2.
Block and Tackle:
The block and tackle is a system of three pulleys. It reverses the direction of the effort so that
a downward pull can be used to lift an object. The mechanical advantage is 3 so that 40
pounds of effort is needed to lift an object weighing 120 pounds. (The distance of the rope
pulled is tripled.)
Examples:
Using a block and tackle pulley system, determine the number of strands that will be
needed to lift a weight of 1040 lbs. by applying 80 pounds of force.
Using a block and tackle pulley system, with 7 strands and an input force of 45 pounds,
what is the maximum weight you can lift?
POE
0204
Imagine that you have the arrangement of a 100 pound (45.4 kilogram) weight suspended from a
rope, as shown below:
In the above figure, if you are going to suspend the weight in the air then you have to apply an
upward force of 100 pounds to the rope. If the rope is 100 feet (30.5 meters) long and you want to
lift the weight up 100 feet, you have to pull in 100 feet of rope to do it. This is simple and
obvious.
Now imagine that you add a pulley to the mix, as shown below:
POE
0204
Does this change anything? Not really. The only thing that changes is the direction of the force
you have to apply to lift the weight. You still have to apply 100 pounds of force to keep the
weight suspended, and you still have to reel in 100 feet of rope in order to lift the weight 100 feet.
The following figure shows the arrangement after adding a second pulley:
This arrangement actually does change things in an important way. You can see that the weight is
now suspended by two ropes rather than one. That means the weight is split equally between the
two ropes, so each one holds only half the weight, or 50 pounds (22.7 kilograms). That means
that if you want to hold the weight suspended in the air, you only have to apply 50 pounds of
force (the ceiling exerts the other 50 pounds of force on the other end of the rope). If you want to
lift the weight 100 feet higher, then you have to reel in twice as much rope - 200 feet of rope must
be pulled in. This demonstrates a force-distance tradeoff. The force has been cut in half but the
distance the rope must be pulled has doubled.
Solve (show all work):
Load/Weight
20 lbs
Effort
50 lbs
30 lbs
3.
120 lbs
210 lbs
15.
225 lbs
5.
6
3
7.
80 lbs
10 lbs
9.
200 lbs
140 lbs
M.A./Strands
2
1
1.
11.
7
4
13.
35 lbs
25 lbs
17.
Type of Pulley
2.
4.
6.
8.
10.
12.
14.
16.
18.
POE
0204
19. Using a block and tackle pulley system, determine the number of strands that will be
needed to lift a weight of 720 lbs. by applying 120 pounds of force.
20. Using a block and tackle pulley system, with 8 strands and an input force of 53 pounds,
what is the maximum weight you can lift?
21-25. List five examples of where you might find or use a pulley:
21.
22.
23.
24.
25.
_____________________
_____________________
_____________________
_____________________
_____________________