But how does a pulley lift? Why does this particular mechanical configuration allow us to lift so much weight so easily?
A pulley is just a grouping of one or more wheels over which is looped a rope or sturdy chain, for heavier objects. Using a pulley greatly multiplies the force of your physical efforts. With a two-wheel pulley, you reduce the effort you exert to lift the same amount of weight.
This type of system is good if you are trying to raise an object located below you to your level. In a variation, if both sides of a movable pulley system are fixed and the rope is taut between the fixed points, the system becomes like a wheel and axle because the object can ride along the rope if a force is applied to it for example, a zip line.
Figure 7. A pulley system with a mechanical advantage of two. Figure 8. A pulley system with a mechanical advantage of four because it has four load-supporting rope segments. Using a system of pulleys can be much more complex and provide a powerful mechanical advantage — greatly reducing the amount of force required to move an object.
If one movable pulley is used Figure 6 , the amount of force required to raise the object attached to the movable pulley is cut in half. The pulley system seen in Figure 7 does not change the mechanical advantage from Figure 6, however, it does change the direction of the necessary force.
The trade-off is that the amount of rope required increases and the amount of rope that you must pull to raise the object is also increased.
If two fixed pulleys are added to the system and a second movable pulley is attached to the object, the amount of force needed to raise the object becomes one-fourth of the object's weight, and four times as much rope is required see Figure 8 and Pulleys and the Pyramids PowerPoint presentation.
The powerful mechanical advantage of a pulley is in using many pulleys at once. Combining multiple pulleys decreases the amount of force necessary to move an object by increasing the amount of rope used to raise the object. That means, do not count ropes that are only used for redirecting, see Figures 6, 7 and 8. Watch this activity on YouTube. How can pulleys make our lives easier? Pulleys are powerful simple machines. They can change the direction of a force, which can make it much easier for us to move something.
If we want to lift an object that weighs 10 kilograms one meter high, we can lift it straight up or we can use a pulley, so we can pull down on one end to lift the object up. It is much easier to use the pulley because, as long as we weigh more than 10 kilograms, we can just hang onto the end of the rope and take advantage of gravity so our weight provides all the necessary force to lift the object.
Pulleys can also provide us with a mechanical advantage when we use several together and more rope. This process lessens the amount of force required to lift something. While we do not know if pulleys were used by ancient pyramid builders, we know pulleys are an ideal simple machine for many of the tasks required to build a pyramid.
In today's highly-technical world, engineers still use pulleys to make difficult tasks easier. Without them, our lives would be much more difficult. Conduct summary assessment activities as described in the Assessment section. Conclude by finishing the KWL Chart and assigning Word Problems in which students calculate the mechanical advantage of an inclined plane see the Assessment section.
In other lessons of this unit, students study each simple machine in more detail and see how each could be used as a tool to build a pyramid or a modern building.
Making the task easier which means it requires less force , but may require more time or room to work more distance, rope, etc. For example, applying a smaller force over a longer distance to achieve the same effect as applying a large force over a small distance. The ratio of the output force exerted by a machine to the input force applied to it.
Usually consists of a grooved wheel in which a pulled rope or chain runs. For example, a wedge, wheel and axle, lever, inclined plane, screw, or pulley. Brainstorming: As a class, have the students engage in open discussion. Remind students that in brainstorming, no idea or suggestion is "silly.
Take an uncritical position, encourage wild ideas and discourage criticism of ideas. Have them raise their hands to respond. Write their ideas on the board. Ask the students:. On a large sheet of paper or on the classroom board, draw a chart with the title "Simple Machines: Pulleys. Fill out the K and W sections during the lesson introduction as facts and questions emerge. Fill out the L section at the end of the lesson.
List all of the things students learned about pulleys and their mechanical advantages. Were all of the W questions answered? What new things did they learn? Based off of their observations of an object's motion can a pattern be used to predict future motion? Word Problems: Assess students' understanding of the lesson concepts by assigning the following word problems.
Try making a human pulley. You need board, a strong rope and a spot with an overhead support, such as a soccer goal or playground equipment. Wrap one end of the rope around a 2 x 4 or something strong such as a seat from a swing and wrap the other end of the rope around the goal, letting the trailing end hang to the ground. Allow one child to sit on the 2 x 4 while two other children try to lift them by pulling down on the free end of the rope.
Keep wrapping the rope around the goal or support bar until two children can easily lift the sitting child up and down. It may be helpful to start the sitting child from a standing position both feet on the ground. If the students are unfamiliar with a zip line, have them research this on the internet.
A zip line is a fun example of a movable pulley. Challenge more advanced students to calculate the mechanical advantage of using multiple pulleys, requiring division with remainders or fractions. Accessed January 25, However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.
Why Teach Engineering in K? Find more at TeachEngineering. Quick Look. Simple Machines Pulleys. Print this lesson Toggle Dropdown Print lesson and its associated curriculum. Suggest an edit. Discuss this lesson. Curriculum in this Unit Units serve as guides to a particular content or subject area. Wheeling It In! TE Newsletter. Subscribe to TE Newsletter. Summary Students continue to explore the story of building a pyramid, learning about the simple machine called a pulley.
To hold the weight in the air you must apply only 25 pounds of force, but to lift the weight feet higher in the air you must now reel in feet of rope. For example, a lever is an example of this phenomenon:. In this diagram a force F is being applied to the left end of the lever. The left end of the lever is twice as long 2X as the right end X.
Therefore on the right end of the lever a force of 2F is available, but it acts through half of the distance Y that the left end moves 2Y. Changing the relative lengths of the left and right end of the lever changes the multipliers. In this diagram the left-hand gear has twice the diameter of the right-hand gear.
For every turn of the left-hand gear, the right-hand gear turns twice. If you apply a certain amount of torque to the left-hand gear through one rotation, the right-hand gear will exert half as much torque but will turn two revolutions. Assume that you have two cylinders full of water with a pipe connecting the two cylinders together as shown.
If you apply a force F to the left-hand plunger, it creates a pressure in the left-hand cylinder. Let's say you apply a 10 pound downward force to the left-hand cylinder. Let's also say that the radius of the left-hand cylinder is 0. If the radius of the right-hand cylinder is 4 times greater, or 2. If you push the left-hand piston down through 16 inches with a force of 10 pounds, then the right-hand piston will rise 1 inch with a force of pounds.
Hydraulic cylinders of all sorts take advantage of this simple force-multiplying effect every day. You can see that a block and tackle, a lever, a gear train and a hydraulic system all do the same thing: they let you magnify a force by proportionally diminishing the distance through which the magnified force can act.
It turns out that this sort of force multiplication is an extremely useful capability! Here are some of the devices that use these simple principles:. Sign up for our Newsletter!
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