Teaching Guidance Class practical: Friction spoils the downhill and uphill experiment of a rolling ball on a curved track. This is Galileo's Practical Activity In this activity, students explore how high a ping pong ball bounces when dropped by itself and then with a golf ball. You can Classroom Activity Conservation of Energy. They learn about the wheel and axle as a means to transport materials from rock quarry to construction site.
General knowledge of pyramids. Familiarity with the six simple machines introduced in Lesson 1 of this unit. We are well underway in learning about simple machines and building the ancient Egyptian pyramids that we have been hired as engineers to design and construct.
Now, we are going to go deeper into our understanding of pulleys to see if we can use this knowledge to help make our work easier. No one knows for sure if pulleys were one of the simple machines that ancient cultures used to build pyramids.
A few people believe that the beautiful and massive pyramids could not possibly have been built using something as simple as simple machines. Some people have wild theories about how the pyramids came to be — that perhaps aliens from another planet came to Earth and built them.
Well, we do not know about that, but we do know that human beings are very creative and resourceful when they want to be. As long as we limit ourselves to materials and technologies that would have been available to the ancient Egyptians, it is acceptable for us to use our knowledge to construct pulley systems to build our pyramid.
Figure 2. A pulley on a fishing boat. All rights reserved. A pulley is a simple machine consisting of a string or rope wrapped around a wheel sometimes with a groove with one end of the string attached to an object and the other end attached to a person or a motor. Pulleys may seem simple, but they can provide a powerful mechanical advantage so lifting tasks may be done easily. Pulleys are used in many ways, everyday.
What kind of pulleys can you think of? An elevator is a modern engineering use of a pulley system that performs much like the raising of a large stone for pyramid building. Without the use of pulleys, an elevator would require a large motor to pull the cable straight up. Instead of using a large motor, some elevators use a large weight that takes advantage of gravity to help raise the elevator car see Figure 3. In this situation, the powering motor can be much smaller and only be used to determine the direction the elevator should go.
Figure 3. Adding a counterweight and two pulleys with a motor in the middle makes an elevator easier to move. But how can a wheel with string over it help us move the huge stones required to build a pyramid?
Well, pulleys help us by changing the direction of the force we use to lift an object. Is it easier for you to pull up on a rope or pull down on a rope? By using a pulley, we do not have to pull up on a rope to lift a heavy object attached to it, but instead we can pull down on it. Think of a flagpole as an example. When you pull down on the rope of a flagpole, the flag goes up the pole to wave in the air.
That's because a flagpole has a pulley on it. By using pulleys to redirect force , a stone could be lifted off the ground, allowing more people to grab on to the rope, and adding weight so workers have to pull less. To simplify this effort even more, workers using a pulley could move a large stone up a ramp by pulling on a rope while walking down the ramp, using gravity to their advantage.
The real mechanical advantage of a pulley is in using many pulleys at once. Using multiple pulleys decreases the amount of force necessary to move an object by increasing the amount of rope used to raise the object.
The mechanical advantage MA of a pulley system is equal to the number of ropes supporting the movable load. That means, do not count ropes that are only used for redirecting, see Figures 6 and 7. We know from other lessons on simple machines that to gain a greater mechanical advantage, there is a trade-off. With a pulley, the trade-off is distance. So, if two pulleys are used together, the amount of force required is cut in half, but twice the amount of rope is needed to be pulled to raise the object to the same desired height.
Illustrate this concept to students by conducting the following classroom demonstration; see Figure 4. Figure 4. Tie the rope to one of the brooms broom 1 and wrap the rope around the other broom broom 2. Have two students stand about a meter apart each holding one broom, and try to keep the brooms separated while the third student pulls on the free end of the rope; it should be a difficult task to pull the broom sticks together.
Next, wrap the rope around each of the brooms again. This is an example showing the power of mechanical advantage. Refer to the associated activity Pulley'ing Your Own Weight to further students involvement in understanding pulley systems by illustrating how a pulley can be used to easily change the direction of a force, making the moving of large objects easier.
Pulleys can be much more complicated. Engineers combine many pulleys into a pulley system that significantly reduces the amount of force required to lift an object. They often use pulley systems to move extremely heavy objects.
A block and tackle is an example of a pulley system that can be attached to anything. It may take a lot of cable or rope, but a human using enough pulleys could lift several tons. Engineers use the block and tackle along with motors and electronics to create modern devices that operate with very low power requirements, such as cranes and elevators.
At Disneyland, engineers even use a pulley system to move Tinkerbell across the sky. We are unsure if the Egyptians used pulleys, and have yet to find any evidence that they did, but we do know, that if they had used them, life would have been easier than if they did not. Now, since we understand pulleys and have modern materials, we can build the pyramids much easier. Today we are going to look at engineering a pulley system and see if we can design a way to get our heaviest stones to the top of our pyramid with the help of this simple machine.
Use the Pulleys and the Pyramids PowerPoint presentation as a helpful classroom tool. Show the PowerPoint presentation, or print out the slides to use with an overhead projector.
The presentation is animated to promote an inquiry-based style; each click reveals a new point about each machine; have students suggest characteristics and examples before you reveal them.
The pulley, a simple machine, helps to perform work by changing the direction of forces and making easier the moving of large objects. When thinking of pulleys, most people think of the type of pulley that allows a person to redirect the direction of a force. With this type of pulley — called a fixed pulley — pulling down on a rope makes an object rise off the ground.
There are also movable pulleys and pulley systems. Thousands of years ago, early engineers used pulleys to help with construction and many useful everyday tasks. Many obelisks were erected using pulleys and wells have pulleys to help retrieve water. Figure 5. A fixed pulley with no mechanical advantage. The most commonly understood concept of a pulley is that it is a simple machine that redirects force.
This means that by looping rope around a pulley and attaching the rope to an object, one pulls down on the rope to raise the object, instead of having to lift the object see Figure 5; imagine raising a flag. Although this is a helpful and convenient use for pulleys, it has a major limitation: the force you must apply to lift the object is the same amount as if you were just lifting the object without the pulley which is acceptable for raising a flag, but not helpful enough if trying to raise a pyramid stone.
Also, Read: What are Garden Forks? As in the figure below, we have to move an object with a single pulley, and we will calculate the force without friction between the wheel and the rope. Types of Spanner Types of Wrench. Pulleys are a simple machine composed of a wheel on an axle that supports the change of speed and direction of a taut cable or belt or the transfer of power between the axle and the cable.
Pulleys may have a groove or grooves between the flanges around their circumference for running along the cable or belt.
The drive element of a pulley system may be a cable, rope, belt, or chain. A fixed pulley system consists of a wheel that is attached to a surface such as a wall or a floor and a rope that hangs free.
The pulley remains stationary. A fixed pulley minimizes the amount of force needed to move a load but allows a person to redirect the necessary force. So instead of lifting a heavy object directly with upward force, a person can use a pulley to lift the object by pulling down on the rope. They are used in flagpoles, curtain binders, and wells.
Pulley can be made of wood, plastic, or metals, depending on the application. A moving pulley, also known as a movable pulley, consists of wheels that are not attached to a particular surface, but instead, a rope or cable is attached to a stationary surface.
This provides a mechanical advantage, meaning that the pulley reduces the amount of force that a person must apply to move the load. The load attaches to the wheels instead of the ropes; the act of pulling the rope slides the wheel over the rope, bringing the load with it.
For this action, less force is required than to lift the load directly. They are used in cranes, utility lifts, and lifting machines. The compound pulley system consists of both a movable pulley and a fixed pulley. This combination of pulleys means that not only is less force required to lift a load, but a person can also change the direction of the force. While this type of configuration makes it possible to move heavy loads very easily, the tradeoff is that it requires significantly more speed to do the job.
A block and tackle pulley is a variant of a composite pulley that can dramatically reduce the force required to move a heavy object. A block-and-tack pulley system is comprised of a number of fixed and movable pulleys that are arranged parallel to each other.
The fixed pulley is aligned with other fixed pulleys and is movable with the movable. Each of these compound pairs is connected to the next pair, and each set reduces the total force required. They are used in sailboats, rock climbing, shops, and garage shutters, and theater curtains. Cone pulleys consist of several pulley wheels that decrease in circumference as they are stacked on top of each other to form a cone shape.
This cone shape allows the winch operator to transfer the speed of the pulley, with a smaller circumference requiring less force but also generating less force. Multi-gear bicycles essentially work on this system. Conveyor pulleys in a conveyor system change the direction of the conveyor belt, drive the belt, and provide tension to the belt.
They are made of rolled shells with flexible end discs and locking assemblies. They are usually located at one end of the conveyor or near the motor. Also, Read: What Is a Rivet? How Is Riveting Done? Riveting Definition Types of Rivet. A flat belt pulley is used in transmission systems that are driven by flat belts, typically in high speed, low power applications in textiles, paper making, and office machinery such as computer printers.
Flat belts are also used for conveyors. Flat belts are also less expensive than the belts used in snake belt pulleys. One safety factor is that in high torque situations, the belt may slip, causing damage to equipment other than the belt.
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