The lever, third millennium b.c. – The Egyptians had not yet discovered the wheel when they built their pyramids; they are thought to have relied heavily on levers.
Lever: A straight rod or board that pivots on a point known as a fulcrum. The fulcrum can be moved depending on the weight of the object to be lifted or the force you wish to exert. Pushing down on one end of a lever results in the upward motion of the opposite end of the fulcrum. Examples: Door on Hinges, Seesaw, Hammer, Bottle Opener
A lever is an object that is used with a pivot point, or ‘fulcrum’, to multiply the force applied to another object. Levers are often long and skinny and made of rigid material. Levers are one of the six simple machines.
There are actually three types of levers! They are called first-class levers, second-class levers, and third-class levers.
A see-saw is an example of a first-class lever. A first-class lever the fulcrum is located between the force pushing down- the input force-(on a see-saw that would be the person going down) and the output force (the person going up).
A wheel barrow is an example of a second-class lever. more…
Mechanical advantage – a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever. Machine components designed to manage forces and movement in this way are called mechanisms.
Law of the Lever – proven by Archimedes using geometric reasoning. It shows that if the distance a from the fulcrum to where the input force is applied (point A) is greater than the distance b from fulcrum to where the output force is applied (point B), then the lever amplifies the input force. In other words, it is possible for you to lift something with a lever that is too heavy for you to lift on your own.
Machines of war
Trebuchet at Warwick Castle (video 1:19) – a trebuchet launching a cannon ball of about 80 pounds over two football fields. The cannon ball was dipped in oil and set ablaze. another view 1:12
The trebuchet played an important role in battles fought in the middle ages. They were made of wood so they could be made anywhere. They could launch large heavy objects like rocks over distances of thousands of feet.
Although it evolved over several hundred years, the [/Engineering%20Design%20Process Engineering Design Process] was followed to produce this effective weapon.
- Ask In medieval times, castles were attacked by men on foot and horses. The defenders had bows and arrows to keep attackers away. Throwing large rocks and fire balls from a safe distance was a good strategy. Wood and rock were available everywhere and could be used to build structures. Based a lever, the trebuchet design was simple, portable and effective. What materials were available? How close could they safely approach a castle?
- Imagine A trebuchet is a type of catapult that works by using the energy of a raised counterweight to throw a projectile. How could they throw heavy rocks? How could they throw rocks a long way?
- Design, Build The long end of the beam is pulled down, raising the counterweight. A sling, which has a pouch containing the projectile, is attached to the long end of the beam. When the trigger is released, the sling and the beam swing upward, pivoting on the axle. At the top of the swing, the sling releases the projectile to fly towards the target. The stored energy of the counterweight is transferred efficiently to the projectile. How much weight was required for the counter weight? How long did the beam need to be?
A long beam is attached to an axle. At the short end of the beam is attached the counter weight. How was the pivot made? What was the trigger release mechanism?
- Improve The trebuchet was used for hundreds of years so there were many improvements made over its history. The earliest versions were simple and stationary. Later, huge wheels were attached so the structures could be moved along with the advancing armies. The accuracy and range were impressive, even by today’s standards. How did they improve the accuracy of the trebuchet?
- Projectile at an angle (video 12:46) – The men launching the trebuchet were remarkably accurate in hurling rocks at enemy castles and fortresses. They figured out what worked and did it. Later on, someone discovered the math and physics that explained it. They knew the performance characteristics of the machine – length of beam, force of the release. They determined the distance to the castle and the weight (mass) of the rock. From that they were able to determine the angle of the path for the rock to reach the target. They adjusted the trebuchet so the rock traveled that path once it was released.
- energy, counterweight, projectile, beam, axle, pivot
Here are some challenges for you to work on…
- Seige engine (video 0:51) – create a model trebuchet and test different configuration by varying the arm length, pivot point, load and sling setup.
- research historical use of trebuchets – design variations, battles, sieges, locations and time periods
- build models to demonstrate the inclusion of levers in simple and complex machines
- Lever an Obelisk – you’re asked to lift one end of a relatively small obelisk three feet. You have a lever, a movable fulcrum, weights, and supporting stones at your disposal. Your goal is to lift the obelisk using as few of the weights and support stones as possible. requires Shockwave
- Destroy the Castle – trebuchet was one of the most devastating weapons of the Middle Ages. During medieval sieges, this destruction machine crushed castle walls. Using trial and error, medieval engineers adjusted the trebuchet to make sure its missile hit the castle wall—and destroyed it. This game is very much like the real thing. There are five elements you can adjust in your trebuchet, requires Shockwave