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A Bridge Too Far…. What is this unit about?. In this unit you will: Learn about the different types of basic forces. Identify these forces within the machines in the department. Learn about the different types of motion. Identify some of these in the department.
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What is this unit about? • In this unit you will: • Learn about the different types of basic forces. • Identify these forces within the machines in the department. • Learn about the different types of motion. • Identify some of these in the department. • Identify where structures have failed in the past. • Understand the need for engineers to design structures that are safe. • Build a model of a bridge and identify some of the forces within the structure. So, to start with, we have to understand how forces work, and what they are called……….
What is a Force? … or extremely large as in the rocket motors of the Space Shuttle. However, in both cases the principle is the same. Somehow, something called a force is being used to push upwards. At its simplest, a force is simply a push or a pull. Forces can do many things, and can be extremely small, as in the wings of a fly, …….. Forces can do all of these things:
Simple Forces Compression is the scientific name for forces that cause squashing Tension is the scientific name for forces that cause stretching These tie straps are in TENSION The woodwork vice is COMPRESSING the wood
Forces can do more than stretch or squash 1.-- Forces can cause things to bend. The force of the wind bends the flagpole
Forces can do more than stretch or squash They can cause things to twist. Forces that cause twisting are called Torsion Forces Eg – loosening or tightening a nut …Opening and locking a door Are there any other forces going on in these pictures? (Tension, Compression or Bending?) Or… when peddling the crank of your bicycle
Forces can do more than stretch or squash Forces can also cut or slice. These forces are called shear forces Scissors The axle on the forks of your bicycle is in shear. The forks push down on the axle, whilst the wheel pushes up between the forks. If it were made of Plasticene it would just slice through. The guillotine in the metal workshop
Workshop Task 1 Reminder:- The basic forces are: Tension Compression Bending Torsion Shear. Take the handout sheet ‘Practical Exercise 1’. Look at the machines in the pictures. Take a trip around the machines in the workshop, and write in the names of the forces that you think exist.
Motion You have now looked at the basic forces that exist on the earth. Our inventions are often subjected to forces. These can be natural or man made. Sometimes, we want our inventions to stay still………….. ……… and at other times we want forces to make things move
Linear Motion This is the simplest form of motion. Something goes in a straight line. An example of this would be shooting an arrow. The arrow goes from the bow to its target in a straight line…..Or Does IT? What causes it to curve? What else might travel in basically a straight line? True linear motion is not particularly useful, as objects generally travel in one direction only……We usually want to get things back!!…. So, what is more useful is:
Reciprocating Motion These machines reciprocate....This means that they move backwards and forwards in a straight line. Any more? There are many more Reciprocating machines than Linear ones!
Rotational Motion This is by far the most common type of motion in our world. All of these Rotate Are there any more? The point about which an object rotates is called its AXIS
Oscillating Motion Oscillating motion occurs when something swings back and forwards Are there any more?
Workshop Task 2 Choose 6 machines in the workshop. You will be allowed to go and study these. On the handout sheet, there are 6 blank spaces. In each of the spaces draw a part of a machine that shows one of the types of motion. Label the picture with one of the motion symbols, and include the name of the machine. Two examples have been done to give you the idea..
Bridge Video Introduction • Show edited parts of video as introduction to bridge design
Concentrating on Structures:- Basic structural elements All structures have forces acting on them. Gravity pulls down on the LOAD. The TIE transfers this TENSION force to the wall. The STRUT pushes outwards to keep the LOAD in place. Because it pushes, the STRUT is in COMPRESSION . Without the STRUT, the LOAD would fall towards the wall. The part of the structure that has a tensile force acting on it is called a TIE. The part that has a compressive force acting on it is called a STRUT.
Roof design Struts and Ties are used in many applications…. Here is one of them: The beams in the roof of your house are under pressure from the weight of the tiles above. This COMPRESSIVE force is transferred to the STRUTS within the roof timbers. Without a TIE at the bottom, the house walls would be pushed apart.
Making Structures Rigid When forces are applied to a simple four-sided structure it can be forced out of shape quite easily. A structure which behaves in this way is said to be non-rigid. By adding an extra bar or member, corners A and B are prevented from moving apart. The structure then cannot be forced out of shape, and is said to be rigid. Notice that the additional member has formed two triangles in the structure. This strengthening process is called TRIANGULATION An alternative to triangulation is to use a gusset plate. A gusset is simply a piece of material used to brace and join the members in a structure. A triangular gusset plate has been used here but they can be made in a variety of shapes.
Where is Triangulation Used? Motorbike or Bicycle Frame A Tower Crane The Ghurkin Building in London The Entrance Hall to Napier University The Roof Structure at Fountain Park
Triangulation in action Imagine the simple frame loaded as shown in the picture. The elements of the frame have been labelled A, B, C, D and E. Both A and B have been removed from this diagram as they will not carry any of the load. They are known as REDUNDANT members. The Load will be carried by C, D and E only. Starting with E, decide whether the element is a STRUT or a TIE A is REDUNDANT REDUNDANT B is TIE C is CLICK for ANSWERS a STRUT D is E is a STRUT
What happens when designers get it wrong • Show Disaster powerpoint:
Bridge building task • You will work in teams of 4 • You will select a company name for your team • Your teacher will select a bridge for your team to build. • Each bridge has a target time for the build. • You will incur penalties for every minute you go over this time. • When you have completed the bridge, you will complete the worksheet – This will include Calculating the cost of the bridge, Finding its weight, Finding three places in the world where this bridge is used, (add more details) • Finally, you will present your bridge design to the class. Your presentation should follow the order on the Presentation Advice Handout Sheet. • Your peers will grade the quality of your presentation.
Moments & Levers • F*d=f*D • D=????
Numeracy • Put this into the costings sheet
There are two main types of basic force. Tension The rope is under tension as the two people pull on it. This stretching puts the rope under tension. Compression Weightlifters find that their body is compressed by the weights they are holding above their head.
