Science 7 Unit B: Structures and Forces. Topic 4. Forces, Loads, & Stresses. pp WORKBOOK. Name:

Transcription

1 Science 7 Unit B: Structures and Forces Topic 4 Forces, Loads, & Stresses pp WORKBOOK Name:

2 Every object that provides support is a structure. A structure may be made up of one or more parts, and it may be any size. Structures vary in their strength and stability. o Structural strength refers to a structure s capacity to hold itself up, as well as the weight added to it. resists forces acting on it without squishing or bending o Structural stability is a structure s ability to maintain its position even when it is being acted on by a force. the ability of a structure to keep from toppling over Read pp Read Did You Know - p. 306 Forces act on structures in many different ways. Engineers divide these forces that affect structures into two main categories: forces External Force stress applied to the outside of a structure Example: collisions, kicking a ball, wind, gravity, etc. 1) permanent force acting on a structure o Also called static load, which does not change position or location o usually part of the structure o Example: weight of structure (gravitational force) 2) changing or non-permanent force acting on a structure o Also called dynamic load, which moves or changes position and is not always in the same place on the structure o Example: people, furniture, snow, rain, wind TASK: Consider the following loads that are acting on a tree. Classify them as static (S) or dynamic (D) loads on the line provided. Wind blowing against a tree The weight of the tree The weight of a bird in the tree 1

3 Read p. 307 Internal Force stress that acts on the inside of a structure Occurs when one part of a structure exerts force on other parts of the same structure Often caused by external forces Can change the shape or size of the structure describes the internal damage caused by external forces Materials that make the structure are unable to resist the load acting on them When too much deformation occurs, the entire structure fails 1) acts to pull apart an object o Pull and stretch a material o measures the largest amount of tension force that a material can stand before breaking Example: spider silk is a material that has high tensile strength Read Did You Know - p ) acts to squish an object o Pressing, squeezing, and pushing materials o measures the largest compression force the material can stand before losing its shape or breaking 3) acts to push parts of an object that are in contact with each other in opposite directions o different parts forced in opposite directions at the same time o tearing materials o measures the largest shear force the material can stand before ripping apart 4) acts to twist parts of an object in different directions o Twist a material by turning the ends in opposite directions o measures the largest torsion force the material can stand and still spring back to its original shape Example: twist tie, nut and bolt, twist off cap 2

4 TASK: Identify the type of strength shown by the following objects chain connecting a ship to its anchor beef jerky that you are chewing bolt being tightened with a wrench legs of a chair you are sitting on Read p. 308 combination of compression (push) and tension (pull) forces that result in a temporary curving change in the share of a structure Load TASK: Label the following illustrations with the correct internal force. 3

5 Discuss Bend That Bike Activity - p. 308 Discuss Figure 4.38 p. 309 Read Cool Tools - p. 309 conditions that a structure, once it is built and operational, must meet in order to show that it is performing to certain standards. Structures must be made to support a particular load and perform other functions TASK: What are three specific performance requirements that braces must meet? Read p. 314 Steel is an example of a material with high tensile strength. o strong forces pull particles together and difficult to break Graphite is also an example of a material with low shear strength. o Layers of material have relatively weak connections and slide past each other. Rubber has a high torsion strength since it can easily be twisted without breaking. 4

6 Complete Topic 4 Review Questions p. 314 #1-5 Please record your answers below or on an attached sheet of loose leaf. 5