Forces.

 =Forces. =

All successful structures have to withstand [|forces] acting upon them (External loads), the body load (weight of the structure) and any internal forces acting within the structure.

When the internal forces within the structure equal the external loads acting upon the structure, the structure is said to be in [|equilibrium].

The mobiles you designed and made and the one we made as a class all work so well because we understand and can control equilibrium.

To find out more about the inspiration for this project [|here].

There are [|five forces] that can act within a structure when a load is applied.


 * Compression. || Tension. || Bending || Torsion. || Shear. ||
 * [[image:http://tbn0.google.com/images?q=tbn:DMlogmumegl05M:http://www.bloodlessturnip.com/images/squeezing_blood_out_of_a_turnip.gif width="129" height="107" align="center" link="http://images.google.co.uk/imgres?imgurl=http://www.bloodlessturnip.com/images/squeezing_blood_out_of_a_turnip.gif&imgrefurl=http://www.bloodlessturnip.com/&usg=__KIhAz4UC3vDIRiU7Sd-PYhj7Y8g=&h=400&w=481&sz=44&hl=en&start=5&sig2=QFahOaY5dvuU0zZqE_Toog&um=1&tbnid=DMlogmumegl05M:&tbnh=107&tbnw=129&ei=KNewSbejEIeI1QWTksmwAg&prev=/images%3Fq%3Dsqueezing%26hl%3Den%26lr%3D%26rlz%3D1G1GGLQ_ENFR273%26sa%3DG%26um%3D1"]] || [[image:http://tbn3.google.com/images?q=tbn:-viT2-zI9wRz7M:http://abnormalimage.com/images/tension.view.jpg width="150" height="117" align="center" link="http://images.google.co.uk/imgres?imgurl=http://abnormalimage.com/images/tension.view.jpg&imgrefurl=http://blackandreformedministries.com/2008/10/21/the-grace-tension-a-case-for-erroring-on-it/&usg=__Y00TxFZS3dcVENRqIcZDQVYqOHw=&h=1082&w=1382&sz=127&hl=en&start=2&sig2=jCR0Zg67fQBgMCJA0XYJ2A&um=1&tbnid=-viT2-zI9wRz7M:&tbnh=117&tbnw=150&ei=p9awSbC8Hoq80AWi4fSWAg&prev=/images%3Fq%3Dtension%26hl%3Den%26lr%3D%26rlz%3D1G1GGLQ_ENFR273%26sa%3DG%26um%3D1"]] || [[image:http://tbn2.google.com/images?q=tbn:1ARSXwggdtX5rM:http://www.usinenouvelle.com/industry/img/precision-bending-000051498-4.jpg width="143" height="85" align="center" link="http://images.google.co.uk/imgres?imgurl=http://www.usinenouvelle.com/industry/img/precision-bending-000051498-4.jpg&imgrefurl=http://www.usinenouvelle.com/industry/sofraser-1531/precision-bending-p25685.html&usg=__RnvcBtgJeHqzDvrd1aXRS5hYGG0=&h=467&w=784&sz=53&hl=en&start=52&sig2=1QiAczRBGUcdBcVVLbqoWg&um=1&tbnid=1ARSXwggdtX5rM:&tbnh=85&tbnw=143&ei=btewScDPOZW-0AXC-cSWAg&prev=/images%3Fq%3Dbending%26ndsp%3D20%26hl%3Den%26lr%3D%26rlz%3D1G1GGLQ_ENFR273%26sa%3DN%26start%3D40%26um%3D1"]] || [[image:http://tbn2.google.com/images?q=tbn:AFwyxEpdLNdVEM:http://homepage.mac.com/cluelessmorgan/Twist%2520Tutorial5.jpg width="132" height="132" link="http://images.google.co.uk/imgres?imgurl=http://homepage.mac.com/cluelessmorgan/Twist%2520Tutorial5.jpg&imgrefurl=http://homepage.mac.com/cluelessmorgan/Twist%2520Tutorial.html&usg=__aqUquzNlLm67ENypmUFzwXdFGfw=&h=540&w=540&sz=37&hl=en&start=9&sig2=dXwcHFWxWa-mLbHEYEKOow&um=1&tbnid=AFwyxEpdLNdVEM:&tbnh=132&tbnw=132&ei=JtiwSeHIOYqk0QW4_MyOAg&prev=/images%3Fq%3DTwist%26ndsp%3D20%26hl%3Den%26lr%3D%26rlz%3D1G1GGLQ_ENFR273%26sa%3DN%26um%3D1"]] || [[image:http://tbn3.google.com/images?q=tbn:yOlj8LihUqdAHM:http://www.timber.org.au/ewebeditpro4/upload/20020117_P1170071_Margaret_R_rail_bridge_shear.JPG width="128" height="120" align="center" link="http://images.google.co.uk/imgres?imgurl=http://www.timber.org.au/ewebeditpro4/upload/20020117_P1170071_Margaret_R_rail_bridge_shear.JPG&imgrefurl=http://www.timber.org.au/NTEP/menu.asp%3Fid%3D199&usg=__Xs-uteduPLC2gsyqoUyXSDf3kfQ=&h=425&w=454&sz=99&hl=en&start=2&sig2=0Vhg0ZFMcM-od2UcUQv33A&um=1&tbnid=yOlj8LihUqdAHM:&tbnh=120&tbnw=128&ei=QdiwSZSFIpa-0AWF4L2pAg&prev=/images%3Fq%3DShear%26hl%3Den%26lr%3D%26rlz%3D1G1GGLQ_ENFR273%26sa%3DG%26um%3D1"]] ||

Compression and tension are the two forces that most interest us at the moment. Within a structure, any member (part) of the structure designed to withstand __compression__ is called a **__strut__** and any member designed to withstand **__tension__** is called a **__tie__**.


 * Concrete Struts being build for a bridge. || Metal Ties holding up a suspension bridge. ||
 * [[image:http://testcf.wsdot.wa.gov/projects/sr16narrowsbridge/photolibrary/newbridge/tower/enlarged/2007tw_26.jpg width="247" height="171" align="center"]] || [[image:http://www.williamsform.com/Post_Tensioning_Systems/post-tensioning_bridge.jpg width="180" height="172" align="center"]] ||

Remember there is a difference between [|weight] and [|mass.] Weight is a force measured in newtons. Mass is measured in Kg.