Compressing a spring

Air bags are activated when a severe impact causes a steel ball to compress a spring and electrically ignite a detonator cap. This action causes sodium azide (NaN3) to decompose explosively according to the following reaction ... 

First we assume that a force similar to that produced by a spring joins the bonded atoms, the bond term. It requires energy to pull the atoms further apart and to push them closer together than the equilibrium bond distance, just as you need to supply energy to stretch or compress a spring. 

Elevation of the priming lever compresses a spring above the base of the MDI. A mechanical obstruction prevents movement of the aerosol canister within the plastic moulding until the patient inhales. Inhalation rotates a small vane within the mouthpiece, removing the obstacle and allowing the aerosol canister to move down sufficiently to actuate the MDI. Unscrewing the removable sleeve can open the Autohaler. This allows the MDI to be removed and the mouthpiece adapter to be washed (68). 

Fig. 1.21 Spring roll DEA. Rolls are fabricated by compressing a spring between two end caps and rolling a DE/electrode/DE/electrode layered strip around it. Increasing the number of layers wrapped around the spring cotc increases the output force [251]. Proc SPIE 2006, reprinted with permission...

There is a free energy penalty for distortion of a nematic phase, much like the energy required to compress a spring. For a nematic, there are three fundamental types of distortion, splay, twist, and bend, and three elastic constants K, AT, and... 

This naming convention is standard but can be confusing note that AwH is parallel to the vector n that points from one bubble to another, while Au- is perpendicular to n, hence parallel to the interface at the contact. Under the convention of Equation 12.23, normal motion (AwH) stretches or compresses a spring, while tangential motion (A - ) occurs when two bubbles slide past each other. 

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