Definitions of the Concepts

1.  Vector projection of 'Fab' onto 'Fac':    
It is a vector whose magnitude is the scalar projection of Fab on Fac with the same direction as Fac. 
(Fab)ac = Fab * Uac where Fab is the vector Fab and Uac is the unit vector of Fac, and Uac= Fac / (Length of Fac)
  
2.  Toughness: 
Amount of energy per unit volume, which a material can absorb before fracturing. Its unit is J per cubic meter and  

3.  Specific gravity: 

It is the ratio of the density of a substance to the density of a well-known substance. Here well-known substance is usually chosen as H2O at a specified temperature. 

Specific Gravity: SG= P of substance /P of H2O

4.  Density: 

It is the ratio of mass to the volume for a substance. Density: p = m/V

5.  Elastic limit: It is the point on the stress-strain curve beyond which the material is permanently deformed after load is removed. When stress is applied till the elastic limit, and released, the material should come back to its first size and shape.

6.  Ultimate strength: 

It is the maximum stress that a material can endure without any alongation.

7.  Proportional limit: It is the highest stress where the stress is proportional to the strain. In other words, the maximum stress limit where the slope of the stress strain curve is still constant.

8.  Yield strength: It is the point where the material exceeds the elastic limit and can not return to original shape even if the stress is removed.

9.  First-angle projection: In the first-angle projection, the object is imagined to be in the first quadrant, and it lies between the observer and the projection plane. This is commonly used in most of the world except the United States.

10.Third-angle projection:  In the third-angle projection, the object is imagined to be in the third quadrant, and projection plane lies between the observer and the object. When views are drawn, the right side view is placed on the right side of the front view, and the top view is placed above the front view. This is used in the United States.