c)If three forces (vectors) are to be in equilibrium, then the sum of magnitudes of any two forces must be greater than the magnitude of third force.
d)Lami's theorem:
If a body is in equilibrium under the action of three coplanar forces P,Q,R at angles as shown in the figure,
18. A body of mass 'm' is suspended by a string of length 'l' from a rigid support. It is pulled aside by distance 'x' so that it makes an angle with the vertical by applying a horizontal force F. When the body is in equilibrium.
19. PARALLELOGRAM LAW OF VECTORS (OR FORCES):"If two vectors acting at a point making an angle with each other are represented both in magnitude and direction by the adjacent sides of a parallelogram, then the diagonal drawn from the same point will give the resultant both in magnitude and direction" .
22. POLYGON LAW OF VECTORS :" If number of vectors acting at a point in the same plane in different directions are represented both in magnitude and direction by the adjacent sides of a polygon taken in order, then the closing side taken in the reverse order will give the resultant both in magnitude and direction".APPLICATIONS OF POLYGON LAW
1. If 'n' equal forces act on a body such that each force makes an angle 2∏ / n with the previous one and the polygon is closed, then the resultant is zero.
If each force of magnitude 'F' makes an angle Θ with previous one, then
a) the resultant is zero, if the number of forces is 2∏/ Θ
b) the resultant is 'F', if the number of forces are 2∏/ Θ - 1
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