0If two vectors acting simultaneously on a particle are represented in magnitude and direction by the two adjacent sides of a parallelogram drawn from a point, then their resultant is completely represented in magnitude and direction by the diagonal of that parallelogram drawn from that point.
Parallelogram Law of Vectors explained:
Let two vectors P and Q act simultaneously on a particle O at an angle 
. They are represented in magnitude and direction by the adjacent sides OA and OB of a parallelogram OACB drawn from a point O.Then the diagonal OC passing through O, will represent the resultant R in magnitude and direction.
On a Gravesand's apparatus, if the body of unknown weight (say S) is suspended from the middle hanger and balancing weights P and Q are suspended from othe two hangers then,
The unknown weight can be calculated from the above equation .
On a Gravesand's apparatus, if the body of unknown weight (say S) is suspended from the middle hanger and balancing weights.
And Q are suspended from the other two hangers then,
Now construct a parallelogram OACB by assuming a scale (say 1cm = 50 gwt) corresponding to the weights P and Q. The diagonal of the parallelogram OC will give the resultant vector. The weight of the unknown body,
S = OC * scale
If W is the actual weight of the body, then the percentage error in the experiment can be calculated using the equation,
Percentage error = (Actual weight - caliculated weight) * 100/Actual weight.
= (W - S) * 100/W
