To find the weight of a given body using the parallelogram law of vectors

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Aim :

To find the weight of a given body using the parallelogram law of vectors.

Apparatus :

A vertical wooden board with 2 pulleys weight, two hangers, a string for sheets of paper, drawing pins, a strip of mirror, drawing pins, pencil, metre scale, set square, wooden block, protractors, string, thread and a spring balance


Law of parallelogram of forces:

When two forces react simultaneously at a point are represented in magnitude and direction by two adjacent sides of a parallelogram, then their resistant is represented both in magnitude and direction by the diagonal of the parallelogram passing through the point of intersecting of two forces.


  1. Clamp the board in vertical position by a plumb line and pin the sheet of paper on it with drawing pins.
  2. The short string in middle of first string to make a knot at 0. Tie the unknown weight at other end. Arrange them on pulleys.
  3. Adjust the weight so that know is in middle of the paper.
  4. When the know comes to rest, mark it on paper.
  5. Repeat the procedure a couple of times.
  6. Remove hangers along with weights and carefully weigh each hanger and slotted weight.
  7. Keep board flat and join the three points to meet at 0. Select a suitable scale and cut lengths OA and OB.
  8. Repeat above set of observation for three more sets of weight.

Sources of Error:-

  • Pulleys may have friction.
  • Weights may not be accurate.
  • Points may not be marked correctly.
  • Weight measured by spring balance may not be much accurate.


Forces Slides Resultant for R (g wt) Unknown weight S(g wt) Weight of Spring (g wt)balance Error (g wt)
P (g wt) Q (g wt) OA (cm) OB (cm)
200 4 200 4 5.9 295 290 5



– The unknown weight of given body is found to be 290 g.

Viva Questions:-

______ represents the resultant of the vector addition in parallelogram law of vector.

Diagonal of the parallelogram

Friction in the pulleys is one of the sources of error in the experiment using Gravesand’s apparatus. True or False.


Mass is a vector quantity. True or False.


Define Vector

Physical quantities, having both magnitude and direction are called vector quantities.

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