APPLIED MECHANICS- Verification of the parallelogram law of forces



To verify the parallelogram law of forces with the help of Gravesand’s apparatus.


Gravesand’s apparatus, paper sheet, slotted weight, thread, pans, set square, pencil,  mirror strips, cello tape, geometrical box etc.


Parallelogram law of forces states that if two concurrent forces acting on a particle be represented in magnitude and direction by the two sides of a parallelogram, then their resultant may be represented in magnitude and direction by the diagonal of the parallelogram drawn from the same point.





  • Fix the paper sheet with cello tape on the board set in a vertical plane such that it should be parallel to the edge of board.
  • Take a thread and pass it over two pulleys. The end of the threads are to be attached to the slotted weight.
  • Take another thread and tie it to the middle of the thread and attach other end to the  slotted weight.
  • Place slotted weight in the three hooks in such a manner that the knots comes in the centre of the paper.
  • Displace them from the position and note if they come to the original position to ensure that the pulley have minimum friction.
  • To mark the line of action of these forces place plain mirror below each thread coinciding with image of the thread in the mirror.
  • Write down the magnitude of the force along the line to meet at a point O.
  • Choose a suitable scale for weight and draw the line OB, OA and OC proportional to the scale.
  • Complete the parallelogram drawing BC parallel to OA and AC parallel to OB meeting at point C. Now OC is the diagonal of the parallelogram.
  • Convert the length into force (R) and check whether it is equal to P1 or not.
  • If R is not equal to P1 then calculate the error.
  • Repeat the experiment at least two times.



S. No.


Force along OA
(in grams)
Force along OB
(in grams)
Force along OD
(in grams)
Observed value of resultant OC
(in grams) (graphically)
Percentage error between OC and OD
= (OC – OD)/ OC x 100




  • Positions of points should be marked very carefully with pointed pencil with the help of the plane mirror.
  • There should not be any knot on the thread expect the centre knot which should be small and fine.
  • Weights should be attached at the ends of the threads.
  • Pulleys should be lubricated properly to minimize friction.
  • Weights or pans should not touch the wooden board.














  1. Owan michael