APPLIED MECHANICS- Verification of the Polygon Law of Forces




To verify the law of polygon of forces with the help of universal force table apparatus.



Universal Forces table with five pulleys, rings with five string weight, slotted weight, plain paper and geometry box.



Polygon law of forces states that if a number of concurrent forces are acting simultaneously on a body, are represented in magnitude and direction by the sides of a polygon, taken in order, then the resultant is represented in magnitude and direction by the closing side of the polygon, taken in opposite order.




  • Before starting the experiment make the graduated table horizontal with the help of leveling screw.
  • Clamp the pulley to the graduated disc of the force table.
  • Tie one and of thread to the circumference of small rings and place it around the central axis of the apparatus.
  • Attach other end of the string to the weight hangs which will carry small slotted weight.
  • Pass one string over each pulley.
  • Place slotted weight on the weight hanger in such a manner that ring is placed around the center.
  • Note down the angle between different threads and corresponding weights.
  • Draw a space diagram of the force acting a part such that the time of action of each force corresponding to the angles of forces between them.
  • Chose a suitable scale and make mark a point on a paper and draw the vector diagram of force system acting at a point by drawing parallel line of each force proportional to the chosen scale.


Procedure to draw vector diagram:

  • Draw a line OA parallel and equal to W1 assuming a suitable scale.
  • Draw a line AB parallel and equal to W2 from point A assuming the same scale.
  • Similarly draw BC and CD parallel and equal to the force W3 and W4.
  • Join OD i.e. the closing side by dotted line, which represents the resultant of polygon (graphically).
  • Measure the line OD and compare it with W5 by using the same scale.
  • Find the error by using the formula given in the observation table. Error should be positive.



FORCE (in grams)

ANGLE ( in degree)

S. No. W1 W2 W3 W4 W5  θ1  θ2  θ3  θ4




(W5 –R)/W5 x 100


(R- W5)/R x 100



  • Ring should not touch the control axle of the disc and should be at the center of the table.
  • Graduated disc should be made kept perfectly horizontal.
  • Positions of the strings should be at the center of the table.
  • Pulley should be frictionless.