# EXPERIMENT

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

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

THEORY:
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. PROCEDURE:

• 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.

OBSERVATION TABLE:

 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

PRECAUTIONS:

• 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.

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