. A text book of physics, for the use of students of science and engineering . V7777777777777777777 Fig. 98.—Model derrick crane. 90 DYNAMICS CHAP. C merely changes the direction of the cord without altering the force init; hence P =W (Fig. 99 (a) ). The polygon of forces (Fig. 99 (b) ) is drawnby making ab represent W, and be represent P ; lines are then drawnfrom a parallel to T, and from c parallel to Q, ; these intersect at d. Q andT may be scaled from cd and da Fig. 99 —Forces in a derrick crane. The values of Q, and T so found will not agree very well with those shownby th
. A text book of physics, for the use of students of science and engineering . V7777777777777777777 Fig. 98.—Model derrick crane. 90 DYNAMICS CHAP. C merely changes the direction of the cord without altering the force init; hence P =W (Fig. 99 (a) ). The polygon of forces (Fig. 99 (b) ) is drawnby making ab represent W, and be represent P ; lines are then drawnfrom a parallel to T, and from c parallel to Q, ; these intersect at d. Q andT may be scaled from cd and da Fig. 99 —Forces in a derrick crane. The values of Q, and T so found will not agree very well with those shownby the spring balances. This is owing to the weights of the parts of theapparatus not having been taken into account. Approximate correctionsmay be applied to the spring balance readings by removing W from thescale pan and noting the readings of the spring balances ; these will givethe forces in the jib and tie produced by the weights of the parts, andshould be deducted from the former readings, when fair agreement will befound with the results obtained from the polygon of forces. Exercises on Chapter VII. 1. A nail is driven into a board and two strings are attached to it. Ifthe angle between the strings is 60°, both strings being parallel to theboard, and if one string is pulled with a force equal to 4 lb. weight and theother with a force of 8 lb. weight, lind by construction the resultant forcecm the nail. 2. Answer Question I supposing a rod is substituted for the first
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