physics

A uniform plank weighing 50N and 12m long is pivoted at a point of 4m from one end.A boy weighing 30N sits on the plank 7m from pivot.Where must his elder brother whose weight is 80N sit to balance the plank horizontally

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  1. Draw a free body diagram and set the total moment about the fulcrum/pivot equal to zero. Let x be the distance of the elder brother from the pivot.

    80*x = 50*2 + 30*7 = 310 N*m

    x = 3.87 m (on the opposite side of the pivot from the 30 N brother)

    This location is quite near the edge of the plank

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