In the diagram on the right, a plank weighing 120 N is supported by two trestles at points A and B. A man weighing 480 N is standing on 1m mass centre of of plane the plank.. a Redraw the diagram, showing all the forces acting on the plank. b Calculate the total clockwise moment of the two weights about A. c Use the principle of moments to calculate the upward force from weight of man 480 N weight of plank 120 N the trestle at B. d What is the total downward force on the tresties? tist e What is the upward force from the trestle at A? f The man now walks past A towards the left-hand end of the plank. What is the upward force from the trestle at 8 at the instant the plank starts to tip? g How far is the man from A as the plank tips

People with normal vision cannot focus their eyes underwater if they aren't wearing a face mask or goggles and there is water in contact with their eyes. in a simplified model of the human eye, the aqueous and vitreous humors and the lens all have a refractive index of 1.40, and all the refraction occurs at the cornea, whose vertex is 2.60 cm from the retina.with the simplified model of the eye, what corrective lens (specified by focal length as measured in air) would be needed to enable a person underwater to focus an infinitely distant object? (be careful-the focal length of a lens underwater is not the same as in air! assume that the corrective lens has a refractive index of 1.62 and that the lens is used in eyeglasses, not goggles, so there is water on both sides of the lens. assume that the eyeglasses are 2.05cm in front of the eye.)