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Motion In A Plane

Question
CBSEENPH11018373

If a particle is in equilibrium under the action of three forces, then show that ratio of each force and sine of angle between the other two forces is constant?

Solution

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#6 {main}</pre> acting at point O be in equilibrium. 


Taking cross product of on both sides, we get
open parentheses straight A with rightwards harpoon with barb upwards on top space plus space straight B with rightwards harpoon with barb upwards on top space plus space straight C with rightwards harpoon with barb upwards on top close parentheses cross times straight C with rightwards harpoon with barb upwards on top space equals space 0 space cross times space straight C with rightwards harpoon with barb upwards on top space equals space 0 space

rightwards double arrow space straight A with rightwards harpoon with barb upwards on top space cross times space straight C with rightwards harpoon with barb upwards on top space plus space straight B with rightwards harpoon with barb upwards on top space cross times space straight C with rightwards harpoon with barb upwards on top space plus straight C with rightwards harpoon with barb upwards on top cross times straight C with rightwards harpoon with barb upwards on top space equals space 0 space

rightwards double arrow space minus space space straight C with rightwards harpoon with barb upwards on top space cross times straight A with rightwards harpoon with barb upwards on top space plus space straight B with rightwards harpoon with barb upwards on top space cross times space straight C with rightwards harpoon with barb upwards on top space plus space 0 space equals space 0 space

rightwards double arrow space straight C with rightwards harpoon with barb upwards on top space cross times straight A with rightwards harpoon with barb upwards on top space equals space space straight B with rightwards harpoon with barb upwards on top space cross times space straight C with rightwards harpoon with barb upwards on top

rightwards double arrow space fraction numerator straight A over denominator sin space straight alpha end fraction space equals space fraction numerator straight B over denominator space sin space straight beta end fraction space space space space space space space space space space space space space space space space... space left parenthesis 2 right parenthesis space

Now comma space taking space the space cross space product space of space
equation space left parenthesis 1 right parenthesis space with space straight B with rightwards harpoon with barb upwards on top comma space we space have

space space space space open parentheses straight A with rightwards harpoon with barb upwards on top space plus space straight B with rightwards harpoon with barb upwards on top space plus space straight C with rightwards harpoon with barb upwards on top close parentheses space cross times space straight B with rightwards harpoon with barb upwards on top space equals space 0 space cross times space straight B with rightwards harpoon with barb upwards on top space equals space 0

rightwards double arrow straight A with rightwards harpoon with barb upwards on top space cross times straight B with rightwards harpoon with barb upwards on top space plus space space straight B with rightwards harpoon with barb upwards on top space cross times space straight B with rightwards harpoon with barb upwards on top space plus straight C with rightwards harpoon with barb upwards on top space cross times straight B with rightwards harpoon with barb upwards on top space space equals space 0 space

rightwards double arrow space straight A with rightwards harpoon with barb upwards on top space cross times straight B with rightwards harpoon with barb upwards on top space plus 0 space minus space straight B with rightwards harpoon with barb upwards on top space cross times space straight C with rightwards harpoon with barb upwards on top space equals space 0

rightwards double arrow space straight A with rightwards harpoon with barb upwards on top space cross times straight B with rightwards harpoon with barb upwards on top space equals space space straight B with rightwards harpoon with barb upwards on top space cross times space straight C with rightwards harpoon with barb upwards on top

rightwards double arrow space space space AB space sin space straight gamma space equals space BC space sin space straight alpha space

rightwards double arrow space fraction numerator straight A over denominator sin space straight alpha space end fraction space equals space fraction numerator straight C over denominator sin space straight gamma end fraction space space space space space space space space space space space space space space space space space space space space space space... space left parenthesis 3 right parenthesis thin space

From space equation space left parenthesis 2 right parenthesis space and space left parenthesis 3 right parenthesis comma space we space get

fraction numerator straight A over denominator sin space straight alpha space end fraction space equals space fraction numerator straight B over denominator sin space straight beta end fraction space equals space fraction numerator straight C over denominator sin space straight gamma end fraction space space space space space