A truck is moving with a constant velocity of `54 km h^-1`. In which direction (angle with the direction of motion of truck) should a stone be projected up with a velocity of `20 ms^-1`, from the floor of the truck of the truck, so as to appear at right angles to the truck, for a person standing on earth ?

Options

A. `cos^-1 (-(3)/(4))`
B. `cos^-1 (-(1)/(4))`
C. `cos^-1 (-(2)/(4))`
D. `cos^-1 ((3)/(4))`

Answer

A

Explanation

(a) Let the stone be projected at an angle `prop` to the direction of motion of truck with a speed of `v = 20 ms^-1`. Since the resultant displacement along horizontal is zero, the velocity along horizontal is `0`.
`15 + 20 cos prop = 0`
`rArr cos prop = - (3)/(4)`
`rArr prop = cos^-1 ((-3)/(4)) = 138^@ 35'`.
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