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Two particles are projected horizontally from a tower with velocity 3m/s and 4m/s respectively but in opposite direction. After what time their velocities

will make an angle 90 deg. With each other?
t = 1 s
t = root 0.12 s
t = root 2 s
t = 2 s



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Trick here is to write the velocities in component form.
Let (3,v_1) be the velocity of first particle.
Let (-4,v_2) be the velocity of second particle.

Since the vertical component of initial velocity is 0 for both the particles, we have v_1=v_2=gt=10t

We want to know the time at which the velocities are perpendicular, so simply set the dot product of velocities equal to 0  and solve t

(3,10t)\cdot~(-4,10t)=0\implies -12+100t^2=0\implies~t=\sqrt{0.12}s
0 0 0
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