You and a friend are standing on the edge on the lake throwing pebbles into the

Question

You and a friend are standing on the edge on the lake throwing pebbles into the lake. The surface of the lake is 7.3 m below the ground you are standing on and you throw the pebbles 1.2 m above the ground. Both of you throw your pebbles with the same initial speed of 17 m/s but in different directions. You throw your pebble upward while your friend throws it downward. The angles between the initial velocity and the horizontal are the same which is 30°. What are the speed of the pebbles when they strike the surface of the lake? (Neglect air resistance.)

Explanation / Answer

vertical displacement y = -7.3 - 1.2 = -8.5 m, a = -9.8 m/s2
v = 17 m/s, = 30°. find final speed u

ux = vx

uy2 = vy2 + 2ay

so u2 = ux2 + uy2 = vx2 + vy2 + 2ay = v2 + 2ay

u = (v2 + 2ay) = 21.3 m/s

both have same speed

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