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 7.2 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 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 degrees. What are the speed of both pebbles when they strike the surface of the lake? (neglect air resistance)

Explanation / Answer

a)Let my pebble strike the lake after time t.

then using 2nd equation of motion,

-7.7=16sin35*t-4.9t2

t=2.50s

vy=16sin35-9.8*2.50=-15.3m/s

speed=sqrt(15.32+(16cos35)2)=20.16m/s

b)Let the friend's pebble strike the lake after time t.

then using 2nd equation of motion,

-7.7=-16sin35*t-4.9t2

t=0.628s

vy=-16sin35-9.8*0.628=-15.3m/s

speed=sqrt(15.32+(16cos35)2)=20.16m/s

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