8. A ball rolls horizontally with a speed 0f 7.6 m/s off the edge of a tall platform. If the ball lands 8.7 m from the point on the ground directly below the edge of the platform, what is the height of the platform?
9. A stone thrown off a bridge 20 m above a river has an initial velocity of 12 m/s at an angle of 45° above the horizontal.
What is the range of the stone?
At what velocity does the stone strike the water?
Solution.
"8. v=7.6 m\/s;"
"L=8.7m;"
"L=vt\\implies t=\\dfrac{L}{v};"
"t=\\dfrac{8.7}{7.6}=1.44s;"
"h=\\dfrac{gt^2}{2};"
"h=\\dfrac{9.8\\sdot1.44^2}{2}=10.16m;"
"9.y_0=20m;"
"v=12 m\/s;"
"\\alpha=45^o;"
"y=y_o+v_{0y}t-\\dfrac{gt^2}{2};"
"0=20+12\\sdot sin45^ot-\\dfrac{9.8\\sdot t^2}{2};"
"t=3.06s;"
"L=v_{ox}t;"
"L=12\\sdot0.7071\\sdot3.06=26m;"
"v_{y}=v_{0y}-gt;"
"v_y=12\\sdot0.7071-9.8\\sdot3.06=-21.5m\/s;"
Answer: "8.h=10.16m;"
"9. L=26m; v_y=-21.5 m\/s."
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