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Question
A ball is thrown vertically upwards from the ground and a student gazing out of the window sees it moving upward past him at 10 ms−1. The window is at 15 m above the ground level. The velocity of ball 3 s after it was projected from the ground is (take g=10 ms−2)
A ball is thrown vertically upwards from the ground and a student gazing out of the window sees it moving upward past him at 10 ms−1. The window is at 15 m above the ground level. The velocity of ball 3 s after it was projected from the ground is (take g=10 ms−2)
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Solution
The correct option is D 10 ms−1, down
Let the ball be thrown with initial velocity of 'u'.
After travelling 15 m, its velocity is 10 m/s.
Thus v2−u2=−2gh (acceleration acts downwards, thus -g)
or, 102−u2=−300
giving u=20m/s
After 3 sec, v=u−gt=20−30=10m/s downwards
Let the ball be thrown with initial velocity of 'u'.
After travelling 15 m, its velocity is 10 m/s.
Thus v2−u2=−2gh (acceleration acts downwards, thus -g)
or, 102−u2=−300
giving u=20m/s
After 3 sec, v=u−gt=20−30=10m/s downwards
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