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Old Jan 6th 2019, 06:01 AM   #1
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How to solve this throwing ball off cliff question

Dear all,

For b) I was curious why i couldn't use s = ut + 0.5at^2 as my equation as i had all the necessary known values.. instead to get the correct answer i had to use v=u + at


why is this so. Thank you for your time

Picture of Problem: https://imgur.com/a/GruxPwe
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Old Jan 6th 2019, 06:47 AM   #2
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The question is ambiguous. Does "height" refer to the height of the ball above the top of the cliff or the height of the ball above the sea? Assuming it is "height above the top of the cliff" then using "s= ut+ 0.5at^2" with u= 20 m/s and a= -g, the acceleration due to gravity, or "v= u+ at" should give the same thing. Since you are specifically asked for "maximum height" completing the square in the former is more direct. To use the latter, you would have to argue that the ball loses speed until its speed is 0 at the maximum height so solve u+ at= 20- 9.8t= 0 to find the time, then use s= u+ 0.5at^2 to find the height
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