Physics Help Forum How to solve this throwing ball off cliff question

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Jan 6th 2019, 06:01 AM #1 Junior Member   Join Date: Jan 2019 Posts: 2 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
 Jan 6th 2019, 06:47 AM #2 Senior Member   Join Date: Aug 2010 Posts: 403 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 topsquark likes this.

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