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Fosterthepeople
 2 years ago
Best ResponseYou've already chosen the best response.0A rock that had been dropped from a high platform is moving with a kinetic energy of 180 J when it strikes the ground. Ignore air resistance. What kinetic energy did the rock have when it had fallen onethird of the distance to the ground? (Points : 1) 120 J 20 J 45 J 60 J

Fosterthepeople
 2 years ago
Best ResponseYou've already chosen the best response.0@CreAtivSpelErr

agent0smith
 2 years ago
Best ResponseYou've already chosen the best response.2use v^2 = 2as and KE = 0.5mv^2. When it hits the ground, \[v _{F}^2 = 2as\]1/3rd of the way to the ground, \[v^2 = 2a \frac{ s }{ 3} = \frac{ 1 }{ 3 } 2as = \frac{ 1 }{3 } v _{F}^{2}\] so notice that the velocity squared 1/3rd the way down is 1/3rd the velocity squared when it hits the ground. KE when it hits the ground: \[KE _{F} = 180= 0.5 m v _{F}^{2}\] KE 1/3rd the way down: \[KE = 0.5 m * (\frac{ 1 }{3 } v _{F}^{2}) = \frac{ 1 }{ 3 } 0.5m v _{F}^{2} = \frac{ 1 }{ 3 }KE _{F}\]

yunus
 2 years ago
Best ResponseYou've already chosen the best response.1the correct answer is 120 j. there i added a drawing which shows the solution.

agent0smith
 2 years ago
Best ResponseYou've already chosen the best response.2@yunus your working is fine, but your answer is incorrect  the question asks "What kinetic energy did the rock have when it had fallen onethird of the distance to the ground?" 1/3rd of the distance TO the ground, not from the ground. You gave the kinetic energy when it has fallen 2/3rds of the distance to the ground.

agent0smith
 2 years ago
Best ResponseYou've already chosen the best response.2I still gave you a medal for the method/drawing :)

yunus
 2 years ago
Best ResponseYou've already chosen the best response.1Oh thanks. my English is not good thats why i couldnt understand it well. It still can be solved by using just what i did in the drawing instead of takin y value different. am i right?

yunus
 2 years ago
Best ResponseYou've already chosen the best response.1if you take y=6.12/m then you find 59.976 joule which can be rounded to 60j.

agent0smith
 2 years ago
Best ResponseYou've already chosen the best response.2Yep, your method works, as long as you use y/3 not 2y/3. Yours works the same as mine, just uses PE instead of velocity. Both give 60J
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