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Summersnow8
 one year ago
John stands on the edge of a deck that is 40.0 m above the ground and throws a rock straight up that reaches a height of 15.0 m above the deck. What is the initial speed of the rock in m/s?
which i solved correctly and it is 17.1
Assuming the rock in the previous question misses the deck on the way down, how fast (in m/s) will the rock be moving when it hits the ground? @ybarrap @lightgrav
Summersnow8
 one year ago
John stands on the edge of a deck that is 40.0 m above the ground and throws a rock straight up that reaches a height of 15.0 m above the deck. What is the initial speed of the rock in m/s? which i solved correctly and it is 17.1 Assuming the rock in the previous question misses the deck on the way down, how fast (in m/s) will the rock be moving when it hits the ground? @ybarrap @lightgrav

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Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0i used the equation \[2as = V _{f}^{2}  V{i}^{2}\]

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0so, use that correct answer in the same statement (equation) as before, but solve it for v_f ... remember that (now) a is in the same direction as s (both downward) so 2as is positive.

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0(or, you could start with v_i = 0, but watch the rock fall 55m

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0so, \[2 (9.8)(40) = Vf ^{2}  (17.1)^{2}\]

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0so vf would be 32.8?

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0yes, 55m is almost 4x the upward 15m (so v should be almost double)

ybarrap
 one year ago
Best ResponseYou've already chosen the best response.0You got it! Another way to look at this  $$ V_f^2 = V_i^2 + 2gs $$ Where \(V_i=0\), the speed of the rock at the highest point. \(s\) is is the distance the rock is above the deck plus the height the deck is above the ground: 15+40=55. Now you have everything to solve. You've got the right approach either way!

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0so is 32.8 the answer? i was told to use the equation i listed

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0His eq'n is the same as yours, just rearranged for v_f

ybarrap
 one year ago
Best ResponseYou've already chosen the best response.0Yep! $$ V_f^2 = V_i^2 + 2gs=02\times9.81\times55\\ \implies V_f=32.8~m/s $$

ybarrap
 one year ago
Best ResponseYou've already chosen the best response.0*Negative, because rock is going down: $$ V_f^2 = V_i^2 + 2gs=02\times9.81\times55\\ \implies V_f=\color{red}{}32.8~m/s $$

ybarrap
 one year ago
Best ResponseYou've already chosen the best response.0Just indicates direction, downward. Up is positive

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0the same direction as the acceleration.

Summersnow8
 one year ago
Best ResponseYou've already chosen the best response.0it says it's wrong

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0ooh, they asked for _speed_ (how fast) not velocity :(

ybarrap
 one year ago
Best ResponseYou've already chosen the best response.0speed is magnitude, velocity includes directional information. So speed would just be 32.8 m/s
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