Kinematics Qs From Kaplan

[Hexon]

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From the Kaplan book, p37, q3:

A hang glider runs off a cliff with a speed of 3m/s. What is its overall velocity after 5 seconds?

a) 3m/s
b)-5m/s
c) 5m/s
d) 10m/s

The answer says D but i got C; i used the V(final)=V(Initial)+at but still got the wrong answer.

also, question 17, p40:

A punter kicks a ball over a goalpost 28m away from him. he kicks with an initial speed of 20m/s and leaves his foot at an angle of 45 degrees. The low post on the goal is 2.5m high. When the football reaches the level of the low post, how far above or below is the ball?

a) 3.25m under
b) 3.25m over
c) 5.75m under
d) 5.75m over

answer is B, but i don't know why🙁


thanks for your help guys

 

[Morsetlis]

vf = vi + at
Vertical v after 5s:
vf = 0 + 10(5) = 50m/s downward
Horizontal v after 5s:
vf = 3 + 0(5) = 3m/s forward
Hypotenuse:
vf = sqrt(50^2 + 3^2) = sqrt(2500 + 9) = sqrt(2509) ~ 50 m/s

As we all know, hang gliders GLIDE because of air resistance and the whole floating on currents thing. So, there's an upward component that slows the vertical falling acceleration due to gravity. I don't know what that is but the final answer shouldn't be 50 m/s.

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20m/s at 45 degrees angle means equal vi for horizontal and vertical components (sin or cos 45 degrees times 20m/s). .7 x 20 = 14 m/s.

A goal 28m away will take 28m x 1s / 14m = 2 seconds to reach horizontally.

Vertically, the ball will be at position d = di + vit + 1/2 at^2 = 0 + 14(2) + 0.5(-10)(2^2) = 28 - 5(4) = 8m above ground after 2 seconds.

Since the goal post is 2.5m above ground, the ball is 8m - 2.5m = 5.5m above the post.

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Conclusion: your book is ******ed

 

[Pre-Med Village]

Morsetlis' computations seem to be correct. I would move on.

Errata in MCAT books are like the puzzle trap Data came up with one time on Next Generation to defeat the Borg. Even the possibility of an error in the learning materials can waste a lot of time and hurt confidence. Keep faith in yourself and move on.

Kaplan has top shelf question writers, but even they make mistakes. Don't let it impact your confidence in yourself or your book.

 

[Hexon]

thanks for your help guys🙂

yeah, after following through on your calculations, it does seem the kaplan guys made some errors; i still like the kaplan books though😛 easy to understand😉