Kinematics Free fall problems part 2 help

AI Thread Summary
In the discussion about a free fall problem involving a book dropped from a descending elevator, the key points revolve around calculating the time it takes for the book to reach the elevator floor and its speed upon impact. The initial attempts at solving the problem involved using kinematic equations, but the user encountered errors, particularly regarding the sign of the initial velocity due to the elevator's descent. Clarification was provided that the initial velocity should indeed be negative, and further guidance was given on how to correctly set up the equations to solve for time and final velocity. The conversation emphasizes the importance of accurately considering the direction of motion when applying kinematic principles. The user is encouraged to follow the revised approach to arrive at the correct answers.
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Homework Statement


While riding on an elevator descending with a constant speed of 3.3 m/s, you accidentally drop a book from under your arm.

(a) How long does it take for the book to reach the elevator floor, 1.0 m below your arm?

(b) What is the book's speed relative to you when it hits the elevator floor?

Homework Equations


v = v0 + a*t
x = x0 + v0 *t + 1/2*a*t^2
x= x0 + v*t-1/2*a*t^2
V^2 = v0^2 +2*a*(x-x0)


The Attempt at a Solution



I tried to solve for time by first solving for final velocity using V^2 = v0^2+2*a*(x-x0) =
V^2 = (3.3m/s)^2 +2(-9.80m/s^2)(0-1m)
V^2 = 30.49 m^2/S^2
square root of 30.49 = 5.522 m/s

v = v0 +a*t
5.522 = 3.3 + (9.80)*t
t = 0.2267

part b) substract the book's velocity 5.522 - my velocity 3.3 = 2.222

I got both answers wrong. I was wondering if someone could help me explain what i did wrong? thank you so much!
 
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Shouldn't your initial velocity be negative, considering that you're descending?
 
Since you're dropping the book with an initial velocity, and the rest of the falling path is accelerating, the velocity at any moment can be calculated, as you know, with:
v = v0 + at
This, however, is the velocity. If we want the distance, we have to multiply by t (vt=s)
v = v0 + at /*t
s = v0t + at^2/2
Now you can substitute s with the height of 1:
1 = 3.33t + 9.81t^2/2
Go on from here. If you have trouble extracting t which you need, it can be substituted:
v = 3.33 + 9.81t
v - 9.81t = 3.33
t = v-3.33/9.81
Insert it into the previous expression:
1 = 3.33(v-3.33/9.81) + 9.81*(v-3.33/9.81)^2/2
Hope you can work it out from here (you'll get the final v and from that the t).
Good luck!
 
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