Kinematics Free fall problems part 2 help

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SUMMARY

The forum discussion focuses on solving kinematics problems related to free fall, specifically involving a book dropped from a descending elevator at a constant speed of 3.3 m/s. The key equations utilized include v = v0 + a*t and V^2 = v0^2 + 2*a*(x-x0). The user initially calculated the final velocity incorrectly and misapplied the initial velocity, failing to account for the downward direction of motion. Correcting these assumptions leads to accurate calculations for both the time taken for the book to reach the elevator floor and its speed relative to the user upon impact.

PREREQUISITES
  • Understanding of kinematic equations: v = v0 + a*t, V^2 = v0^2 + 2*a*(x-x0)
  • Basic knowledge of acceleration due to gravity (9.81 m/s²)
  • Concept of relative velocity in a descending frame of reference
  • Ability to manipulate algebraic equations for solving variables
NEXT STEPS
  • Review the derivation and application of kinematic equations in free fall scenarios
  • Study the concept of relative velocity in non-inertial reference frames
  • Practice solving similar problems involving objects in free fall from moving platforms
  • Explore advanced kinematics topics, such as projectile motion and air resistance effects
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Students studying physics, educators teaching kinematics, and anyone seeking to improve their problem-solving skills in mechanics related to free fall and motion in non-inertial frames.

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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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