Kinematics Free fall problems part 2 help

In summary, the conversation discusses the scenario of dropping a book while riding on an elevator and asks for the time it takes for the book to reach the elevator floor and its velocity at impact. The equations used to solve for these values are v = v0 + at and x = x0 + v0*t + 1/2*a*t^2. However, the initial velocity should be negative since the elevator is descending. By correctly substituting values and using the equation v = v0 + at /*t, the correct time and velocity values can be calculated.
  • #1
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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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  • #2
Shouldn't your initial velocity be negative, considering that you're descending?
 
  • #3
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!
 

What is Kinematics Free Fall?

Kinematics Free Fall is a branch of physics that studies the motion of objects under the influence of gravity. It specifically deals with objects that are falling towards the ground with no other forces acting on them.

What is the difference between Free Fall and Constant Acceleration?

Free Fall and Constant Acceleration are similar in that they both involve objects falling under the influence of gravity. However, Constant Acceleration also takes into account other forces acting on the object, while Free Fall assumes that there are no other forces present.

How do you calculate the velocity of an object in Free Fall?

The velocity of an object in Free Fall can be calculated using the equation v = gt, where v is the velocity, g is the acceleration due to gravity, and t is the time the object has been falling.

What is the acceleration due to gravity on Earth?

The acceleration due to gravity on Earth is approximately 9.8 m/s². This means that for every second an object falls, its velocity will increase by 9.8 meters per second.

How do you solve Kinematics Free Fall problems?

To solve Kinematics Free Fall problems, you must first identify the known variables, such as initial velocity, time, and acceleration due to gravity. Then, you can use equations such as d = 1/2gt² and v = gt to calculate the unknown variables.

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