How Long Does It Take a Dropped Book to Hit the Floor in a Descending Elevator?

[caksez.14]

While riding on an elevator descending with a constant speed of 3.2 , you accidentally drop a book from under your arm.How long does it take for the book to reach the elevator floor, 1.0 below your arm?

 

[JJBladester]

You need to show some formulas and some kind of attempt at a solution before you can receive assistance.

 

[caksez.14]

I can't solve the problem how can I attempt at a solution??

 

[JJBladester]

What is going on in the problem?
- Constant acceleration (due to gravity)... Here are your constant acceleration equations:

v_f²=v_i²+2a(x₂-x₁)

v_f=v_iat

Okay... So let's see what the problem gives you.

Firstly, you know that gravity accelerates things down at 9.81m/s²... That's what "a" is.

You know the initial velocity and the change in height (x₂-x₁) from the problem. Can you find v_f?

Once you know v_f, let's see if you can find t with the equations given...

 

[DaveC426913]

I can't solve the problem how can I attempt at a solution??

Why do you think that? Did you not attend the class? Do you not have your textbook?

 

[caksez.14]

time is 0.45 I found that then Of course I can found final velocity however my result is wrong. What is the book's speed relative to you when it hits the elevator floor? asks question

 

[DaveC426913]

We can't help you if you don't post your work!


Also, you erased the template provided. Why did you do that?

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[JJBladester]

Show your work. How did you find time? How did you find velocity?

This is the formula for relative velocity. It says that the velocity of A equals the velocity of B plus the velocity of A with respect to B.

[URL]http://upload.wikimedia.org/math/2/f/5/2f5a92923c17aea5a83160b242065b10.png[/URL]

 

[caksez.14]

I'm finding velocity from 'vf2=vi2+2a(x2-x1)' as 5.5 m/s however this result is wrong...

our Vi should be 3.2
a=9.8
and x=1

 

[caksez.14]

what is A what is B?

 

[JJBladester]

Elevator
v_i = -3.2 m/s
a = 0m/s²
y_i(floor)=0m
Using the constant acceleration equation y_f = y_i + v_i(t)=(1/2)a(t)²
Set up an equation for y_f(floor)

Book
v_i = -3.2 m/s
a = -9.81 m/s²
y_i(book) = 1m

Using the constant acceleration equation y_f=y_i+v_i(t)+(1/2)a(t)²
Set up an equation for y_f(book)

________________________________________________
You know that the book and the floor are in the same final position at the end of the drop,right? I mean, the book rests on the floor so they must have the same y_f. Knowing this, how could you solve for t after setting the equations above up with actual values?