# Help with a mechanics problem from Kleppner's book

## Homework Statement

An elevator departs the ground at t=0 With uniform velocity. At T1 a boy drops a marbel through the floor of the lift that falls under constant acceleration g=9.8 ms^-2. If it takes marbel T2 time to fall down, what was the height at the T1.

## Homework Equations

h=vT1
h= vT2 -.5g(T2)^2

## The Attempt at a Solution

I simply replaced v in second equation by h/T1. And i got the h in terms of T1, T2. But the problem is, there is a clue given with this problem in the book. That is when T1=T2=4 h=39.2 now, in my equation, T1=T2 gives a height 0.

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PhanthomJay
Homework Helper
Gold Member
you have to relook your 2nd equation for proper signage.

## Homework Statement

An elevator departs the ground at t=0 With uniform velocity. At T1 a boy drops a marbel through the floor of the lift that falls under constant acceleration g=9.8 ms^-2. If it takes marbel T2 time to fall down, what was the height at the T1.

## Homework Equations

h=vT1
h= vT2 -.5g(T2)^2

## The Attempt at a Solution

I simply replaced v in second equation by h/T1. And i got the h in terms of T1, T2. But the problem is, there is a clue given with this problem in the book. That is when T1=T2=4 h=39.2 now, in my equation, T1=T2 gives a height 0.
the height that the elevator rises at T1 is the same as the height the marble falls at T2. The sum total of those 2 distances is 0 (h - h =0). Check signage.