Calculating the Velocity and Time of a Freely Falling Rock

AI Thread Summary
To solve the problem of a rock thrown from a 30-meter building, the initial velocity is given as 18 m/s. The equations of motion are applied, leading to the calculation of the time and final velocity before impact. The correct time of flight is determined to be approximately 3.67 seconds, and the final speed just before hitting the street is about 66.04 m/s. The initial miscalculation of 9.8 seconds was clarified once the initial velocity was confirmed. Accurate application of kinematic equations is essential for solving projectile motion problems.
ThiagoG
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Homework Statement


A small rock is thrown vertically upward with a speed of
from the edge of the roof of a 30.0-m-tall building. The
rock doesn’t hit the building on its way back down and lands in the
street below. Air resistance can be neglected. (a) What is the speed
of the rock just before it hits the street? (b) How much time elapses
from when the rock is thrown until it hits the street?


Homework Equations


0=30+18t-4.9t2
Vx=30-9.8t


The Attempt at a Solution


For the time I got 9.8s and for the velocity I got -66.04 m/s. I'm not sure where I went wrong.
 
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You didn't provide the initial velocity.
 
Lamebert said:
You didn't provide the initial velocity.

Sorry about that. The initial velocity is 18 m/s.
 

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