Question

A small rock is thrown vertically upward with a speed of 21.0 m/sm/s from the edge of the roof of a 21.0-mm-tall building. The rock doesn't hit the building on its way back down and lands in the street below. Ignore air resistance.A. What is the speed of the rock just before it hits the street?

Express your answer with the appropriate units.
B. How much time elapses from when the rock is thrown until it hits the street?
Express your answer with the appropriate units.

Answer 1

Step-by-step explanation:

A.

Given:

Vo = 21 m/s

Vf = 0 m/s

Using equation of Motion,

Vf^2 = Vo^2 - 2aS

S = (21^2)/2 × 9.8

= 22.5 m.

B.

Given:

S = 22.5 + 21 mm

= 22.521 m

Vo = 0 m/s

Using the equation of motion,

S = Vo × t + 1/2 × a × t^2

22.521 = 0 + 1/2 × 9.8 × t^2

t^2 = (2 × 22.521)/9.8

= 4.6

t = 2.14 s