Question

Question A rock is dropped from a 18 m -high ledge. What's its speed when it hits the ground? Express your answer to two significant figures and include the appropriate units. I got -19 m/s and this worked. Now... What's its height when its speed is half the value found in previous part? Express your answer to two significant figures and include the appropriate units. I got 4.6 m but this is not the right answer Neither is anything around 4.6 I do not know what I am doing wrong.

Answer 1

(a) When it hits the ground, speed of the rock will be given as :

Using an equation, we have

v_f = 2 g h                                                          { eq.1 }

where, g = acceleration due to gravity = 9.8 m/s²

h = height of ledge from ground = 18 m

inserting these values in above eq.

v_f = 2 (9.8 m/s²) (18 m)

v_f = 352.8 m²/s²

v_f = 18.7 m/s

(b) Now, when its speed is half the value found in part-a. Then height of the ledge will be given as :

again, using eq.1

v_f² = 2 g h

[(18.7 m/s) / 2]² = 2 (9.8 m/s²) h

(9.35 m/s)² = (19.6 m/s²) h

h = (87.4 m²/s²) / (19.6 m/s²)

h = 4.45 m