Consider the upward direction of motion as positive and downward direction of motion as negative.
a = acceleration due to gravity in downward direction = - 9.8 
v₀ = initial velocity of rock in upward direction = ?
v = final velocity of rock at the highest point = 0 
t = time to reach the maximum height = 4.2 sec
Using the kinematics equation
v = v₀ + a t
inserting the values
0 = v₀ + (- 9.8) (4.2)
v₀ = 41.2 
Answer:
The duration of the impact is 0.005384 seconds
Explanation:
Given
m = 0.43 kg
v = 5.2 m/s
x = 0.014 m
Knowing the formulas

Answer:
7.24 Ω
Explanation:
Power = energy / time = 195 J / 9.81 = 19.88 W
and power = V² / R
R resistance = 12 V² / 19.88 W = 7.24 Ω