John throws a rock down with speed 20 m/s from the top of a 30-m tower.If g = 9.8 m/s² and air resistance is negligible,what is the rock's speed just as it hits the ground? A) 12 m/s B) 28 m/s C) 31 m/s D) 78 m/s

Question

Quizplus · Accepted Answer

Using the kinematic equations, we can find the final speed of the rock just before it hits the ground.
First, we need to find how long it takes for the rock to hit the ground:
S1 = 30 m (distance traveled)
U1 = 20 m/s (initial velocity)
a = g = 9.8 m/s^2 (acceleration due to gravity)
Using the kinematic equation S1 = U1t + (1/2)at^2, we can solve for t:
30 = 20t + (1/2)(9.8)t^2
Simplifying and solving for t, we get t = 2.04 s.
Now we can use the kinematic equation V1 = U1 + at to find the final velocity V1 just before impact:
V1 = 20 + (9.8)(2.04) = 31.592 m/s ≈ 31 m/s.
Therefore, the best choice is C, 31 m/s.

John Throws a Rock Down with Speed 20 M/s from the Top