✔ 最佳答案
1. Take the downward direction be positive.
Displacement, s = 80 m
Acceleration due to gravity, a = 10 m/s²
Initial speed, u = -10 m/s
So, by the equation, s = ut + 1/2 at²
80 = (-10)t + 1/2 (10)t²
t² - 2t - 16 = 0
t = 5.12 s or -3.12 (rejected)
So, it takes 5.12 s for the stone to strike the water 80 m below.
2. Assume the vertical speed of the stone is uniform when entered in water.
By the equation, v² = u² + 2as
v² = (-10)² + 2(10)(80)
v² = 1700
v = 41.23 m/s
So, the vertical speed of the stone is 41.23 m/s when entered into water.
圖片參考：
http://i182.photobucket.com/albums/x4/A_Hepburn_1990/1.jpg?t=1182579681
So, total speed = √(vertical speed² + horizontal speed²)
total speed = √(41.23² + 30²)
total speed = √(1700 + 900)
total speed = 51.0 m/s
3.
圖片參考：
http://i182.photobucket.com/albums/x4/A_Hepburn_1990/11.jpg?t=1182579728
The velocity can be resolved into two components.
For the vertical speed, v = 10sin30°
v = 5 m/s
Assume the upward direction to be positive.
So, displacement, s = 0
initial speed, u = 5 m/s
Accleration due to gravity, a = -10 m/s²
So, by the equation, s = ut + 1/2 at²
0 = (5)t + 1/2 (-10)t²
t² - t = 0
t(t - 1) = 0
t = 1 s or 0 (rejected)
So, the total flight time is 1 s.
