化學-求H?

You can apply the concept of conversation of energy.
Energy of the water bottle consists of potential energy and kinetic energy.
At the point of dropping, the bottle has only potential energy since it has no velocity. At the height of 2.0 m above ground, the bottle has both potential energy and kinetic energy. The sum of both energy at 2.0 m should equal to the potential energy at the dropping height.

At 2.0 m above ground,

Potential energy = mgh = 0.567 kg x 9.81 ms^-2 x 2.0 m = 11.125 J
Kinetic energy = 1/2 mv^2 = 1/2 x 0.567 kg x 69.7^2 = 1377.269 J
Total energy = 11.125 J + 1377.269 J = 1388.394 J


The total energy should equal to the potential energy at dropping height H,

mgH = 1388.394 J
H = 1388.394 J / (0.567 kg x 9.81 ms^-2)
H = 249.609 m.=


The bottle is dropped from 249.609 m above the ground.

u = 0, v = 69.7 m s-1, s = ?
Take a = g = 10 m s-2

v2 = u2 + 2as
(69.7)2 = (0)2 + 2(10)s
s = 243 m

Distance above the ground
= s + 2
= 243 + 2
= 245 m

2008-02-11 14:59:53 補充：
If g is taken as 9.87 m s^-2, the answer is slightly different.Alternative method :Loss of P.E. = Gain of K.E.i.e., mgh = (1/2)mv^2Then, distance above the ground = (h 2) m

2008-02-12 17:38:10 補充：
The height above the ground is (H+2) m, because H is measured 2 m above the ground. The answer is imcomplete and I wonder why this is the best answer.