PHYSICS!!!

2008-09-12 8:56 pm
A rocket moves straight upward, starting from rest with an acceleration of 9.4 m/s^2 (with gravity). It runs out of fuel at the end of 4.0 seconds and continues to coast upward before reaching a maximum height and falling back to earth.

a.) find maximum height about the ground.
b.) Find velocity of the rocket the moment before it hits the ground.

回答 (1)

2008-09-13 12:31 am
✔ 最佳答案
a)
Taking all upward quantities as positive.

Consider the journey of the first 4 seconds:
u = 0 m/s
a = 9.4 m/s2
t = 4 s

v = u + at
v = (0) + (9.4)(4)
Final velocity = 37.6 m/s

s = ut + (1/2)at2
s = (0)(4) + (1/2)(9.4)(4)2
s = 75.2 m


Consider the journey just after 4.0 s and just before the maximum height.
u = 37.6 m/s
a = -10 m/s2
v = 0 m/s

v2 = u2 + 2as
(0)2 = (37.6)2 + 2(-10)s
s = 70.7 m

The maximum height
= 75.2 + 70.7
= 145.9 m

=====
b)
Consider the journey just after 4.0 s and just before it hits the ground.
u = 37.6 m/s
a = -10 m/s2
s = -75.2 m

v2 = u2 + 2as
v2 = (37.6)2 + 2(-10)(-75.2)
v2 = 2917.76
v = -54 m/s

The velocity of the rocket just before it hits the ground
= 54 m/s (downwards)
=


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