兩條物理題目.20分(聽日要交.急)

1. kmh^-1=1000mh^-1
=1000/3600ms^-1
so,100kmh^-1=250/9ms^-1

用公式 v=u+at
250/9=0+6.5t
t=4.27s
The shortest time is 4.27s.

2.(a)opposite direction to her velocity
(b)a skater is slowing down at a rate of 2ms^-1 (<-----這是不是2ms^-2呢?) and it takes 3s for her to slow down to 9ms^-1.

如果是2ms^-2,用公式 v=u+at
9=u+2(3)
u=1.5ms^-1
The initial velocity of the skater is 1.5ms^-1.

2007-09-26 19:23:21 補充：
(如果是2ms^-2,用公式 v=u at 9=u 2(3) u=1.5ms^-1 The initial velocity of the skater is 1.5ms^-1. )個acceleration冇加返負號,下面2樓正確(thx ^.^)

1.
1 kmh^-1 = 5/18 ms^-1
first of all 1km =1000m
1 hour = 3600 seconds

consider a car travel 1km in hour =1kmh^-1
this means that the car travel 1000m in 3600s =1000/3600
= 5/18 ms^-1

the shortest time to accelerate from rest to 100kmh^-1
=the shortest time to accelerate from rest to 100*5/18 ms^-1
=250/9 ms^-1

t =(v-u)/a
t =(250/9-0)/6.5
t =4.27 seconds

2.
a)
in order to slow down, the direction of the her acceleration must be opposite to the
velocity

b)
since the direction of acceleration is opposite to the velocity
the acceleration is -2ms^-2
the initial velocity of the skater
t =(v-u)/a
3 =(9-u)/-2
u =15ms^-1

2007-09-26 19:03:30 補充：
樓上果位有D 問題, 話明slow down 既, 無理由final velocity 會大過initial velocity 架. 佢可能無考慮埋opposite 個condition