1. a gun is fired straight up from ground. the bullet takes 30 s to return to the ground. what is the speed of the bullet when it is fired.
take g=10m/s/s and ignore air resistance
只得time 同accelaration...唔識.....
2.a stone is thrown straight down with a speed of 8 m/s from a height of 120m above the ground. how long does it take to reach the ground
如果u=8 a=10 ,點解s唔等於120???
兩條F.4 physic 問題
2010-10-02 2:49 am
回答 (2)
2010-10-02 6:20 am
✔ 最佳答案
1. Use equation of motion: s = ut + (1/2).a.t^2with s = 0 m, a = -g(=-10 m/s^2), t = 30 s, u = ?
hence, 0 = 30u + (1/2).(-10).(30)^2
solve for u gives u = 150 m/s
2. Use equation of motion: s = ut + (1/2).a.t^2
with u = -8 m/s, s = -120 m, a = -g(=-10 m/s^2), t = ?
hence, -120 = (-8).t + (1/2).(-10).t^2
i.e. 5t^2 + 8t -120 = 0
solve for t gives t = 4.16 s or -5.76 s(rejected)
Hence, the time to reach the ground = 4.16 s
2010-10-02 3:09 am
Take upward direction as positive.
1)Given:a=10ms^-2, t=30, v=0
v=u+at
0=u+10x30
u=-300m s^-1
2)Given:u=8ms^-1, s=120m, a=10
v^2-u^2=2as
v^2=2(10)(120)+8^2
v=49.6ms^-1
v=u+at
t=(v-u)/a
t=4.16s
1)Given:a=10ms^-2, t=30, v=0
v=u+at
0=u+10x30
u=-300m s^-1
2)Given:u=8ms^-1, s=120m, a=10
v^2-u^2=2as
v^2=2(10)(120)+8^2
v=49.6ms^-1
v=u+at
t=(v-u)/a
t=4.16s
收錄日期: 2021-04-29 17:36:22
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20101001000051KK01263