请帮我解决难题，谢谢！

无知小子92

A stone falls from rest from the top of a high cliff. A second stone is thrown downward from the same height 2.00s later with an initial speed of 30ms^-1. If both stones hit the ground simultaneously, how high is the cliff?

[Answer: 73.8m]


我找到的答案是12Xm
肯定错了咯

有哪位会做的吗？
请帮帮小弟！
谢谢！

soulcrying

可能会有一点乱..看不懂可以问我
a=9.81m/s^2
assume moving downward be positive

S=ut+1/2at^2
S=4.905t^2-------1

S=uT+1/2aT^2
S=30(t-2)+4.905(t-2)^2-----2

Sub 1 to 2
4.905t^2=30t-60+4.905t^2-19.62t+19.62
10.38t=40.38
t=3.89s

Sub t=3.89s to 1
S=4.905(3.89)^2
S=74.22m

我做到是这样..可以指点我下..

UTAR生

可能会有一点乱..看不懂可以问我
a=9.81m/s^2
assume moving downward be positive

S=ut+1/2at^2
S=4 ...
soulcrying 发表于 27-8-2010 09:25 PM

首先我先说声谢谢！

不过我还是有点不明白之处，希望大家可以为小弟解答！

题目注明
【thrown downward from the same height 2.00s later with an initial speed of 30ms^-1】

velocity is vector
downward should be -30ms^-1 啊

然后因为stone falling downward，gravity acceleration is +，this one i can accept la

如果我用velocity = -30ms^-1
我就得不到这个答案咯

而且没错好像是200多

谢谢！

soulcrying

assume moving downward be positive
我已经设向下为正了..

无知小子92

谢谢soulcrying为我解答！！！

也谢谢UTAR生为我提出疑问！！！

小弟完全明白了解！