phy~從不知怎麼是對的

小霞的fans · 發表於 26/2/2010 11:16 AM

1.the motion of a toy car is recorded by a ticket-tape tine with a frequency of　　
100Hz.the distance between the first two successive dots and the final two successive dots is 1cm and 7cm respectively.also,there are 5 dots on the tape.

find the initial velocity,final velocity and hence the acceleration of the toy car.

initial velocity 應該係1/1/100 =100cms^-1
final 應該係 7/1/100 = 700cms^-1

acceleration係 7ms^-1 - 1ms^-1/ time
個time應該係幾多.. 唔識搵個時間.. 果2個velocity既時間係幾多...?

2.如個1舊石頭被氣球條線縛住,,慢慢由地下升上空中
如果向上係positive
姐係石頭上面既tension既magnitude大過weight既magnitude.
咁佢個acceleration係acceleration due to gravity=10ms^-2 ?
咁佢係向上加速定向上減速...?搞唔清楚呢個問題..好似加又好似減- - " thx

鴨 · 發表於 26/2/2010 02:28 PM

本帖最後由 RX78-2 於 26/2/2010 02:36 PM 編輯

第一題時間是有給提示的
the motion of a toy car is recorded by a ticket-tape tine witha frequency of　　
100Hz.the distance between the first two successive dots and the final two successive dots is 1cm and 7cm respectively.also,there are 5 dots on the tape.

既然向上為positive
acceleration due to gravity必為-10ms^-2
但是acceleration due to gravity大多是在free fall 情況下才會用的
因這個代表的是W=mg的g
所以其實在計net force的時候你已經把其考慮在內
一般有剩力的acceleration是由net force來計的

小霞的fans · 發表於 26/2/2010 04:47 PM

第一題時間是有給提示的
the motion of a toy car is recorded by a ticket-tape tine with a frequency of　　
100Hz.the distance between the first two successive dots and the final two successive dots is ...
RX78-2 發表於 25/2/2010 06:28 PM

登錄/註冊後可看大圖

thx你...
第1題唔知點解時間係0.03s
tine for 1 dot應該係1/100=0.01s
由於5個dots之間有4個空位..咁應該係0.01x4 = 0.04 s ??
點解少左0.01s ..??

第2題,,其實佢係得1個acceleration (10ms^-2)
定係2個(T-mg=ma)
姐係佢係向上加速定減速t^t

hk56740 · 發表於 26/2/2010 10:03 PM

2. take upward direction as positive.
let the mass of stone be m kg, T N be the tension between the stone and the string, the acceleration of stone be A m/s^2.
Weight of stone=mg
=m(-10)
=-10m N
Apply F=ma(Newton's Second Law),
T-(-10m)=mA
T+10m=mA

其實你要留意既係1個叫accelerationdue to gravity,另1個叫acceleration.
Weight係地球拉個object既力,=mass of object x acceleration due to gravity所以有需要包括埋用黎計net force.而acceleration係真實中個net force引致既加速.

orb · 發表於 27/2/2010 10:51 AM

Tension
↑
Ｏ (Stone)
↓
mg

考量free body diagram, 就是如上
只要tension大過mg, stone永遠都向上加速
(tesion=mg, no acceletarion for stone: 原本向上升的繼續向上升)
(tension<mg, 連氣球一起向下拖)

小霞的fans · 發表於 27/2/2010 02:40 PM

第2題明左啦謝謝~~

問另外1題似第1題的東東..

登錄/註冊後可看大圖

an experiment is set up th find the acceleration due to gravity g on metal ball M.the ticket time-tape timer produces 50 dots per second.the above figure shows a paper tape obtained from the experiment.

1.find the time interval represented by AB.
ans:1/50x5=0.1s

2.find the average speed of the metal ball M during the time interval represented by part of strip
1.between A and B
2.between C and D
ans:1.u=0.132/0.1=1.32ms^-1
2.v=0.318/0.1=3.18ms^-1

我的問題1:點解AB果段時間既速度係u唔係v ? direction of paper tape唔係由左去右?點解調轉左?

3.hence,find the value of g from the experiment
ans:g=3.18-1.32/0.2 =9.3ms^-2

我的問題2:點解個t係0.2唔係0.3 ? A同B之間係0.1s..C同D之間係0.1s..B同C之間都有0.1s架喎‥‥點解會係0.2既??

小霞的fans · 發表於 2/3/2010 09:32 AM

本帖最後由小霞的fans 於 1/3/2010 04:42 PM 編輯

另外再想問一問..
wave length of visible violet light in glass is greater than that in air.

我睇哂成本書都搵唔到有關資料..
點樣先知佢
wave length同speed既轉變呢 - -?

同埋呢,,electromagnetic wave唔係係vacuum既speed先係3x10^8ms-1架咩?
點解係空氣度都係相同speed架?空氣唔係應該各有不同架咩-.-?
咁點解the speeds of electromagnetic waves are equal to the speed of light呢句說話係岩??

小霞的fans · 發表於 31/3/2010 11:59 AM

又有新問題啦~...

A small metal ball of mass 4kg is dropped from rest tnto a sand pit.it is initally at a height of h=1.8m above surface of the sand.as it hits the sand,it moves further over a distance of s=0.2m before it stops.find the average force acting on the ball by the sand.

4x10x1.8=F(0.2)
F=360N
但我唔明點解答案係400N ,, 點解要加埋weight呢? weight應該係gravity比個ball既力黎架喎..而家唔係搵sand比佢既力架咩??

Requiem · 發表於 31/3/2010 12:12 PM

又有新問題啦~...

A small metal ball of mass 4kg is dropped from rest tnto a sand pit.it is initally at a height of h=1.8m above surface of the sand.as it hits the sand,it moves further over a dista ...
小霞的fans 發表於 31/3/2010 11:59 AM

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Net force acting on the ball=360N
The small metal ball still experiences gravity during its deceleration.
Hence, net force acting on the ball=force exerted on the ball by the sand-weight of the ball.

小霞的fans · 發表於 31/3/2010 12:58 PM

Net force acting on the ball=360N
The small metal ball still experiences gravity during its deceleration.
Hence, net force acting on the ball=force exerted on the ball by the sand-weight of the ...
Requiem 發表於 30/3/2010 04:12 PM

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force exerted on the ball by the sand係向上定向下既力?
如果係向上..呢個力大過weight..咁個波唔係向上運動咩?
佢仲向下行??
如果係向下既力..姐係同weight平行?? 唔係好明act係ball既力

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[求助] phy~從不知怎麼是對的

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