quiz 既mc題 =[

小霞的fans · 發表於 11/12/2008 04:23 PM

1. BALLS A AND B ARE THROWN VERTICALLY UPWARDS AND DOWNWARDS RESPECTIVELY

if their initial speeds are the same,which of the following statements is/are correct when they meet ?

1) their displacement should be the same
2)their speeds should be the same
3)their accelerations should be the same

a.1 only
b.3 only
c.1 and 2 only
d.1 and 3 only

ms 我已經錯...唉 .. phy 又可能唔合格啦

abc666 · 發表於 11/12/2008 08:04 PM

D

1.same initial position,meet at same point-->same displacement
3.accelerations equal to g

renz · 發表於 12/12/2008 08:06 AM

我的理解是這樣...
A
↓

O

↑
B

B往上的speed在一直減
A往下的speed在一直加
2)否決
如是...接觸點不是中間點
1)否決
只有3)
兩球的a都是g=10

我選B

小霞的fans · 發表於 12/12/2008 04:14 PM

原帖由 renz 於 11/12/2008 12:06 PM 發表

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我的理解是這樣...
A
↓

O

↑
B

B往上的speed在一直減
A往下的speed在一直加
2)否決
如是...接觸點不是中間點
1)否決
只有3)
兩球的a都是g=10

我選B

係呀..真係B呀 .
但係我想問呢 ..
唔係1個係10..1個係-10咩 ?
方向唔同喎 .. 下係POSITIVE..

sapphire · 發表於 12/12/2008 04:26 PM

1) their displacement should be the same
This is true if and only if the starting point of two objects are the same

2)their speeds should be the same
This is true if and only if their meeting point is at the ground, i.e. both of the speed of the objects = 0 ms^(-2)

3)their accelerations should be the same
Acceleration due to gravity at the same place for any objects should be the same. Its direction is always towards the ground, no matter the object is moving up or moving down.

So 3 is always true.

[ 本帖最後由 sapphire 於 12/12/2008 05:25 PM 編輯 ]

小霞的fans · 發表於 12/12/2008 04:41 PM

原帖由 sapphire 於 11/12/2008 08:26 PM 發表

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1) their displacement should be the same
This is true if and only if the starting point of two objects are the same

2)their speeds should be the same
This is true if and only if their meeting poi ...

唔係向下果個正10 .. 向上果個-10咩 ? 佢反地心吸力- ,-

sapphire · 發表於 12/12/2008 05:18 PM

原帖由 小霞的fans 於 12/12/2008 04:41 PM 發表

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唔係向下果個正10 .. 向上果個-10咩 ? 佢反地心吸力- ,-

Remember that a negative acceleration means that the objects is accelerating in the opposite direction.

Same as your case, consider downward as positive.
Consider the motion of the object which is moving upward, its speed towards upward is in fact decreasing. That means the object is upward decelerating (accelerating downward).
So, if you consider the object moving upward has a negative acceleration. It means that this object is upward accelerating. Clearly it is impossible unless there is a great force provided to the object against the gravity. (Try to think if you throw a ball to the sky and it never falls down again......)
The object is actually accelerating downward. Its acceleration is also positive.

For any object in the air, its acceleration is always toward the ground if there is no other force.

[ 本帖最後由 sapphire 於 12/12/2008 05:24 PM 編輯 ]

小霞的fans · 發表於 12/12/2008 05:36 PM

原帖由 sapphire 於 11/12/2008 09:18 PM 發表

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Remember that a negative acceleration means that the objects is accelerating in the opposite direction.

Same as your case, consider downward as positive.
Consider the motion of the object which ...

咁TAKE XXX DIRECTION AS POSITIVE 有咩用..
點都係正 =_= "" 我太笨 ..

sapphire · 發表於 12/12/2008 06:01 PM

原帖由 小霞的fans 於 12/12/2008 05:36 PM 發表

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咁TAKE XXX DIRECTION AS POSITIVE 有咩用..
點都係正 =_= "" 我太笨 ..

For calculation, direction can be defined as you like.
If you define upward as positive, then its acceleration is -10ms^(-2)
If you define downward as positive, then its acceleration is 10ms^(-2)

小霞的fans · 發表於 12/12/2008 06:19 PM

原帖由 sapphire 於 11/12/2008 10:01 PM 發表

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For calculation, direction can be defined as you like.
If you define upward as positive, then its acceleration is -10ms^(-2)
If you define downward as positive, then its acceleration is 10ms^(-2)

咁我如果take downward direction of ballA 係positive
咁佢既a 就-10^-2 因為佢向上升
咁我take downward direction of ball B 係 positive
咁佢既a咪係10^2 囉 ..因為佢向下跌

咁2個既acceleration咪唔同lor ..咁點解3)會岩 ..
呢個係我一直想問既問題

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