F2 science 考試題..the mini 4WD runs faster if the weight of the mini 4WD is..

john is very keen on racing mini 4WD(四驅車) with other kids. He always wants
    his 4WD run faster on the racing track .After studying the chapter about fiction ,he
    knows that object can move faster on a smooth surface.

     His friend David suggested that the mini 4WD runs faster if the weight of the mini 4WD is
     reduced. Do you agree? Explain.

考試吾識做.求其答左NO.

呢題係YES啊 當一件物件越大 佢所成受既力就越大 你要令一件重既物件郁得快 咁你咪要用更多力 如果架車細D 咁佢用既力咪少D 郁得咪快D囉

一件物件越大 佢所成受既力就越大
質量越大定咩.

旦系E題應該講緊摩擦力.
甘講法.咪變左吾系幾關摩擦力事囉0.0

friction between the 4WD and the ground depends on the area of contact surface(i.e the tyres)    but not the weight of the 4WD ! this situation is just like pushing a cube and a ball with the same weight .it 's always easier to push the ball because the contact surface of the ball to the ground is  smaller!   please correct me if there is any mistake!

根據F=ma
於同一個重力場內,,, weight/mass = constant
減少weight的方法只有減少mass (除非不在地球進行)
若改裝後的摩打及摩擦力不變, 但增加"加速度", 自然會移動較快,,,

[quote]原帖由 [i]夜遊[/i] 於 2009-6-19 16:06 發表 [url=http://3boys2girls.com/redirect.php?goto=findpost&pid=41558720&ptid=701750][img]http://3boys2girls.com/images/common/back.gif[/img][/url]
friction between the 4WD and the ground depends on the area of contact surface(i.e the tyres)    but not the weight of the 4WD ! this situation is just like pushing a cube and a ball with the sam ... [/quote]


有兩樣野.分別10kg&1kg.
佢地GE area of contact surface 一樣.
佢地GE 摩擦力又一樣0.0?

[[i] 本帖最後由 森0.0 於 2009-6-20 14:32 編輯 [/i]]

[quote]原帖由 [i]講不出聲[/i] 於 2009-6-19 20:11 發表 [url=http://3boys2girls.com/redirect.php?goto=findpost&pid=41575103&ptid=701750][img]http://3boys2girls.com/images/common/back.gif[/img][/url]
根據F=ma
於同一個重力場內,,, weight/mass = constant
減少weight的方法只有減少mass (除非不在地球進行)
若改裝後的摩打及摩擦力不變, 但增加"加速度", 自然會移動較快,,, [/quote]

減少mass.摩擦力吾會減少0.0?

你又說得對,,, 減少mass的確會減低friction,,,
因為f = uN (u係constant, N係normal reaction, 若在平地行駛, 大致等於weight,,,)
即weight的減少會引致friction的降低
根據F=ma --> M - f = ma
假設摩打力(M)不變, f和m減少，
合力(F)將會增加, 而a(加速度)亦會增加, 換句話說理應是開快了

但這些是比較高年級的物理,,, 我也只是問人,,,=)

Gravitational force will decrease
Acceleration increases but highest velocity keep constant since it depends on the motor

1.decreasing mass

R=mg ------1
F=μR  ------2

mass細
R細
F細

friction細,向前NET FORCE 大左,加速度effective左
faster!



2.on smooth surface

F=μR
μ細
F細

friction細,向前NET FORCE 大左,加速度effective左
faster!

μ depends on surface material..
smooth 細 rough 大

[[i] 本帖最後由 昌昌 於 2009-7-4 18:18 編輯 [/i]]

[quote]原帖由 [i]森0.0[/i] 於 2009-6-20 13:04 發表 [url=http://www1.3boys2girls.com/redirect.php?goto=findpost&pid=41611956&ptid=701750][img]http://www1.3boys2girls.com/images/common/back.gif[/img][/url]



有兩樣野.分別10kg&1kg.
佢地GE area of contact surface 一樣.
佢地GE 摩擦力又一樣0.0? [/quote]

NO , DEPENDS ON MASS AND SURFACE MATERIAL

[quote]原帖由 [i]夜遊[/i] 於 2009-6-19 16:06 發表 [url=http://www1.3boys2girls.com/redirect.php?goto=findpost&pid=41558720&ptid=701750][img]http://www1.3boys2girls.com/images/common/back.gif[/img][/url]
friction between the 4WD and the ground depends on the area of contact surface(i.e the tyres)    but not the weight of the 4WD ! this situation is just like pushing a cube and a ball with the sam ... [/quote]
typical mistake
it should depends on weight not area

試下用支針好大力吉係table到
同一張薄紙放係table...
邊個易推D?

[[i] 本帖最後由 昌昌 於 2009-7-4 18:11 編輯 [/i]]

why not use E=1/2 mv^2?
if energy is constant, mass is reduce, then v would increase.
therefore the car run faster.