Mechanics - Newton's laws

By law of conservation of Momentum動量守恆定律

CASE 1:

(mass of car + mass of rain droplet) x velocity = constant,

however, mass of rain droplet is increasing,

so velocity of the car decreases.

[如 (2 + 3) x 4 = 18, 若3改為7, 那麼4要改為2才能繼續令結果是18.]

In order to maintain uniform velocity,

a net force is required.


CASE 2:

(mass of car + mass of rain droplet) x velocity = constant,

however, mass of rain droplet is decreasing,

so velocity of the car increases.

[如 (2 + 3) x 4 = 18, 若3改為1, 那麼4要改為6才能繼續令結果是18.]

That's why net force is NOT required.

唔係好明條問題,嘗試下
uniform velocity的話,架車應該o係smooth horizontal plane(without friction)上面行

係咪肯定用Newton's laws?以下係用momentum做
case i
By conservation of momentum,
m1u1+m2u2=m1v1+m2v2
(Taking direction of u1 as positive,ie. u1>0)
object 1係車,object 2係雨
一開始,雨o既horizontal velocity係0(vertical motion)
After collision,雨同車變成一個object,所以v1=v2>0 (since u1>0)
So,m1u1+m2*(0)=m1v1+m2v2
m1*(u1-v1)=m2v2
m2>0 and v2=v1>0
Therefore,u1>v1
所以架車唔可以再以uniform velocity行走,即係要用力去compensate個deceleration


case ii
一樣,by conservation of momentum
m1u1+m2u2=m1v1+m2v2
u1=u2
由於有inertia,v2=u2
So,u1=u2

case i

因為一路都落緊雨
而且個題目都話個mass一路上升 [可能係因為d雨有mass掛 - -]
如果一路都係uniform velocity既話,
Force=Friction
但mass大左
fiction都會大左
咁根據Newton's second law,
F-f=ma
F-f=m(0) << a=0 coz uniform velocity
F=f
ie. Force = friction
所以就要有Force先可以走倒


case ii

相反
因為架車一直漏水
mass decreases,
咁如果係uniform velocity既話
fiction細左
咁根據Newton's second law,
F=ma
F=m(0) << a=0 coz uniform velocity
F=0
但mass係咁decreases,
that means Force > Friction
所以就唔駛力都走倒


唔明可以再問 =]]

2007-09-20 20:23:10 補充：
Correction for case i因為一路都落緊雨而且個題目都話個mass一路上升 [可能係因為d雨有mass掛 - -]如果一路都係uniform velocity既話,但mass大左fiction都會大左咁根據Newtons second law,F-f=maF-f=m(0).. a=0 coz uniform velocityF=fie. Force = friction所以就要有Force先可以走倒