Phy 問題 [mechanics]

首先第2個..點解係唔conserved呢??

As you can find out, it is not conserve

initially, p=0(at rest)

after all it gains momentum(to the right direction)

既然唔conserved又點可以用到conservation of momentum果條式呢??

Common trap in CE

The momentum of the whole system is always conserved

The momentum of one object in the whole system may not be
conserved

be careful when taking the whole system which is Grace+Kitty+ball.

而第3個..點解會係decrease呢???

the statement is “when Kitty caught the ball”

so you have to compare the KE before and after Kitty just caught the
ball

before she caught the ball

KE=0.5(2)(20^2)=400J

after she caught the ball

KE=0.5(42)(0.95^2)=19.0J

2009-12-28 22:52:59 補充：
Some energy is converted to internal energy and sound energy.
Kitty’s muscle have is compressed during the collision.
Energy is used to deform her muscle.





點解velocity唔係??
throwing a stone upwards
velocity -> up
acceleration(weight)->always down

no one says it accelerates from rest

2009-12-28 22:54:11 補充：
by definition, momentum must be in the same direction as velocity.
again, by the case: throwing a stone upwards
you know the answer~

2009-12-29 00:00:07 補充：
個u點搵架???點解係20既???
conservation of momentum when Grace threw the ball

2009-12-29 00:00:14 補充：
你既例子係throwing up a ball..咁佢應該係減緊速wo...
in physics, acceleration is a vector; it has direction
when it is in the direction of motion, the object speeds up
when it is in the oppostie direction of motion, the object slows down

ie. +ve a -> acceleration -ve a ->decelerationi

2009-12-29 00:00:17 補充：
in PHY
"acceleration" does not mean speeding up
it means change in velocity　only

2009-12-29 01:19:09 補充：
initial momentum=0
so
after throwing
momentum of Grace+momentum of the ball=0
50(-0.8)+2v=0
(taking right as positive)
v=20, which is the velocity of the ball when Grace just thorw it out

2009-12-29 02:18:49 補充：
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