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千千万万遍
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此套试卷由25道选择题以及3道大题组成
每道大题含有不同数量的小题
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Part A选择题部分:
Part B简答题部分
Solution to Problem 1:
Let x be the horizontal distance from the point where the ball was released, to the point where a ray originating from the instantaneous position of the ball at depth y crossesthesurface andreaches the observer.
From Snell’s law we have sin才 = nw sin万, where nw =1.33 is the index of refraction of water.
From geometry, wecanal so write:
x = d − h tanα
tanβ = x/y
Then
(a)
The initial acceleration a0 can be calculated from the firs velocity point in the data table. Velocity changed from 0 to0.091m/sin 0.01s,which gives a0 =9.10 m/s2 .
The terminal speed can be read off the table as approximately vf =0.916 m/s2 .
The total instantaneous downward force F acting on the ballwhen ithas speed v is
F = Mg − ρwVg − bv (1)
where Mg isthegravitationalforce, ρwVg istheupward buoyancy force if the ball has volume V, and bv is the drag force. By Newton’s second law, F = Ma, where a is theacceleration oftheball..
Initially, v is small enough that the drag force can be ignored, andwe can write
F = ρballVg − ρwVg = ρballVa0
with ρball the densityof theball. Thus,
ρw
ρ=
ball
1 − a0 /g
1000 kg/m3
= 1 − 9.10/9.80
=1.4 × 104 kg/m3
Ontheotherhand,whenthespeed v has reacheditsfina value vf,theacceleration vanishes andeq.(1) becomes
ρw
0= Mg − ρwVg − bvf = (1 − )Mg − bvfρball
Solvingfor b yields
Mg ρ
w
b =1 − vfρball
(1.00 kg)(9.80 m/s2 )1.00 × 103 kg/m3
=1 −
0.916 m/s1.4 × 104 kg/m3 =9.9 kg/s
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