# physics mock mc

http://imm.io/YEsh

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1. Apply the equation: s = ut + (1/2)at^2 to the vertical motion from R to S

with s = -10m, u = 0 m/s, a = -g(=-9.81 m/s^2), t =?

hence, -10 = (1/2).(-9.81)t^2

t = 1.428 s

Hence, horizontal speed of monkey = 7/1.428 m/s = 4.9 m/s

2. Let b be the angular position of the 4th order maximium, and c be the angular position of the 2nd order maximium.

Hence, a.sin(c) = 2入

and a.sin(b) = 4入

where a is the grating slit separation and 入 is the wavelength of light used.

Dividing, sin(c)/sin(b) = 2入/ 4入 = 1/2

i.e. sin(c) = sin(b)/2

The max vlaue of sin(b) = 1, hence, sin(c) < 1/2

i.e. c < 30 degrees

The answer is thus option B (i.e. choose the nearest angle below 30 degrees)

[ Option A is wrong because when the 2nd order max is at 15 degrees, the max order of fringe is 7, not 4]

3. Suppose the electron is at distance d from the -ve plate

Change in potential of electron = (Vo).(d/D)

where Vo is the voltage between the two plates, with separation D.

Hence, drop in potential energy of electron = e.(Vo)(d/D)

where e is the electronic charge

By conservation of energy,

(KEo) + e(Vo)(d/D) = KE

where (KEo) is the initial kinetic energy, and KE is the kinetic energy at distance d.

Thus, KE = [e(Vo)/D].d + (KEo)

The graph of KE against d is a straight line with +ve slope of value [e(Vo)/D].

4. The answer is option C.

Statement 1 is wrong because the straight line of M doesn't pass through the origin. Hence, V and I are NOT proportional.

5. When S is closed,

99 = 100R/(100+R)

R = 9900 ohms

When S is open, the 100-ohm is in parallel with 2R

Equivalent resistance = 100 x 2R/(100+2R) = 99.5 ohms

(Since the "word limit" is reached, I give answer to Q6 and 7 in the opinion (意見) section.)

2013-03-09 17:50:32 補充：

6. Magnetic force at X due to Y and Z, Fx = [-k(2).(1)/(2) + k(2).(3)/4] = 0.5k

where k is a constant

Magnetic force at Y due to X and Z, Fy = [k.(1).(2)/2 k(1).(3)/2] N = 2.5k

Magnetic force at Z due to X and Y, Fz = [k(2).(3)/4 + k(1).(3)/2] = 3k

2013-03-09 17:51:09 補充：

6(cont'd) Hence, the greatest magnetic force is experienced by Z, and the least force by X.

2013-03-09 17:51:36 補充：

7. There are 3 forces acting on the kite, the kite weight of 0.2g, the string tension 15 N and the wind force F. The 3 forces are in equilibrium, and hence forms a triangle (Principle of Triangle of Forces). The angle bewteen the kite weight and the string tension is (90+35) degrees = 125 degrees.

2013-03-09 17:52:03 補充：

7(cont'd) By the cosine rule,

F^2 = [(0.2g)^2 + 15^2 - 2 x 0.2g x 15.cos(125) ] N^2

where g is the acceleration due to gravity

F = 16.2 N