# 2005 - JAMB Physics Past Questions & Answers - page 1

1
Marching soldiers crossing a suspension bridge are usually advised to break their steps to avoid damaging the bridge owing to
A
oscillation
B
resonance
C
swinging
D
vibration
CORRECT OPTION: b
Marching soldiers crossing a suspension bridge are usually advised to break their steps to avoid damaging the bridge owing to resonance. The steps of the marching soldiers can set the bridge into vibration, and when the frequency of the bridge is equal to that of the steps of the soldiers, the resonance occurs,and at this resonance, the bridge vibrates violently with maximum amplitude, and can collapse.
2
The sharpness of the boundary of the shadow of an object is determined by the
A
intensity of light striking the object
B
rays of light passing through the object
C
nature of the object
D
opacity of the object
CORRECT OPTION: b
The sharpness of the boundary of the shadow of an object is determined by the rays of light passing through the object. The smaller the rays passing through , that is from a point source, the more sharp the shadow becomes, but the more the rays, that is rays from a large source, the more blurred the shadow becomes.
3
A thin converging lens has a power of 4.0 diopters determined its focal length.
A
2.50m
B
0.03m
C
5.00m
D
0.25m
CORRECT OPTION: d
the power of a lens is defined as the reciprocal of the focal length ,expressed in metre.Mathematically, the power of a lens in dioptres is given by:(100)/f(in cm).
thus for lens of power of 4.0 dioptres we have that: (100)/(f) = +4.0
f= (100)/(4.0) = 25cm
= 0.25cm
4
A real image three times the size of an object is formed 24cm from a converging mirror. what is the focal length of the mirror.
A
16cm
B
6cm
C
8cm
D
12cm
CORRECT OPTION: b
for a real image 3 times the size of the object, it implies that the magnification 3 = v/u, 3 = 24/u, therefore 3u = 24, u = 8cm thus we have from the relation:m = (f)/(u)-f; that 3 = (f)/(8) - f. therefore f = 3(8-f) f = 24 - 3f ∴4f = 24; f = 6cm OR: (1)/(u) + (1)/(v) = (1)/(f).
(1)/(8) + (1)/(24) = (1)/(f)
therefore (3+1)/(24) = (1)/(f)
4/24=1/f;therefore 4f = 24
f = 6cm
5
The pair of physical quantities consisting of vectors only are
A
momentum and power
B
velocity and distance
C
displacement and torgue
D
accelaration and speed
CORRECT OPTION: c
6
In an ideal wheel an axle system, R stand for the radius of the wheel and r is the radius of the axle. the mechanical advantage is
A
(R/r)2
B
r/R
C
(r/R)2
D
R/r
CORRECT OPTION: d
In a wheel and axle system of mechanic, the mechanical advantage is given by its ratio of the radius of the wheel R, to the radius of the axle r. that M.A = R/r
7
An object is projected for the hieght of 80 m above the ground with a velocity of 40ms-1 at the angle of 30° to the horizontal. The time of flight
A
8 s
B
4 s
C
16 s
D
10 s
CORRECT OPTION: b
for a body projected at an angle θ to the horizontal, the time of the flight is given by T = (2u sin θ)/g =(2x 40)x sin (30)/10 =(2 x 40) x (0.5)/10 = 4.0secs.
8
In an a.c circuit that contains only a capacitor, the voltage lags behind the current by
A
180°
B
90°
C
60°
D
30°
CORRECT OPTION: b
In a purely capacity circuit, voltage lags behind current by 90°, or current leads voltage by 90°
9
the time it will take a certain radioactive material with a half-life of 50 days to reduce to 1/32of its original number is
A
250days
B
150days
C
300days
D
200days
CORRECT OPTION: a
Since 1/32, is the fraction left of any radioactive material after 5half lives, that is:
1/32= (1/2)5. Thus if a half life is 50 days, 5 half lives will be 50 x 5= 250 days
10
a proton moving with a speed of 1.0 x 106ms-1 through a magnetic field of 1.0T experiences a magnetic force of magnitude 8.0 x 10-14 N. The angle between the proton''s velocity and the field is
A
60°
B
90°
C
30°
D
45°
CORRECT OPTION: c
The force acting on moving charge in magnetic field is given by: F=q VBSin θ
sin θ =F=(8.0x10)-14/1.6x10-19x1.0x106x1.0 = (8.0x10)14/16 x 1013
(8.0x 10)-14/16x10-14
=0.5000
∴ θ= sin-1 0.5000
=30°
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