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JAMB Physics 2021 Objective Past Questions

All 40 questions from the Joint Admissions and Matriculation Board (JAMB) Physics 2021 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.

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Physics 2021 Objective — Question 1

The half life of a radioactive substances is 5 days. What percentage will remain after 20 days?

  • A. 12.5%
  • B. 25%
  • C. 6.25%Correct
  • D. 42%

Explanation

n=20/5=4 half-lives; (½)⁴=1/16=6.25%.

Physics 2021 Objective — Question 2

Which is true for a coin rolling down an inclined plane?

  • A. centre is rotational, other points translational
  • B. centre and all other points translational
  • C. centre is translational, other points rotationalCorrect
  • D. motion of centre is translational

Explanation

Centre undergoes pure translation; all other points rotate about the centre.

Physics 2021 Objective — Question 3

Energy stored in elastic material when force 1000 N causes extension of 1 cm:

  • A. 9×10⁻¹¹ J
  • B. 5 JCorrect
  • C. 10⁴ J
  • D. 9×10¹¹ J

Explanation

E=½Fe=½×1000×0.01=5 J.

Physics 2021 Objective — Question 4

Reading on Kelvin scale when Fahrenheit scale reads 188°F:

  • A. 359.7 KCorrect
  • B. 273 K
  • C. 90 K
  • D. 156 K

Explanation

°C=(188-32)×5/9=86.67°C; K=86.67+273=359.7 K.

Physics 2021 Objective — Question 5

Given M*L^xT^z is the dimension for G. Values of x, y & z:

  • A. −1, 3, −2Correct
  • B. −2, &3
  • C. 2, −1 & 1
  • D. 2, 1 & −1

Explanation

G=[M⁻¹L³T⁻²]; x=3, y=−1, z=−2 (varies by notation).

Physics 2021 Objective — Question 6

Load 20 N on wire (area 8×10⁻⁷ m², extension 10⁻⁴ m, length 3 m). Young's modulus:

  • A. 9×10¹¹ Nm⁻²
  • B. 8.5×10¹¹ Nm⁻²
  • C. 7.5×10¹¹ Nm⁻²Correct
  • D. 7×10¹¹ Nm⁻²

Explanation

Y=Fl/Ae=(20×3)/(8×10⁻⁷×10⁻⁴)=7.5×10¹¹ Nm⁻².

Physics 2021 Objective — Question 7

Work to move 500μC charge from A to B is 0.05 J. Potential difference A to B:

  • A. 100 VCorrect
  • B. 50 V
  • C. 200 V
  • D. 500 V

Explanation

V=W/q=0.05/(500×10⁻⁶)=100 V.

Physics 2021 Objective — Question 8

Which has the shortest wavelength?

  • A. Infra-red rays
  • B. Radio waves
  • C. Gamma raysCorrect
  • D. Ultraviolet rays

Explanation

Gamma rays have the shortest wavelength in the EM spectrum.

Physics 2021 Objective — Question 10

14 kg body released on smooth incline at 60° to horizontal. Acceleration down the plane: (g=10 ms⁻²)

  • A. 10 m/s²
  • B. 5 m/s²
  • C. 8.66 m/s²Correct
  • D. 4.33 m/s²

Explanation

a=g sinθ=10×sin60°≈8.66 m/s².

Physics 2021 Objective — Question 11

Which is used for construction of a searchlight?

  • A. parabolic mirrorCorrect
  • B. concave mirror
  • C. convex mirror
  • D. spherical mirror

Explanation

Parabolic mirror produces a parallel beam — ideal for searchlights.

Physics 2021 Objective — Question 12

Force of repulsion between 4μC and 5μC charges 0.02 m apart:

  • A. 9×10² N
  • B. 4.5×10² NCorrect
  • C. 1.8×10³ N
  • D. 2.7×10³ N

Explanation

F=kq₁q₂/r²=9×10⁹×20×10⁻¹²/4×10⁻⁴=450 N=4.5×10² N.

Physics 2021 Objective — Question 13

Ebonite rod rubbed with fur attracts glass rod rubbed with silk because:

  • A. ebonite negative, glass positiveCorrect
  • B. ebonite positive, glass negative
  • C. both negative
  • D. both positive

Explanation

Ebonite becomes −ve with fur; glass becomes +ve with silk; unlike charges attract.

Physics 2021 Objective — Question 14

Iron core of induction coil made from bundles of wires to:

  • A. obtain high secondary voltage
  • B. minimize eddy-currentsCorrect
  • C. prevent sparking
  • D. generate eddy-currents

Explanation

Lamination/bundling increases resistance to eddy currents, minimising losses.

Physics 2021 Objective — Question 15

When metallic surface illuminated by light of given frequency and intensity:

  • A. number and energy depend on frequency
  • B. number and energy depend on intensity
  • C. number depends on intensity, energy depends on frequencyCorrect
  • D. number depends on intensity, energy depends on intensity

Explanation

Number of photoelectrons ∝ intensity; KE depends on frequency.

Physics 2021 Objective — Question 16

Velocity V=5t²+11. Instantaneous acceleration after 2 seconds:

  • A. 20 m/s²Correct
  • B. 10 m/s²
  • C. 5 m/s²
  • D. 17 m/s²

Explanation

a=dV/dt=10t; at t=2: a=20 m/s².

Physics 2021 Objective — Question 17

Which is a first class lever?

  • A. sugar tong
  • B. wheel barrow
  • C. nutcracker
  • D. scissorsCorrect

Explanation

First-class lever: fulcrum between load and effort (e.g. scissors).

Physics 2021 Objective — Question 18

In p-type semiconductor, majority carriers are:

  • A. electrons
  • B. holesCorrect
  • C. electrons and holes
  • D. ions

Explanation

Holes are majority carriers in p-type.

Physics 2021 Objective — Question 20

Equation of wave y=0.15 sin200π(x−5.5t). Frequency and wavelength:

  • A. 500 Hz and 0.25 m
  • B. 550 Hz and 0.01 mCorrect
  • C. 400 Hz and 0.4 m
  • D. 325 Hz and 2 m

Explanation

ω=1100π → f=550 Hz; k=200π → λ=0.01 m.

Physics 2021 Objective — Question 21

Mirror rotated through 40° with incident angle 30°. Angle through which reflected ray rotates:

  • A. 70°
  • B. 10°
  • C. 40°
  • D. 80°Correct

Explanation

Rotating mirror by θ rotates reflected ray by 2θ=80°.

Physics 2021 Objective — Question 22

First three harmonics of stretched string (length 2m, v=150 m/s):

  • A. 37.5, 75, 150 Hz
  • B. 37.5, 75, 112.5 HzCorrect
  • C. 187.5, 56.25, 93.75 Hz
  • D. 18.75, 37.5, 56.25 Hz

Explanation

f₁=v/2L=37.5 Hz; f₂=75 Hz; f₃=112.5 Hz.

Physics 2021 Objective — Question 23

Sound wave (85.5 Hz, 343 m/s) reflected from wall. Distance from wall to first antinode:

  • A. 1 mCorrect
  • B. 2 m
  • C. 3 m
  • D. 4 m

Explanation

λ=343/85.5≈4 m; first antinode at λ/4=1 m from wall.

Physics 2021 Objective — Question 24

Driving mirror radius of curvature 1 m. Vehicle 4 m from mirror. Image distance:

  • A. 1.143 m
  • B. 0.444 mCorrect
  • C. 0.571 m
  • D. 4.5 m

Explanation

f=+0.5 m (convex), u=−4 m; 1/v=2+0.25=2.25→ v≈0.444 m.

Physics 2021 Objective — Question 25

Angles of inclination at pole and equator are respectively:

  • A. 90° & 0°Correct
  • B. 0° & 90°
  • C. 90° & 45°
  • D. 45° & 90°

Explanation

Pole: dip=90° (vertical field); Equator: dip=0° (horizontal field).

Physics 2021 Objective — Question 26

A satellite is in a parking orbit when:

  • A. period > earth's period
  • B. period < earth's period
  • C. period = earth's periodCorrect
  • D. period = half earth's period

Explanation

Geostationary orbit: satellite period = Earth's rotation period.

Physics 2021 Objective — Question 27

Two parallel plane mirrors with reflecting faces towards each other. Number of images:

  • A. 6
  • B. infiniteCorrect
  • C. 2
  • D. 4

Explanation

Parallel facing mirrors → infinite images.

Physics 2021 Objective — Question 28

If object placed between two parallel plane mirrors with reflecting faces facing each other, images formed:

  • A. 6
  • B. infiniteCorrect
  • C. 2
  • D. 4

Explanation

Infinite reflections → infinite images.

Physics 2021 Objective — Question 29

V∝√θ; if V₂=2V₁, then:

  • A. θ₂=4θ₁Correct
  • B. θ₂=2θ₁
  • C. θ₂=θ₁
  • D. θ₂=0.5θ₁

Explanation

V∝√θ → V²∝θ; doubling V quadruples θ.

Physics 2021 Objective — Question 31

Submarine rises from real depth 80 m to 60 m. Change in apparent depth (n=4/3):

  • A. 45 m
  • B. 60 m
  • C. 15 mCorrect
  • D. 80 m

Explanation

App₁=80/(4/3)=60 m; App₂=60/(4/3)=45 m; change=15 m.

Physics 2021 Objective — Question 32

Glass bottle containing liquid heated. Liquid level falls. This shows:

  • A. liquid expands more than bottle
  • B. bottle and liquid expand equally
  • C. bottle expands more than liquidCorrect
  • D. liquid does not expand

Explanation

If level falls, bottle expands more than liquid.

Physics 2021 Objective — Question 33

Which gas law is equivalent to the workdone?

  • A. Charles' law
  • B. Boyle's lawCorrect
  • C. Gay Lussac's law
  • D. none

Explanation

Boyle's law (PV=const): work done in isothermal compression/expansion.

Physics 2021 Objective — Question 34

If α, β, γ represent linear, area and cubic expansivities, which is correct?

  • A. α=2β
  • B. γ=3β/2
  • C. β=2αCorrect
  • D. β=2γ/3

Explanation

β=2α (area=2×linear expansivity).

Physics 2021 Objective — Question 35

Instrument used by designers to obtain different colour patterns:

  • A. sextant
  • B. Periscope
  • C. microscope
  • D. KaleidoscopeCorrect

Explanation

Kaleidoscope uses mirrors to create symmetrical colour patterns.

Physics 2021 Objective — Question 36

Lenz's law is a law of conservation of:

  • A. energyCorrect
  • B. momentum
  • C. velocity
  • D. charge

Explanation

Lenz's law: induced emf opposes change → conservation of energy.

Physics 2021 Objective — Question 37

Empty density bottle weighs 2 N. Filled with water: 5 N. Filled with olive oil: 4 N. Relative density of olive oil:

  • A. 2/3Correct
  • B. 1/2
  • C. 1/3
  • D. 2/7

Explanation

RD=(4-2)/(5-2)=2/3.

Physics 2021 Objective — Question 38

Short chain on back of petrol tanker to:

  • A. caution driver
  • B. increase friction
  • C. ensure balancing
  • D. conduct excess charges to earthCorrect

Explanation

Chain grounds static charges, preventing sparks near fuel.

Physics 2021 Objective — Question 40

The ability of the human eye to focus objects on the retina is referred to as the power of:

  • A. interference
  • B. superposition
  • C. diffraction
  • D. accommodationCorrect

Explanation

Accommodation is the eye's ability to change focus for different distances.

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