JAMB Physics 1988

22 reviewed questions with answers and explanations.

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Question 2

Which SI units are correctly matched? I. N for force. II. N·m⁻¹ for torque. III. W for power. IV. kg·m·s⁻² for momentum.

  1. I and II only
  2. I. II and III only
  3. I, II and IV only
  4. I and III only.
Answer and explanation

D: I and III only.

Force is measured in newtons and power in watts, so I and III are correct. Torque is measured in N·m, not N/m. Momentum is kg·m/s, not kg·m/s².

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Question 4

Two perpendicular forces have resultant 100 N. One force makes 30° with the resultant. Find that force’s magnitude to one decimal place.

  1. 8.66 N
  2. 50.0 N
  3. 57.7 N
  4. 86.6 N
Answer and explanation

D: 86.6 N

The force making 30° with the resultant is the adjacent component: F = R cos 30° = 100 × √3/2 = 86.6 N to one decimal place.

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Question 5

Which of the following quantities are scalars? I Electrical potential II Torque III Momentum IV Kinetic energy

  1. II and III only
  2. I and II only
  3. III and IV only
  4. I and IV only
Answer and explanation

D: I and IV only

Electric potential and kinetic energy are scalars. Torque and momentum are vectors, so the scalar quantities are I and IV.

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Question 7

A footballer applies an average force of 30.0 N to a stationary ball for 0.05 s. If the ball’s mass is 0.075 kg, what is its speed as it moves off?

  1. 4.50m s-1
  2. 11.25m s-1
  3. 20.00m s-1
  4. 45.00m s-1
Answer and explanation

C: 20.00m s-1

Impulse equals change in momentum. Starting from rest, mv = Ft = 30.0 × 0.05 = 1.5 N·s. Hence v = 1.5/0.075 = 20.0 m/s.

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Question 8

A 20-tooth gear drives a 60-tooth gear. If the smaller gear rotates at 120 rev/s, what is the angular speed of the larger gear?

  1. 3 rev s-1
  2. 40 rev s-1
  3. 360 rev s –1
  4. 2 400 rev s-1
Answer and explanation

B: 40 rev s-1

Meshing gears pass the same number of teeth per second. Therefore 20 × 120 = 60 × n, giving the larger gear’s speed n = 40 rev/s.

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Question 11

A 60 kg man climbs through a vertical height of 22 m with useful lifting power 0.25 kW. How long does the climb take? Use g = 10 m/s².

  1. 5.3 s
  2. 34.5 s
  3. 41.6 s
  4. 52.8 s
Answer and explanation

D: 52.8 s

Useful energy for the climb is mgh = 60 × 10 × 22 = 13200 J. At 0.25 kW = 250 W, time = energy/power = 13200/250 = 52.8 s.

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Question 16

The product PV where P is pressure and V is volume has the same unit as

  1. Force
  2. Power
  3. Energy
  4. Acceleration
Answer and explanation

C: Energy

Pressure × volume has units (N/m²) × m³ = N·m = J. A joule is the SI unit of energy.

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Question 17

The amount of heat needed to raise the temperature of 10kg of copper by1K is its

  1. Specific heat capacity
  2. Heat capacity
  3. Latent heat
  4. Internal heat.
Answer and explanation

B: Heat capacity

Heat capacity refers to the whole body: C = Q/ΔT. The heat required for this 10 kg piece to rise by 1 K corresponds to its heat capacity. Specific heat capacity is per kilogram.

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Question 20

Heating one end of a metal rod causes lattice particles at the other end to vibrate more vigorously. Heat has been transferred through the rod by

  1. Radiation
  2. Convection
  3. Conduction
  4. Evaporation.
Answer and explanation

C: Conduction

Conduction transfers thermal energy through the rod without bulk motion of the metal. In metals, mobile electrons and lattice vibrations both contribute to this transfer.

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Question 21

Which of the following media allow the transmission of sound waves through them? I Air II Liquids III Solids

  1. I and II only
  2. I and III only
  3. II and III only
  4. I, II and III.
Answer and explanation

D: I, II and III.

Sound is a mechanical disturbance that can propagate through gases, liquids and solids. All three provide matter whose particles can pass the disturbance onward.

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Question 23

Which behaviours can both mechanical and electromagnetic waves exhibit under suitable conditions? I. Diffraction. II. Refraction. III. Interference.

  1. I only
  2. III only
  3. I and III only
  4. I, II and III.
Answer and explanation

D: I, II and III.

Mechanical and electromagnetic waves can diffract around openings, refract when their propagation speed changes, and interfere when they overlap. All three are characteristic wave behaviours.

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Question 26

In the usual nondispersive ideal-gas model for air of fixed composition, which factor independently affects sound speed? I. Temperature. II. Pressure at fixed temperature. III. Frequency.

  1. I only
  2. II only
  3. I and II only
  4. II and III only.
Answer and explanation

A: I only

For air treated as an ideal gas, sound speed is √(γRT/M). At fixed composition it depends on temperature. Pressure has no independent effect at fixed temperature, and frequency does not enter this model.

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Question 27

The speed of sound in air at sea-level is 340m s-1 while that of light is 300,000km s-1. How far (to the nearest metre) from the center of thunderstorm is an observer who hears a thunder 2s after a lightning flash?

  1. 170m
  2. 340m
  3. 600m
  4. 680m.
Answer and explanation

D: 680m.

The delay is d/340 − d/(3 × 10⁸) = 2 s. Solving gives d ≈ 680.0008 m, which rounds to 680 m. Neglecting the tiny light travel time gives the same metre-rounded result.

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Question 29

Two uniform strings of equal length and equal tension sound their fundamental notes in frequency ratio 4:1. What is their corresponding mass ratio?

  1. 2:1
  2. 1:2
  3. 1:4
  4. 1:16
Answer and explanation

D: 1:16

For equal length and tension in the same vibration mode, f is proportional to 1/√m. Thus f₁/f₂ = 4 implies m₁/m₂ = (1/4)² = 1/16.

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Question 32

An object is placed 10 cm in front of a concave mirror of focal length 15 cm. What is the position and nature of the image?

  1. 30 cm behind the mirror, virtual
  2. 6 cm in front of the mirror, real
  3. 6 cm behind the mirror, virtual
  4. 30 cm in front of the mirror, real
Answer and explanation

A: 30 cm behind the mirror, virtual

With real object distance positive, 1/v = 1/15 − 1/10 = −1/30. The image is therefore virtual and lies 30 cm behind the mirror. The object is inside the focal length.

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Question 34

When white light is dispersed by a spectrometer, the component having the shortest wavelength is

  1. Orange
  2. Green
  3. Violet
  4. Red.
Answer and explanation

C: Violet

Violet is at the short-wavelength end of the visible spectrum. Green, orange and red all have longer wavelengths than violet.

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Question 35

Which of the following statements is TRUE of ultra- violet, visible and infra-red rays?

  1. They are all electromagnetic waves with the same wavelength
  2. Ultraviolet rays have shorter wavelengths than infrared rays and can produce fluorescence
  3. Infrared rays have shorter wavelengths than visible light and produce a sensation of heat
  4. Wavelengths increase in the order visible, ultraviolet, infrared
Answer and explanation

B: Ultraviolet rays have shorter wavelengths than infrared rays and can produce fluorescence

Ultraviolet has shorter wavelengths than visible light and infrared. It can excite fluorescent materials, which then emit light, often at visible wavelengths.

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Question 40

Connecting a resistance R across a cell reduces its terminal voltage to two-thirds of its open-circuit emf. What is the cell’s internal resistance?

  1. R/3
  2. R/2
  3. 2R/3
  4. R
Answer and explanation

B: R/2

With external resistance R and internal resistance r, terminal voltage is V = ER/(R + r). Setting V/E = 2/3 gives 3R = 2(R + r), hence r = R/2.

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Question 44

At the given voltage and power ratings, which instrument draws the highest current?

  1. Electric iron 250V 1kW
  2. Television set 220V 110W
  3. Torch light 6V 30W
  4. Immersion heater 110V 500W
Answer and explanation

C: Torch light 6V 30W

Using I = P/V: iron = 1000/250 = 4 A; television = 110/220 = 0.5 A; torch = 30/6 = 5 A; heater = 500/110 ≈ 4.55 A. The torch draws the highest current.

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Question 45

Which instruments can directly measure ordinary alternating current without an added rectifier? I. Permanent-magnet moving-coil ammeter. II. Moving-iron ammeter. III. Hot-wire ammeter.

  1. I and II only
  2. II and III only
  3. I and III only
  4. I, II and III.
Answer and explanation

B: II and III only

Moving-iron meters respond to magnetization irrespective of current direction. Hot-wire meters respond to I²R heating. Both can measure AC. An unrectified permanent-magnet moving-coil meter averages opposing torques over an AC cycle.

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Question 46

The principle of operation of an induction coil is based on

  1. Ohm’s law
  2. Ampere’s law
  3. Faraday’s law
  4. Coulomb’s law.
Answer and explanation

C: Faraday’s law

An induction coil uses a changing current to change magnetic flux through another winding. Faraday’s law relates that changing flux to the induced emf.

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Question 49

Which notation represents a neutral atom X with 22 electrons and 43 neutrons?

  1. ⁴³₂₂X
  2. ²²₄₃X
  3. ⁶⁵₄₂X
  4. ⁶⁵₂₂X
Answer and explanation

D: ⁶⁵₂₂X

For a neutral atom, 22 electrons mean 22 protons, so atomic number Z = 22. Mass number A = protons + neutrons = 22 + 43 = 65. The notation is ⁶⁵₂₂X.

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