JAMB Physics 1986

25 reviewed questions with answers and explanations.

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Question 4

Which of the following is a derived unit?

  1. Kilogramme
  2. Metre
  3. Kelvin
  4. Newton
Answer and explanation

D: Newton

The newton is defined as kg·m·s⁻², combining base units to measure force. Kilogram, metre and kelvin are SI base units.

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Question 5

Two objects, one with three times the mass of the other, are dropped together from the same height. Neglect air resistance. While falling above the ground, they have the same

  1. Momentum
  2. Kinetic energy
  3. Potential energy
  4. Acceleration.
Answer and explanation

D: Acceleration.

Without air resistance, both objects accelerate at g and have the same velocity at each instant. Momentum, kinetic energy and gravitational potential energy also depend on mass, so these differ by a factor of three.

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Question 9

A uniform metre rule QR is horizontal, with Q at 0 cm and R at 100 cm. It balances on a knife edge 55 cm from R. A 10 g mass hangs at P, 10 cm from Q. What is the mass of the metre rule?

  1. 550 g
  2. 350 g
  3. 70 g
  4. 35 g
Answer and explanation

C: 70 g

The pivot is at 100 − 55 = 45 cm from Q. The 10 g mass is 35 cm left of the pivot, while the uniform rule’s centre is 5 cm right of it. Balancing moments gives 10 × 35 = M × 5, so M = 70 g.

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Question 10

Three forces F₁, F₂ and F₃ act at O in equilibrium. F₂ = 5.0 N points horizontally right. F₁ = 10.0 N points at 60° above F₂. What is the magnitude of F₃, to one decimal place?

  1. 26.4 N
  2. 15.0 N
  3. 13.2 N
  4. 10.0 N
Answer and explanation

C: 13.2 N

F₃ must oppose the resultant of F₁ and F₂. The resultant’s squared magnitude is 10² + 5² + 2 × 10 × 5 cos 60° = 175 N². Hence |F₃| = √175 ≈ 13.2 N; its direction is opposite the resultant.

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Question 11

When a box of mass 400g is given an initial speed of 5m s-1, it slides along a horizontal floor a distance of 3m before coming to rest. What is the coefficient of the kinetic friction between the box and the floor? (g = 10m s-2)

  1. 5/6
  2. 5/12
  3. 1/3
  4. 2/3
Answer and explanation

B: 5/12

Work done against friction equals the initial kinetic energy: μmg × 3 = ½m × 5². Cancelling mass gives μ = 25/(2 × 10 × 3) = 5/12.

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Question 15

A 1000 kg elevator descends with a downward acceleration of 1.0 m/s². If g = 10.0 m/s², what is the tension in its suspending cable?

  1. 1.0 N
  2. 10.0 N
  3. 9000.0 N
  4. 11000.0 N
Answer and explanation

C: 9000.0 N

Taking downward as positive, mg − T = ma. Therefore T = m(g − a) = 1000(10 − 1) = 9000 N. The downward acceleration makes tension less than weight.

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Question 17

Which mode of heat transfer does not require a material medium?

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

B: Radiation

Thermal radiation travels as electromagnetic waves and can cross a vacuum. Conduction requires interacting matter, and convection requires moving fluid.

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Question 18

Thermal expansion of solids can be a disadvantage in the

  1. Balance wheel of a watch
  2. Fitting of wheels on rims
  3. Fire alarm
  4. Thermostat.
Answer and explanation

A: Balance wheel of a watch

Thermal expansion changes the dimensions and moment of inertia of a watch’s balance wheel, affecting its oscillation period and timekeeping. Thermal compensation is needed to reduce this error.

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Question 20

The specific latent heat of fusion of lead is the amount of heat required to

  1. Melt lead at its melting point
  2. Heat unit mass of lead through 1 °C
  3. Change unit mass of lead from solid to liquid at its melting point
  4. Change the state of unit mass of lead at its boiling point
Answer and explanation

C: Change unit mass of lead from solid to liquid at its melting point

Specific latent heat of fusion is the energy required to melt unit mass at the melting point without changing temperature. Both the unit mass and the solid-to-liquid change are essential.

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Question 21

Which of the following is common to evaporation and boiling? They

  1. Take place at any temperature.
  2. Are surface phenomena.
  3. Involve change of state
  4. Take place at a definite pressure.
Answer and explanation

C: Involve change of state

Both processes change liquid into gas. Evaporation occurs at the surface, while boiling forms vapour throughout the liquid when its vapour pressure reaches the surrounding pressure.

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Question 22

Mercury is suitable as a barometric fluid because it

  1. Expands uniformly
  2. Is opaque
  3. Is several times denser than water
  4. Is a good conductor of heat
Answer and explanation

C: Is several times denser than water

For a barometer, h = p/(ρg). Mercury’s high density means a much shorter column can balance atmospheric pressure than a water column, making the instrument practical.

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Question 23

Which of the following properties makes metals ideal for cooking utensils?

  1. High coefficient of expansion
  2. Good conduction of heat
  3. Low specific heat capacity
  4. Poor radiation of heat.
Answer and explanation

B: Good conduction of heat

Good thermal conduction lets a metal utensil transfer heat efficiently from the heat source to its contents. That is the relevant property for cooking.

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Question 25

When two objects P and Q are supplied with the same quantity of heat, the temperature change in P is observed to be twice that of Q. The mass of P is half that of Q the ratio of the specific heat capacity of P to Q is

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

C: 1 : 1

Equal heat gives mPcPΔTP = mQcQΔTQ. Since mP = mQ/2 and ΔTP = 2ΔTQ, the factors cancel: cP = cQ. The ratio is 1:1.

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Question 27

A ray travelling in air strikes a glass surface at an incidence angle of 60°, measured from the normal. The reflected and refracted rays are perpendicular. Taking the refractive index of air as 1, what is the refractive index of the glass, to two decimal places?

  1. 1.73
  2. 1.50
  3. 0.87
  4. 0.57
Answer and explanation

A: 1.73

The reflected angle equals the 60° incidence angle. Perpendicular reflected and refracted rays imply i + r =90°, so r =30°. Snell’s law gives n = sin 60°/sin 30° = √3 ≈1.73.

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Question 29

In which of the following arrangements is the wavelength in an increasing order?

  1. Gamma rays, infra-red rays, X-rays, radiowaves.
  2. Gamma rays, X-rays, infra-red rays, radiowaves.
  3. Radiowaves, X-rays, gamma rays, infra-red rays.
  4. Infra-red rays, radiowaves, X-rays, gamma rays.
Answer and explanation

B: Gamma rays, X-rays, infra-red rays, radiowaves.

In the electromagnetic spectrum, gamma rays have shorter wavelengths than X-rays, followed here by infrared and then radio waves. This gives gamma → X-ray → infrared → radio.

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Question 30

For light travelling from glass of refractive index 1.5 into air, what is the critical angle?

  1. sin⁻¹(1/2)
  2. sin⁻¹(2/3)
  3. sin⁻¹(3/4)
  4. sin⁻¹(8/9)
Answer and explanation

B: sin⁻¹(2/3)

At the critical angle, the refracted ray in air is at 90°. Snell’s law gives 1.5 sin c = 1, so c = sin⁻¹(2/3). The incident ray must be travelling from glass toward air.

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Question 32

In the geometrical ray model of a pinhole camera, what happens to image sharpness when the hole is enlarged?

  1. Gives a blurred image.
  2. Corrects for chromatic aberration
  3. Magnifies the image
  4. Brings the image into the sharper focus.
Answer and explanation

A: Gives a blurred image.

In the ray model, enlarging the pinhole lets each object point illuminate a wider patch on the screen. These patches overlap more, so the image becomes brighter but more blurred.

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Question 36

Which of the following is true of the loudness of sound? It

  1. Depends on the square of the amplitude of the vibrating body.
  2. Is proportional to the distance of the observer from the source of the sound.
  3. Is greatest in vacuum
  4. Is independent of frequency.
Answer and explanation

A: Depends on the square of the amplitude of the vibrating body.

For otherwise unchanged conditions, sound intensity varies with amplitude squared. Greater intensity is perceived as greater loudness. Loudness also depends on frequency and the listener, so it is not identical to physical intensity.

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Question 38

The vibration of an air column produces the sound in the

  1. Piano
  2. Guitar
  3. Flute
  4. School handbell.
Answer and explanation

C: Flute

A flute sets the air column inside its tube into vibration. A piano and guitar primarily use vibrating strings, while a handbell uses the vibration of the bell itself.

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Question 39

A 9.0 V accumulator with negligible internal resistance drives a 5 Ω resistor in series with a parallel pair of 5 Ω and 20 Ω resistors. What is the total current supplied by the accumulator?

  1. 0.3 A
  2. 0.8 A
  3. 1.0 A
  4. 1.8 A
Answer and explanation

C: 1.0 A

The parallel resistance is(5 ×20)/(5 +20) =4 Ω. Adding the series 5 Ω gives 9 Ω. Since the accumulator’s internal resistance is negligible, its current is I =9.0/9 =1.0 A.

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Question 41

Which is NOT a part of a simple brushed DC electric motor?

  1. Field magnet
  2. Armature
  3. Commutator
  4. Transformer
Answer and explanation

D: Transformer

A simple brushed DC motor has a field magnet, an armature and a commutator. A transformer is a separate device for transferring AC electrical energy between circuits.

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Question 42

Which of the following is stored by a dry Leclanche cell?

  1. Electrical power
  2. Kinetic energy
  3. Electric current
  4. Chemical energy.
Answer and explanation

D: Chemical energy.

A Leclanché cell stores chemical energy in its reactants. During discharge, chemical reactions provide electrical energy to the external circuit.

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Question 44

Which of the following is true of magnetism?

  1. Iron filings cling mainly around the ends of a bar magnet
  2. A freely suspended bar magnet always comes to rest in the geographic north–south direction
  3. Like poles attract
  4. Lodestone is a non-magnetic oxide
Answer and explanation

A: Iron filings cling mainly around the ends of a bar magnet

The magnetic field is strongest near the poles of a bar magnet, so iron filings gather most densely near its ends. Like poles repel, and lodestone is magnetic.

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Question 45

The angle between the magnetic meridian and the geographic meridian at a locality on the earth is the

  1. Longitude of the locality.
  2. Angle of inclination at the locality
  3. Latitude of the locality
  4. Angle of declination of the locality.
Answer and explanation

D: Angle of declination of the locality.

Magnetic declination is the horizontal angle between magnetic north and true geographic north. Inclination instead measures the magnetic field’s tilt relative to the horizontal.

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Question 49

Radiation has wavelength 400 nm. Taking the speed of light in air as 3 × 10⁸ m/s, what is its frequency?

  1. 1.3 × 10⁻¹⁵ Hz
  2. 7.5 × 10⁵ Hz
  3. 1.2 × 10¹¹ Hz
  4. 7.5 × 10¹⁴ Hz
Answer and explanation

D: 7.5 × 10¹⁴ Hz

Convert 400 nm to 400 × 10⁻⁹ m. Frequency f = c/λ = (3 × 10⁸)/(400 × 10⁻⁹) = 7.5 × 10¹⁴ Hz.

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