All 40 questions from the Joint Admissions and Matriculation Board (JAMB) Physics 2020 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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1. A pendulum which has a mass of 200g is pulled sideways by a horizontal distance of 30cm. If the length of the pendulum is 50cm, calculate the potential energy of the bob in this position (g=10m/s2)
h = L - sqrt(L^2-x^2) = 0.5 - sqrt(0.25-0.09) = 0.5-0.4 = 0.1m. PE = mgh = 0.2x10x0.1 = 0.2J.
2. An object of mass 300kg and a density 600kg/m3 was suspended with a string so that half of it is immersed in paraffin of density 900kg/m3. What is the tension in the string? (g=9.8m/s2)
Volume = 300/600 = 0.5m3. Volume immersed = 0.25m3. Upthrust = 900x0.25x9.8 = 2205N. Weight = 300x9.8 = 2940N. Tension = Weight - Upthrust = 2940-2205 = 735N.
3. A car accelerates uniformly from rest at 4m/s2. How far will it travel in the fifth second?
Distance in nth second = u + a(2n-1)/2 = 0 + 4(9)/2 = 18m.
4. The quantity of an object which is a measure of its tendency to continue in motion with a steady velocity is its
Inertia (the tendency of a body to continue in motion at steady velocity, or remain at rest) is known as its physical property, and this depends on mass.
5. Which of the following combinations will increase the stability of an object?
The wider the base and the lower the position of centre of gravity of a body, the more stable it is (and vice-versa).
6. The physical quantity that has the same dimension as impulse is
Impulse = force x time = mass x acceleration x time = mass x change in velocity = momentum. Both have dimension MLT^-1.
7. A disc rotates on a player at constant speed of 200 revolutions per minute. Determine the period of rotation.
Speed = 200 revolutions per minute = 200/60Hz = 3.3333Hz. T = 1/f = 1/3.3333 = 0.3s.
8. A lorry travels 10km northwards, 5km eastwards, 4km southwards and then finally 5km westwards. What is the total displacement?
Net north-south: 10km N - 4km S = 6km N. Net east-west: 5km E - 5km W = 0. Total displacement = 6km due north.
9. A rocket is held fixed so that it ejects material horizontally with a speed of 150m/s. If it ejects a mass of 1.2kg/s, calculate the force needed to hold it fixed.
Force = mass flow rate x velocity of exhaust = 1.2 x 150 = 180N.
10. A hygrometer is an instrument for measuring the
A hygrometer is used for measuring the relative humidity of the air, while a hydrometer is used for measuring the relative density of a liquid by the method of flotation.
11. A uniform metre rule of weight 1.0N is pivoted at the 40cm mark. A weight of 2N is hung from the 15cm mark. Where must a weight of 2N be placed to balance the rule?
Taking moments about the pivot (40cm mark): Clockwise moment = Anticlockwise moment. 1x(50-40) + 2x(x-40) = 2x(40-15). 10+2x-80=50. 2x=120. x=60cm mark.
12. A wheel and axle is used to raise a load of 500N by the use of an effort of 250N. Determine the efficiency of the machine if the radii of the wheel and axle are respectively 0.4cm and 0.1cm
Velocity ratio = radius of wheel/radius of axle = 0.4/0.1 = 4. Mechanical advantage = load/effort = 500/250 = 2. Efficiency = (MA/VR) x 100% = (2/4)x100% = 50%.
13. For what celsius temperature is the reading on the celsius scale equal to that on the fahrenheit scale?
F=9/5C+32. Setting F=C=x: x=9/5x+32 → -4/5x=32 → x=-40. So -40°C corresponds to -40°F.
14. Which of these waves cannot be polarized?
Polarization is an exclusive property of transverse waves. All longitudinal waves (including sound waves) cannot be polarized.
15. A concave mirror of radius of curvature 20cm forms a real image four times as large as the object. The object distance is
f=r/2=10cm. Since magnification M=v/u=4, v=4u. Using mirror formula 1/f=1/v+1/u: 1/10=1/4u+1/u=5/4u. 4u=50. u=12.5cm.
16. What is the resistance that must be placed in parallel with a 25Ω to make a combined resistance of 15Ω
1/15 = 1/25 + 1/x. 1/x = 1/15-1/25 = (5-3)/75 = 2/75. x = 37.5Ω.
17. The distance between the object and the pinhole of a pinhole camera is reduced by a factor of 4. The size of the imaged
Magnification (image size/object size) = image distance/object distance. Since object distance is reduced by a factor of 4, the magnification (and hence image size, for fixed screen/image distance) increases fourfold — the image is quadrupled.
18. Which of the following combinations will not produce white light after mixing?
Red+cyan, blue+yellow, and green+magenta are complementary colour pairs that produce white light when mixed, as does red+green+blue. Blue & magenta is not a standard complementary pair that produces white.
19. If a current of 4A is passed through a thin cable for two hours, how much electrical energy will be converted to heat energy, given that the electrical resistance of the cable is 10 ohms?
H = I^2Rt = 4^2 x 10 x (2x3600)s = 16x10x7200 = 1,152,000J ≈ 1.15MJ.
20. Which of the following statements is true about Coulomb's law?
Coulomb's law states that the force of attraction or repulsion between two charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them — it is an inverse square law.
21. An object initially at a distance of 10cm from a plane mirror moves at a speed of 10cm/s away from the mirror. What would be the distance between the object and the image after 5s?
Additional distance moved in 5s = 10x5 = 50cm. New object distance = 10+50 = 60cm. Image distance (plane mirror) = object distance = 60cm. Distance between object and image = 60+60 = 120cm = 1.2m.
22. Two mirrors in a barber's shop are on the walls opposite to each other. A man seated in between the mirrors will see how many images of himself?
For two mirrors placed exactly opposite (parallel) to each other, the angle between them is 0°, producing an infinite number of images.
23. Which of the following pairs contains one vector and one scalar quantity?
Force is a vector while kinetic energy is a scalar. Displacement and acceleration are both vectors; momentum and velocity are both vectors; power and speed are both scalars.
24. The process of adding impurity to a semi-conductor to increase its conductivity is called
The process of adding impurity to a semiconductor to increase its conductivity is called doping. The resulting semiconductor is known as an extrinsic (impure) semiconductor.
25. A vibrating string has a tension of 40N and produces a note of 200Hz when plucked in the middle. If the length of the string is unaltered and the tension is increased to 160N, the frequency becomes
Frequency is proportional to the square root of tension (length unaltered). f2 = f1 x sqrt(T2/T1) = 200 x sqrt(160/40) = 200 x 2 = 400Hz.
26. The equation of a travelling wave is given by y=2sin(4πx-25πt), the frequency of the wave is
Comparing with y=Asin(kx-ωt): ω=25π=2πf, so f=12.5Hz.
27. The process of adding impurity to a semi-conductor to increase its conductivity is called (repeat reference)
Same as Q24 — this is doping, producing an extrinsic semiconductor.
28. A stone which is dropped into a dry well hits the bottom in 2.2s. How deep is the well?
s = ut + (1/2)at^2 = 0 + 0.5x10x2.2^2 = 0.5x10x4.84 = 24.2m.
29. A boy holds the end of a rubber cord from which hangs a solid metal ball. If the boy whirls the ball in a horizontal circle, keeping his hand still, in what direction will the ball move when the cord breaks?
When the cord breaks, the centripetal force is removed, so the ball moves in a straight line in the direction of the tangent to the circle at the point of break (Newton's first law).
30. 50g of water at 20°C is added to 200g of water at 80°C. If properly stirred, determine the temperature of the mixture.
Heat lost by hot water = heat gained by cold water: 200(80-x) = 50(x-20). 16000-200x=50x-1000. 17000=250x. x=68°C.
31. When a ray of light passes from water to glass, it is
When light passes from a less dense medium (water) to a denser medium (glass), it bends towards the normal.
32. The following frequencies are overtones of 140Hz except
Overtones of 140Hz are whole-number multiples: 280, 420, 560, 700, 840Hz etc. 760Hz is not a whole-number multiple of 140Hz (760/140 = 5 remainder 60), so it is not an overtone of 140Hz.
33. The quality of sound depends on
The pitch of a note depends on its frequency, the loudness depends on the amplitude, and the quality/timbre depends on the overtones/harmonics present.
34. 250g of lead at 170°C is dropped into 100g of water at 0°C. If the final temperature is 12°C, the specific heat capacity of lead is (S.H.C of water = 4200J/kg°C)
Heat lost by lead = heat gained by water: 0.25 x C_lead x (170-12) = 0.1 x 4200 x (12-0). 0.25 x C_lead x 158 = 5040. 39.5 x C_lead = 5040. C_lead = 127.6J/kg°C.
35. A magnetic needle is suspended first at the earth's north magnetic pole and then at the point on the earth's magnetic equator. The respective angles between the needle and the horizontal are
The angle of dip is 90° at the magnetic poles and 0° at the magnetic equator.
36. The linear expansivity of brass is 2x10^-5°C^-1. If the volume of a piece of brass is 10cm3 at 0°C, what will be its volume at 100°C?
Cubic expansivity ≈ 3 x linear expansivity = 6x10^-5°C^-1. V2 = V1(1+3α∆θ) = 10(1+6x10^-5x100) = 10(1.006) = 10.06cm3.
37. The resistance of a wire depends on
Resistance R=ρl/A, so it depends on the resistivity (ρ), length (l), and area/diameter (A) of the wire.
38. A given mass of an ideal gas occupies a volume V at a temperature T and under a pressure P. If the pressure is increased to 2P and the temperature reduced to (1/2)T, the percentage change in the volume of the gas is
Using PV/T=constant: (2P)(V2)/(T/2) = PV/T → V2 = V/4. The volume decreases from V to V/4, a decrease of 3V/4, i.e. a 75% decrease.
39. When a sound wave passes from air into water, its
When sound passes from air to water, only the speed and wavelength change (both increase); the frequency remains the same since it is determined by the source.
40. A wave of frequency 10Hz forms a stationary wave pattern in a medium where the velocity is 0.2m/s. Calculate the distance between adjacent nodes.
λ = v/f = 0.2/10 = 0.02m = 2cm. Distance between adjacent nodes = λ/2 = 1.0cm.
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