All 20 questions from the Post-UTME Screening (Post-UTME) Physics 2016 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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When a rocket at a high altitude passes overhead, only a little sound is heard from it because
As sound travels up through the atmosphere from a high-altitude rocket, much of its energy is absorbed/scattered by the atmosphere before reaching an observer on the ground.
What causes musical notes produced from open pipes to be sharper than those from closed pipes is the
Open pipes produce both odd and even harmonics (overtones), including a fundamental twice the frequency of an equivalent closed pipe, giving a sharper (higher pitch) sound; closed pipes produce only odd harmonics.
The optical device labelled X in the diagram (light rays converging through F to C) is a
The labels F (focal point) and C (centre of curvature) both being used together, with rays converging after reflecting, indicate a converging (concave) mirror rather than a lens.
A sinusoidal wave travelling along a string is described by the equation y = 30 sin[15*pi(4t-2x)]. What is the wavelength of this wave if t is in seconds and x is in metres?
Expanding: y = 30 sin(60*pi*t - 30*pi*x). Comparing to y=A sin(wt-kx), k = 30*pi. Wavelength = 2*pi/k = 2*pi/(30*pi) = 1/15 m = 0.067m = 6.7cm.
A ray of light travels up through a bucket of water of refractive index 1.33. If the angle of incidence is 50 degrees at the surface, the ray experiences
Critical angle = arcsin(1/1.33) = 48.8 degrees. Since the angle of incidence (50 degrees) exceeds the critical angle, total internal reflection occurs.
Calculate the frequency of the first overtone of a closed pipe of length 68.0cm if the velocity of sound in air is 340.0 m/s.
A closed pipe supports only odd harmonics. The fundamental f1 = v/4L = 340/(4x0.68) = 125Hz. The first overtone is the next allowed harmonic, the 3rd harmonic: f3 = 3 x 125 = 375Hz.
In wind instruments, sound is produced as a result of the vibration of
Wind instruments produce sound through the vibration of air columns inside the instrument.
Calculate the focal length of a converging lens which forms a real image of magnification 4 when the object distance is 15cm.
For a real image, v = m x u = 4 x 15 = 60cm. Using 1/f = 1/v + 1/u = 1/60 + 1/15 = 1/60 + 4/60 = 5/60 = 1/12, so f = 12cm.
Diagram: light rays pass through an aperture past an opaque globe (with measurements 6cm, 4cm, 2cm) towards a screen at distance x. From the diagram, what is the distance x if there is no shadow of the globe on the screen?
Using similar triangles based on the geometry of the light source, aperture, and globe (so that the penumbra regions from each edge of the source exactly meet with no umbra reaching the screen), the distance works out to approximately 3.0cm. (Note: precise figure dimensions were only partially legible in the scan.)
A point source in space produces wave fronts whose shapes are
A point source radiates energy equally in all directions in 3D space, producing spherical wavefronts.
A bullet is fired from a point P with an initial velocity of 50 ms-1 at an angle of 45 degrees to the horizontal. At what distance from the point P will the bullet hit the ground assuming that the ground is horizontal throughout the trajectory. (Take g = 10ms-2)
Range = v^2.sin(2theta)/g = 50^2 x sin(90)/10 = 2500/10 = 250m.
Calculate the force on an electron situated in a uniform electric field of intensity 120 000Vm-1
F = qE = 1.6x10^-19 x 120000 = 1.92x10^-14N.
A current of 5A flowing in a circuit falls to zero in 0.1 sec. If the average e.m.f of 200V is induced, what is the self-inductance of the circuit?
emf = L(dI/dt). L = emf/(dI/dt) = 200/(5/0.1) = 200/50 = 4H.
Use the diagram above (an AC circuit with 120V, 60/pi Hz supply, a resistor R and capacitor C = 30uF) to answer this question. In the circuit above, calculate the reactance of the capacitor.
Xc = 1/(2.pi.f.C) = 1/(2 x 60 x 30x10^-6) = 277.78 ohm (using f=60/pi so that 2.pi.f=120).
A step-down transformer reduces the voltage from 2400V to 120V. If the primary winding has 4000 turns, determine the number of turns in the secondary winding.
Ns = Np x Vs/Vp = 4000 x 120/2400 = 200 turns.
I. Travel in straight lines II. Have kinetic energy III. produce heat on collision with metals IV. Can be deflected by a magnetic field. From the above, which of the combinations is a characteristic of cathode rays?
Cathode rays travel in straight lines, possess kinetic energy, produce heat on collision, and can be deflected by magnetic fields - all four properties apply.
A coil having an inductance of 0.14H and a resistance of 12 ohm is connected across a 110V, 25Hz line. Determine the current in the coil.
XL=2.pi.f.L=2x3.14x25x0.14=22 ohm. Z=sqrt(R^2+XL^2)=sqrt(144+484)=25.06 ohm. I=V/Z=110/25.06=4.4A.
What is the time required to electroplate 3.00g of silver onto a brass casting by using a steady current of 15.0A? [e.c.e. of silver = 0.001118gC-1]
t = mass/(e.c.e. x I) = 3.00/(0.001118x15) = 178.89s.
The electron diffraction experiment illustrates
Electron diffraction demonstrates that particles (electrons) exhibit wave-like properties.
An induced e.m.f. is obtained between the ends of a horizontal steel axle of a moving train because the
The vertical component of the earth's magnetic field, combined with the horizontal motion of the conducting axle, induces an emf across it.
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