All 20 questions from the Post-UTME Screening (Post-UTME) Physics 2015 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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The laws of vibration of stretched strings can be verified experimentally using an instrument called
A sonometer is the standard laboratory instrument used to verify the laws of vibrating stretched strings.
Refraction of light in a medium is a result of change in the
Refraction occurs because the speed (velocity) of light changes as it passes from one medium to another of different optical density.
A note has a frequency of 128Hz. What is the frequency of a note two octaves higher?
Each octave doubles the frequency. Two octaves higher = 128 x 2 x 2 = 512Hz.
The length of a pinhole camera is 30cm. An object 2cm high is placed 15cm from the pinhole. Calculate the height of the image produced.
Image height = object height x (image distance/object distance) = 2 x (30/15) = 2 x 2 = 4.00cm.
The concept of no-parallax is often used in light experiments to
The no-parallax method is used to precisely locate the position of an image (real or virtual) by eliminating relative (parallax) movement between a search pin and the image.
Two turning forks have frequencies of 150 Hz and 200 Hz. Calculate the difference in the wavelength of the notes they emit. [speed of sound in air = 330 m/s]
Wavelength1 = 330/150 = 2.2m. Wavelength2 = 330/200 = 1.65m. Difference = 2.2-1.65 = 0.55m.
Which of the following correctly gives the second harmonic of a closed pipe? (Note: a closed pipe supports only odd-numbered harmonics of its fundamental frequency f - i.e. f, 3f, 5f, ... A true 'second harmonic', 2f, does not occur in a closed pipe.)
The options for this question were cut off in the source scan, so no lettered answer can be given. However, the key physics concept is: a closed (one end closed) pipe only resonates at odd harmonics of its fundamental (f, 3f, 5f, ...); it has no true even harmonic such as 2f. Please match this principle to the actual options on your paper.
The force between the molecules of the same substance is
The force of attraction between molecules of the same substance is called cohesive force.
A quantity of liquid has a volume of 80cm3 at 15C. If its cubic expansivity is 10-3K-1, calculate its volume at 45C.
Change in volume = V x gamma x deltaT = 80 x 10^-3 x 30 = 2.4cm3. New volume = 80+2.4 = 82cm3.
20g of water at 2C is added to 35g of water at 65C. Calculate the final steady temperature of the mixture.
Heat balance: 20(Tf-2)=35(65-Tf). Solving gives Tf approximately 42.1C.
The instrument used for measuring the specific gravity of a liquid is
A hydrometer measures the specific gravity (relative density) of a liquid.
Convectional current is a consequence of the fact that
Convection currents arise because a fluid's density changes with temperature, causing warmer (less dense) fluid to rise and cooler (denser) fluid to sink.
A Pyrex glass can be used to boil water because it
Pyrex glass has a low coefficient of thermal expansion, making it resistant to cracking under rapid temperature changes.
The pressure of 760cm3 of gas initially at 120cmHg is changed at constant temperature to 76cm Hg. What is its new volume?
By Boyle's law, P1V1=P2V2: 120x760=76xV2, giving V2=1200cm3.
The dimensions of a rectangular glass block are 4cm by 10cm by 16cm. Calculate the ratio of the maximum pressure to the minimum pressure which the block can exert when placed on a horizontal surface.
Max pressure occurs with the smallest face down (4x10=40cm2); min pressure with the largest face down (10x16=160cm2). Ratio = 160/40 = 4:1.
When a liquid in a container is evaporating, the temperature of the bulk liquid
Evaporation removes the most energetic molecules, lowering the average kinetic energy (and thus temperature) of the remaining liquid.
The upper and lower fixed points on a certain thermometer are marked 155 and -5 respectively. Which mark on the thermometer corresponds to 80C?
Reading = lower + (80/100) x (upper - lower) = -5 + 0.8x160 = -5+128 = 123.0.
Sounds produced by bats are of
Bats produce and detect very high (ultrasonic) frequency sounds for echolocation.
An object is placed 20 cm in front of a plane mirror. If the mirror is moved 5 cm towards the object, what is the distance between the image and the object?
New object distance = 20-5=15cm, so image distance (behind mirror) is also 15cm. Total distance between image and object = 15+15 = 30cm.
If the diagram of the motion of a wave is at right angles to the direction of particle vibration, the wave is
A wave whose direction of travel is at right angles to the direction of particle vibration is, by definition, a transverse wave.
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