WAEC 2018 Physics Practical Questions and Answers
Read below For 2018 WAEC Physics Practical Questions and Answers
Question 1
In the diagram illustrated above, a body of mass m slides on an inclined plane. Show that the coefficient of friction between the surfaces in contrast is tan 0
Observations: This was a popular question among the candidates. The physics and substitution that proved n = tan 6 was well handled by many candidates. However, most candidates did not indicate that the body have uniform speed. The expected answer is
Assumin F = Fp or uniform speed of body
µ = F/R
Mg Sinθ
MgCosθ
= tanθ
Question 2
A spiral spring with a metal extends by 10.5cm in air When the metal Is fully in
water , the spring extends bv 6.8 cm. Calculate the relative density of the metal (Assume Hooke s law Is obeyed)
Observation: This question was poorly attempted and the performance was low. Many candidates tilled to use upthrust which Is (10.5 - 6.8) rather majority of them used 6 8 directly for the division Some candidates erroneously stated that
Relative density = Extension in air
Extension loss in water
The expected answer is:
R.D 38 = Weight of body in air
Upthrust (in water)
NOTE: F = kx
= K(10.5)
K( 10.5-6.8)
105
3.1
= 2.84
Question 3
A ray of light is incident on an air-glass boundary at an angle t? If the angle between the partial reflected ray and the refracted ray is 90°, calculate θ given that the refractive index of glass is 1.50
Observation: This question is unpopular as many candidates avoided it. Those who attempted it failed. The problem was with the trigonometric function of linking the angle of refraction to sin (90- θ) = Cos θ
The expected answer is:
n = Sin θ
Sin r
= Sin θ
Sin (90- θ)
= Sin θ
Cos θ
= tan θ
Θ= (1.5) = 56.31°
Question 4
(a) Explain the term electrodes in electric cells
(b) An electric current passing through an electrolyte for 2minutes deposited200u of a substance If the electrochemical equivalent of the substance is 8.33x10'4gC', calculate the current passed
Observation: The explanation was well attempted but the calculation was poorly rendered by few responding candidates due to poor handling of the exponential figure. Some candidates substituted wrongly. They are supposed to convert the time given from minutes to the S.I unit which is seconds before substituting into the formula. Performance was on the average. The expected answers are:
An electrode is a conductor through which electrons flow into or out of the
Electrolyte (of a cell) Any other correct additional information e.g. electrodes can be positive or negative or metals
(b) m = zlt or 1 = m
zt
= 200
8.33 x 10_4x2x60
=2000.8A
Question 5
Explain why sound waves cannot be plane polarized
This question was popular among the candidates and was well attempted. However, the knowledge that only transverse wave can be plane polarized which is required to solve this problem eluded few candidates who attempted the question.
Observation: The expected answer is:
Polarization is a characteristic of only transverse waves. Since sound waves are longitudinal they cannot be plane polarized
Question 6
Define surface tension
State two methods by which the surface tension of a liquid can be reduced
Observation: This question on surface tension was very popular among the candidates although the part (a) of it was poorly attempted but part (b) was well attempted. Few candidates lost marks in part (b) for not using the correct action word (verb). Some candidates erroneously wrote ‘soap’ 'use soap’ ‘use detergent’ etc. This was vague.
The correct action word is ‘by adding* or by addition’
The expected answers are:
Is the force per unit length acting in the surface at right angles to one side of a line
drawn in the surface of a liquid OR The property of a liquid that makes it behave as if its surface is an elastic skin
Two methods of reducing surface tension e.g by adding -
Detergent
Soap
Alcohol
Oil
Increasing the temperature
Question 7
Explain why it is desirable to install an air conditioner near the ceiling of a room and not close to the floor.
Observation: There was lack of knowledge on the demand of the question. Only few candidates saw it from the air circulation by convection point of view. Performance was poor. The expected answer is
Air molecules near the ceiling get cooled, become denser and sink. Warmer air molecules below being lighter rise to the top, get cooled, become denser and sink again; thus conventional current i set up
Question 8
On what principle does lighting in a fluorescent tube operate?
State two factors which determine the colour of light produced in'a fluorescent tube.
Observation: This is popular among the candidates part(b) posed no problem to most responding however, in pan (a) most of candidates lost the mark for not including very to quality km pressure and high voltage. The expected answers are:
Electrical discharge through a gas at very low pressure and maintained m high voltage
Two factors which determine the colour of light in a fluorescent tube
Nature of gas in the tube
Nature of powder coating inside the tube
Voltage across the tube.
Question 9
This diagram illustrates an arrangement of a cathode ray from an electron gun and a bar magnet placed perpendicularly to the direction of the ray. will the ray bend downward or upward? Explain.
Observation: This is another unpopular question. Majority of the candidates that attempted this question could not get the caned concept(application of flemmings left hand rule) needed to tackle the question. However few candidates got it right as upward but failed in the explanation
The expected answers are:
Upward
The direction of motion of the charge is due to the force acting on it which is mutually perpendicular to the direction of the magnetic field and the moving charge.
Question 10
Explain wave-particle duality of light.
Illustrate your answer in 10(a) with observable phenomena.
Observation: Few candidates attempted this question and it was well answered. However, some got part (b) wrong. They were describing light and matter rather than stating the observable phenomena.
The expected answers are:
Light exists in two forms: wave and particle
(i) wave nature - reflection, refraction, diffraction, polarization and interference
(ii) particle nature - emission and absorption of light
-- photo-electricity
-- momentum
-- radiation of energy from hot bodies.
Question 11
State the principle of conservation of linear momentum.
Explain the mode of action of a propelled rocket
During a training session, two footballers pass a ball repeatedly between themselves. Give two reasons why the to and fro motion of the ball is not simple harmonic.
A ball is dropped from a height, at the same time as another ball is projected horizontally from the same height.
would the balls hit the ground at the same time?
Explain your answer in(i).
A ball of mass 0.10kg is projected horizontally onto a vertical wall with a speed of 17ms-1. The ball makes contect with the wall for 0.15s and rebounds horizontally with a speed of 13ms-1. Calculate the: change in momentum of the ball;
Average force exerted on the ball during its collision with the wall.
Observation: Part (a) Most candidates in their definition forgot to include the key word such as closed system, total momentum or absence of external forces. This made them to lose marks. Performance was average. Part (b) Most candidates knew that rocket propulsion was due to Newton’s third law of motion but they could not give details. Part (c) many candidate* did m* have anything to write due to poor comprehension of the demand of the question. Performance was poor.
Part (d) most candidates knew the balls would hit the ground at the same time but could not explain why due to poor understanding of the concept of free falling objects
Part (e) This was very popular among the candidates and majority of them -calculated correctly. However, few candidates got it wrong due to incorrect substitution as they failed to note that momentum is a vector quantity hence reversal in direction connote a negative sign.
The expected answers are as follow:
Principle of conservation of linear.
State that in a system of colliding objects the total momentum is always conserved provided that there is no net external force acting on the system
OR
If two or more bodies collide in a closed system, the total momentum after the collision is equal to the total momentum before the collision
Principle of action of a propelled rocket
Gases burnt within the combustion chambers of the rocket engine are expelled downwards through the tail nozzle (exhaust) at very high speed. An equal and opposite momentum is given to the rocket which then moves upward.
c. The acceleration of the ball is not directed towards a fixed point.
- These is no equilibrium position
- Acceleration of the ball is not directly proportional
d. Ball projection
(i) Yes or Both balls hit the ground at the same time
(ii) All objects are pulled towards the Earth by gravity and accelerate at a constant rate. This means that all free fall mg objects fails at the same rate.
e. Momentum Change P
P - m(v - u)
= 0.10 (13 +17)
= 3kgms-1
Average force on the ball
F= m(v-u)
t
= Δp
Δt
= 3
0.15
= 20N
Question 12
(a) (i) What is a machine?
(ii) State two uses of gears
(iii) Define the velocity ratio for a pair of gear system
The diagram above illustrates the gears system of a bicycle
Determine its velocity ratio
If the bicycle has an efficiency of 90%, calculate the effort required to overcome a load of 70 N.
Why is the calculated effort less than the actual effort required?
Observation: This question was very popular among the candidates.
Part (a)(i) majority of the responding candidates got the definition correct. Few candidates however, stated the additional information that ought's to be added to the actual as their main definition of machine.
Part a(ii) uses of gears was correctly stated by few candidates. Few candidates wrote where gears are used (e.g. in bicycles, motor cars, pulley systems e.t.c) and not the demand of the question.
Part (a)(iii) many candidates wrote
V. R = Number of teeth in the driving wheel Number of teeth in the driven wheel which was wrong.
Part (a(iv) was poorly attempted as most candidates failed to observe the constructional input in a gear that can achieve the desired goal.
Part (b)(i) to (iii). The problem most candidates had was on how to obtain the velocity ratio. Many of them did not realize that they need to (oc) identify and ((S) physically count the number of teeth on tho driven and diivmg wheels as illustrated in the diagram in the question, Those who succeeded in deriving the velocity ratio had no difficulty in solving the remaining parts of the questions.
Performance was average. The expected answers are:
A machine is a device in which effort applied at one end is used to overcome a greater load/resistance at another point.
It makes work easier
Uses of gears
e.g - to change direction of motion
To change speed
To magnify force
(iii)
Velocity ratio
Velocity ratio is defined as ratio of number of teeth on driven wheel to number of teeth on driving wheel.
OR
Velocity ratio= Time rate of revolution/speed/radius of driven wheel Time rate of revolution/speed/radius of driving wheel
(iv) To increase the mechanical advantage (M.A), the driving gear is made smaller and has fewer teeth than the gear being driven
(b) (i) From the diagram
Number of driving teeth = 18
Number of driven teeth = 12
V.R =
= 
=
Efficiency =
x 100%
x 100%
=
/2/3
E= 116.7N