WAEC Physics 2017
Questions & Answers

1) Distractor check: Options B, C, or D might be selected if you confuse electrical charge with electrical potential, resistance, or power units. 2) Reasoning to the answer: The international standard unit for electric charge is the coulomb (C), which is formally defined as the amount of electricity carried by a current of 1 ampere flowing for 1 second. 3) Common mistake: Confusing the coulomb with volts, ohms, or watts.

Distractor check: A student might choose option A by forgetting to simplify the exponents, or select option D by missing the time component entirely. Reasoning to the answer: Impulse is calculated as the product of force and time. Substituting the dimensional formula for force, which is MLT⁻², and multiplying it by the dimension of time, T, yields (MLT⁻²) × T, which simplifies to MLT⁻¹. Common mistake: Confusing the dimension of force with the dimension of impulse by omitting the time factor.

Distractor check: A student could mistakenly select option A or B by miscalculating the square root or improperly applying the kinematic formula. Reasoning to the answer: To find the final velocity, apply the kinematic equation v² = u² + 2as. Given that the initial velocity u is 0, the acceleration a is 2 ms⁻², and the distance s is 9 m, substitute these values into the equation to get v² = 0 + 2(2)(9) = 36. Taking the square root of both sides gives a velocity of 6.0 ms⁻¹. Common mistake: Forgetting to take the square root of v² at the final step.

Distractor check: Students might mistakenly consider option B or D as incorrect, overlooking the fact that solid molecules only vibrate about fixed positions rather than moving freely. Reasoning to the answer: Molecules of solids are tightly bound and vibrate in place, lacking the freedom of movement found in liquids and gases. In contrast, liquids and gases feature greater molecular freedom, energy is indeed required to break intermolecular forces, and liquid molecules are in constant random motion, making option A the false statement. Common mistake: Assuming that solid molecules have more translational freedom than liquid molecules.

Distractor check: Option A is a common distractor because energy is measured in Joules, leading students to confuse energy with the rate of energy transfer. Reasoning to the answer: Power is defined as the rate at which energy is transferred or work is done, expressed mathematically as Joules per second. This fundamental rate corresponds to the SI unit watt. Common mistake: Confusing the unit of energy (Joule) with the unit of power (Watt).

Distractor check: Option A might be chosen if a student subtracts the forces instead of adding them, or option B if they miscalculate the division by mass. Reasoning to the answer: Assuming both forces act in the same direction, the net force is the sum of 100 N and 60 N, which equals 160 N. Using Newton's second law, acceleration is calculated by dividing the net force by the mass, giving a = 160 / 10 = 16.0 ms⁻². Common mistake: Subtracting the two forces instead of combining them when finding the net force.

Distractor check: A student might pick option D by simply using the weight in air (14 N) or misinterpreting the mass units. Reasoning to the answer: According to Archimedes' principle, the upthrust on a partially immersed body equals the weight of the water it displaces. Converting the mass of the displaced water from 200 g to 0.2 kg and multiplying by the acceleration due to gravity (10 ms⁻²) yields an upthrust of 0.2 kg × 10 ms⁻² = 2.0 N. Common mistake: Failing to convert grams to kilograms before calculating the weight of the displaced liquid.

Distractor check: A student might select evaporation or condensation, assuming all phase changes relate to surface phenomena identically. Reasoning to the answer: Condensation, evaporation, and thermionic emission all heavily rely on surface properties or take place at boundaries, whereas photo emission involves light ejecting electrons from metal surfaces and is not exclusively a surface tension-related phenomenon. Common mistake: Assuming all electron emission or phase processes are strictly surface phenomena.

Distractor check: Students might select option A or D, confusing a chemical balance with equipment used for volumetric or density measurements. Reasoning to the answer: A chemical balance is a precise laboratory instrument specifically designed for measuring mass for analytical purposes. Common mistake: Confusing mass-measuring laboratory instruments with density or volume apparatus.

Distractor check: A student might choose option A by confusing the tangential velocity vector with the radial acceleration vector. Reasoning to the answer: When a body moves in a circular path at constant speed, its velocity vector is always directed tangentially along the path, while the centripetal acceleration acts radially towards the center, making them perpendicular to each other. Common mistake: Mixing up the direction of tangential velocity with centripetal acceleration.

Distractor check: Option B is a strong distractor resulting from forgetting to convert the speed from km/h to m/s. Reasoning to the answer: First, convert the speed from 72 km h⁻¹ to meters per second by multiplying by 5/18, yielding 20 m/s. Multiplying this speed by the time interval of 0.4 s gives the distance covered: 20 × 0.4 = 8.0 m. Common mistake: Multiplying the speed in km/h directly by time without converting to m/s.

Distractor check: A student might guess option A or C by confusing the angle for maximum range with angles used in other trigonometric identities. Reasoning to the answer: The range of a projectile on level ground is maximized when the sine function reaches its peak value of 1. Since the range formula involves sin(2θ), 2θ must equal 90°, making the launch angle θ equal to 45°. Common mistake: Confusing the angle for maximum height (90°) with the angle for maximum range (45°).

Distractor check: Students might select rotational or translational motion, forgetting they are standard physics classifications. Reasoning to the answer: Rotational, translational, and relative motion are all recognized and standard classifications of motion in physics. Random, however, is not classified as a standard fundamental type of motion in this context. Common mistake: Mistaking descriptive terms of movement for formal mechanical classifications.

Distractor check: A student might pick option A by miscalculating the time of fall or misapplying the free-fall equation. Reasoning to the answer: The time it takes to hit the ground is found using the vertical height formula h = (1/2)gt². Substituting h = 20 m and g = 10 m/s² gives 20 = 5t², which simplifies to t² = 4, or t = 2 s. Multiplying this time by the horizontal speed of 10 m/s yields a landing distance of 10 × 2 = 20 m. Common mistake: Forgetting to square root the time equation when solving for t.

Distractor check: Option C is a common trap if a student forgets to convert centimeters to meters before squaring the displacement. Reasoning to the answer: Elastic potential energy is calculated using the formula (1/2)kx². Converting the extension from 4 cm to 0.04 m, substitute k = 2000 Nm⁻¹ and x = 0.04 m into the formula to get 0.5 × 2000 × (0.04)² = 1.60 J. Common mistake: Substituting 4 directly into the formula without converting centimeters to meters.

Distractor check: A student might choose option A by dividing resistance by current instead of multiplying them. Reasoning to the answer: According to Ohm's law, potential difference V is equal to the product of current I and resistance R. Multiplying 2 A by 20 Ω yields a voltage drop of 40 V. Common mistake: Dividing the resistance by the current instead of using the product.

Distractor check: Students might pick option A or B by confusing velocity ratio with mechanical advantage trigonometric functions. Reasoning to the answer: For an inclined plane, the velocity ratio is defined mathematically as the inverse of the sine of the angle of inclination, represented as 1 / sin θ. Common mistake: Confusing sine with tangent when determining the velocity ratio of an inclined plane.

Distractor check: A student might select option C or D by misinterpreting how temperature changes affect surrounding liquid properties or atmospheric pressure. Reasoning to the answer: As bubbles rise through the water, the hydrostatic pressure exerted on them decreases significantly. According to Boyle's law, a decrease in pressure results in an increase in volume, causing the bubbles to expand. Common mistake: Attributing bubble expansion solely to water temperature increases rather than the reduction in pressure.

Distractor check: Option A is a distractor because a bulb converts electrical energy into light energy, reversing the desired process. Reasoning to the answer: Solar cells, also known as photovoltaic cells, directly harness the photoelectric effect to convert incoming light energy into usable electrical energy. Common mistake: Confusing devices that consume electrical energy to produce light with those that generate electricity from light.

Distractor check: Option A represents the wasted energy fraction (25%) rather than the operating efficiency. Reasoning to the answer: Since one-quarter (25%) of the energy is wasted, three-quarters (75%) of the total energy must be useful. Calculating the efficiency as the ratio of useful energy to total energy multiplied by 100% yields (3/4) × 100% = 75%. Common mistake: Reporting the wasted energy percentage instead of calculating the useful efficiency.