WAEC Physics
Past Questions

AI Explanation

**The reasoning** This is pure **Newton's Second Law of Motion**: F = ma (Force = mass × acceleration). You have: - Force (F) = 20 N - Mass (m) = 4 kg - Acceleration (a) = ? Rearrange the formula: a = F/m So: a = 20/4 = **5 m/s²** That's it! When a force pushes an object, the acceleration depends on how heavy it is. Bigger mass = harder to accelerate. **Why the wrong options tempt you** - **A (0.2)**: You divided wrong — 4/20 instead of 20/4. Always put force on top! - **C (16)**: You subtracted (20 − 4). Newton's law uses division, not subtraction. - **D (80)**: You multiplied (20 × 4). That gives you the force if you already knew acceleration — backwards thinking! **Quick takeaway** To find acceleration, always divide force by mass: **a = F/m**. The heavier the object, the slower it accelerates under the same force — just like pushing a bicycle versus pushing a car!

AI Explanation

## The reasoning Sound travels through air by creating compression waves—air molecules bump into each other passing the vibration along. At room temperature (around 20°C, typical for Nigeria), sound moves at approximately **330 m/s** (or more precisely, 343 m/s). This is a fundamental constant you should memorize: sound in air ≈ 330-340 m/s. To get a feel for it: if you see lightning and hear thunder 3 seconds later, the storm is roughly 1 km away (330 m/s × 3 s ≈ 1000 m). ## Why the wrong options tempt you **A) 30 m/s** — This is just a car's speed (about 108 km/h). Too slow! Sound would take forever to travel. **C) 3,000 m/s** — You might confuse this with sound in denser materials like steel, where it travels much faster. **D) 300,000 m/s** — This is the speed of *light* (actually 3×10⁸ m/s), not sound. That's why you see lightning before hearing thunder! ## Quick takeaway **Sound in air ≈ 330 m/s; light is a million times faster at 300,000 km/s—never confuse the two!**

AI Explanation

**The reasoning** Think about energy conversion — what goes in, what comes out: - A **motor** takes electrical energy (from a power source) and converts it into mechanical energy (motion/rotation). That's exactly what the question asks for. When you plug in a fan, blender, or electric car, the motor inside transforms electricity into movement. The principle here is **energy conversion** — devices are defined by their input → output transformation. **Why the wrong options tempt you** - **Generator** does the *opposite* — it converts mechanical energy (like spinning turbines) into electrical energy. It's the motor's reverse twin, so students confuse them. - **Transformer** changes voltage levels but keeps energy as electrical throughout (high voltage ↔ low voltage). No mechanical conversion. - **Battery** stores chemical energy and releases it as electrical energy. It's a storage device, not a converter to mechanical form. **Quick takeaway** **Motor = Electrical → Mechanical** (makes things move). **Generator = Mechanical → Electrical** (makes electricity from motion). They're opposites — remember one, you know both!

AI Explanation

**The reasoning** This is pure **Newton's Second Law**: F = ma (Force = mass × acceleration). You have: - Force (F) = 20 N - Mass (m) = 4 kg - Acceleration (a) = ? Rearrange the formula: a = F/m Substitute: a = 20/4 = **5 m/s²** That's it! When force pushes mass, acceleration is simply force divided by mass. --- **Why the wrong options tempt you** **A) 0.2 m/s²** — You divided mass by force (4/20) instead of force by mass. Classic reversal mistake! **C) 16 m/s²** — You subtracted: 20 − 4 = 16. Newton's law isn't subtraction; it's division. **D) 80 m/s²** — You multiplied: 20 × 4 = 80. This gives you force if you had acceleration, not the other way round. --- **Quick takeaway** **"To find acceleration, always divide the force by the mass—never multiply or subtract."** Remember: heavier objects (bigger mass) accelerate *less* for the same force, so division makes sense!

AI Explanation

**The reasoning** Sound travels through air as vibrations of air molecules bumping into each other. At room temperature (around 20°C – typical Nigerian weather), sound moves at approximately **330 m/s** (or more precisely, about 343 m/s). This is the standard value you need to memorize for physics exams. Think of it this way: sound takes roughly 3 seconds to travel 1 kilometer. When you see lightning and count seconds before hearing thunder, every 3 seconds ≈ 1 km away. This gives you **speed = distance/time = 1000m/3s ≈ 330 m/s**. **Why the wrong options tempt you** - **30 m/s** – This is about the speed of a fast car (108 km/h). If sound were this slow, conversations would have annoying delays! - **3,000 m/s** – You might confuse this with sound in solids (like steel), which is much faster than in air. - **300,000 m/s** – This is the speed of *light* (3 × 10⁸ m/s simplified). Common mix-up, but light is nearly a million times faster than sound! **Quick takeaway** Sound in air ≈ **330 m/s** – roughly the length of 3 football fields per second; always way slower than light but faster than any vehicle you'll ride.

AI Explanation

**The reasoning** Think of it like this: **Electrical energy in → Mechanical energy out = Motor.** A motor takes electricity from a power source and uses it to create rotational motion (mechanical work). This is what makes your fan blades spin, your blender rotate, and electric cars move. The principle is **electromagnetic induction** — when current flows through coils in a magnetic field, it produces a force that causes rotation. The question asks what *converts* electrical → mechanical, and that's exactly what a motor does. **Why the wrong options tempt you** - **Generator**: Does the *opposite* — converts mechanical → electrical (like in NEPA stations where turbines spin to produce electricity) - **Transformer**: Only changes voltage levels; electrical → electrical (steps up or down AC voltage) - **Battery**: Stores chemical energy and releases it as electrical energy when needed; it doesn't create motion itself **Quick takeaway** **"Motor = electricity makes it move; Generator = movement makes electricity."** Remember: motors are in devices that *spin or move* when you plug them in!

AI Explanation

## The reasoning **Weight is a force, not the same as mass!** This is the key principle here. The formula connecting them is: **Weight (W) = mass (m) × gravity (g)** Given: - Mass = 1 kg - g on Earth = 10 m/s² So: W = 1 kg × 10 m/s² = **10 N** Notice the units: kg × m/s² = N (Newton). Weight is always measured in Newtons because it's a force! ## Why the wrong options tempt you **A) 1 N** — You probably just copied the mass value, forgetting to multiply by gravity. Easy mistake when rushing! **C) 100 N** — You might have multiplied 10 × 10, perhaps confusing this with a different calculation. **D) 1000 N** — Way off! Maybe you thought "1 kg = 1000 g" and mixed up mass units with force. ## Quick takeaway **"Mass tells you HOW MUCH stuff; weight tells you HOW HARD gravity pulls it."** Always multiply mass by g (10 m/s² on Earth) to get weight in Newtons. One sentence to remember: *1 kg weighs 10 N on Earth.*

AI Explanation

**The reasoning** Resistivity (ρ) measures how strongly a material opposes current flow, *independent of its shape or size*. It's a property of the material itself. Start with Ohm's Law: R = V/I, so resistance R is measured in ohms (Ω). But resistance depends on the object's dimensions: **R = ρL/A** Where: - R = resistance (Ω) - L = length (m) - A = cross-sectional area (m²) Rearranging: **ρ = RA/L** Substituting units: ρ = (Ω × m²)/m = **Ω·m** That's why resistivity has units of ohm-meters! **Why the wrong options tempt you** **A) Ω** — This is just resistance, not resistivity. The trap: confusing the two related concepts. **C) V/m** — This looks scientific (it's actually electric field strength), so students might guess it relates to "voltage in materials." **D) A/m** — This is magnetic field intensity. The "per meter" makes it seem dimensional, but it's the wrong quantity entirely. **Quick takeaway** Resistivity isn't just resistance — it's resistance *adjusted for size*: multiply Ω by area, divide by length, and you get **Ω·m**.

AI Explanation

## The Reasoning Think of watching waves at Bar Beach, Lagos. **Period (T)** is literally how much *time* it takes for one complete wave to pass you – from crest to crest, or from any point back to that same point again. It's measured in **seconds**. The formula connects everything: **v = fλ** (wave speed = frequency × wavelength), and since frequency f = 1/T, the period is central to wave motion. If a wave completes 5 cycles in 5 seconds, T = 1 second per cycle. ## Why the Wrong Options Tempt You **A) Distance between crests** – This is *wavelength* (λ), not period. Distance vs. time – don't mix them! **C) Max displacement** – This is *amplitude* (A), how high the wave rises. Completely different property. **D) Speed** – That's wave velocity (v), how fast the wave travels forward. These all describe waves, but period specifically answers "How long?" not "How far?" or "How much?" ## Quick Takeaway **Period = Time per cycle** – whenever you see "period," think stopwatch ⏱️, not ruler 📏.

AI Explanation

**The reasoning** Pressure measures how concentrated a force is over a surface. Think of it this way: if you press your palm against a wall versus pressing a needle with the same force — the needle creates way more pressure because that force is concentrated on a tiny area. The formula is: **Pressure = Force / Area** The unit confirms this: Pressure is measured in **Pascals (Pa)**, where 1 Pa = 1 Newton per square meter (N/m²). See? Force (Newtons) divided by Area (square meters). **Why the wrong options tempt you** - **Mass (A)**: Easy mix-up because Force = Mass × Acceleration. But pressure isn't about the object's mass — it's about how that force spreads. - **Volume (B)**: Volume matters in *density* (mass/volume), not pressure. Different formula entirely. - **Time (D)**: Time relates to concepts like impulse or power, but pressure is instantaneous — it doesn't care how long the force acts. **Quick takeaway** Remember: **Pressure is force per unit area** — the smaller the area, the greater the pressure, which is why a sharp knife cuts better than a blunt one!

AI Explanation

**The reasoning** This question is testing your knowledge of **electrostatic charging methods**. When you rub two materials together (like a plastic comb through dry hair, or a balloon on a sweater), electrons are transferred from one material to the other through direct contact and rubbing. This process is literally called **charging by friction** (or triboelectric charging). The friction creates intimate contact between surfaces, allowing electrons to move from the material that holds them loosely to the one that attracts them more strongly. One object becomes negatively charged (gains electrons), the other becomes positively charged (loses electrons). **Why the wrong options tempt you** - **A) Conduction** involves touching a charged object to a neutral one to share charges — no rubbing needed - **B) Induction** charges an object *without* touching it at all, using only the influence of a nearby charged object - **D) Earthing** removes excess charge by connecting to the ground — it's not a charging method but a discharging one **Quick takeaway** When you see "friction" and "rubbing" together in charging, the answer is literally **friction** — it's the most direct method where contact + movement = charge transfer!

AI Explanation

## The reasoning Sound is a **mechanical wave** — it travels by making particles vibrate and bump into their neighbors. The speed depends on how tightly packed and connected those particles are. In **steel**, atoms are locked in a rigid lattice, incredibly close together. When one vibrates, it instantly nudges its neighbor. That's *fast* transmission — about **5,000 m/s**. In **water**, molecules are close but can slide around. Sound moves at roughly **1,500 m/s** — faster than air but slower than solids. In **air**, molecules are far apart and bounce randomly. Sound crawls at about **340 m/s**. In a **vacuum**, there are *no particles at all*. Sound cannot travel. Speed = 0 m/s. **Principle:** Sound speed increases with the stiffness and density arrangement of the medium. Solids > Liquids > Gases. ## Why the wrong options tempt you **A (Air)** — We hear sound in air daily, so it *feels* natural. But familiarity ≠ fastest. **B (Water)** — You might remember sound travels in oceans, but liquids are still looser than solids. **D (Vacuum)** — Sounds futuristic, but space is silent! ## Quick takeaway **"The tighter the particles grip each other, the faster sound flies — solids win every time."**