Physics Basics: Motion, Light & Electricity: questions and answers
Study every question with its correct answer and explanation.
1. What is the SI unit of electric current?
- A. Volt
- B. Ohm
- C. Ampere
- D. Watt
Answer: Ampere
Electric current is measured in amperes (A), one of the seven SI base units. The volt measures potential difference, and the ohm measures resistance.
2. Newton's first law of motion is also known as the law of what?
- A. Inertia
- B. Gravitation
- C. Acceleration
- D. Friction
Answer: Inertia
The first law says an object stays at rest, or keeps moving at a constant velocity, unless an external force acts on it — this property is called inertia. The more mass an object has, the greater its inertia, which is why a loaded truck is harder to stop than a bicycle.
3. Which of these is a vector quantity?
- A. Speed
- B. Mass
- C. Temperature
- D. Velocity
Answer: Velocity
A vector has both magnitude and direction, and velocity is speed in a stated direction. Speed, mass and temperature have magnitude only, so they are scalar quantities; force, displacement and acceleration are other common vectors.
4. Sound cannot travel through which of these?
- A. Water
- B. Steel
- C. Air
- D. A vacuum
Answer: A vacuum
Sound is a mechanical wave that needs particles to vibrate, so it cannot cross empty space — which is why space is silent. Sound generally travels fastest in solids, slower in liquids and slowest in gases; in air at 20 °C it moves at about 343 m/s.
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5. A 12 V battery is connected across a 4 Ω resistor. According to Ohm's law, what current flows?
- A. 48 A
- B. 3 A
- C. 0.33 A
- D. 16 A
Answer: 3 A
Ohm's law states V = I × R, so I = V ÷ R = 12 ÷ 4 = 3 amperes. Resistance is measured in ohms (Ω), named after the German physicist Georg Simon Ohm.
6. Why does a straw standing in a glass of water look bent at the water's surface?
- A. Reflection of light
- B. Refraction of light
- C. Diffraction of light
- D. Dispersion of light
Answer: Refraction of light
Light changes speed as it passes from water into air, so it bends — this is refraction, and it makes the submerged part look shifted. The same effect makes a pond or swimming pool look shallower than it really is.
7. What is the approximate range of frequencies that a healthy human ear can hear?
- A. 20 Hz to 20,000 Hz
- B. 2 Hz to 200 Hz
- C. 200 Hz to 2,000 Hz
- D. 20,000 Hz to 200,000 Hz
Answer: 20 Hz to 20,000 Hz
Humans typically hear sounds between about 20 Hz and 20 kHz, and the upper limit falls with age. Sound below 20 Hz is infrasound (elephants use it to communicate), while sound above 20 kHz is ultrasound, used by bats and in medical scans.
8. Which type of mirror is used as the rear-view (side) mirror in cars and motorbikes?
- A. Concave mirror
- B. Plane mirror
- C. Convex mirror
- D. Cylindrical mirror
Answer: Convex mirror
A convex mirror always forms a small, upright image and gives a much wider field of view, so drivers can see more of the road behind them. Concave mirrors, which can form enlarged images, are used as shaving mirrors and as reflectors in torches and headlights.
9. On an electricity bill, '1 unit' of electrical energy equals what?
- A. 1 kilowatt
- B. 1 kilowatt-hour
- C. 1 joule
- D. 1 volt-ampere
Answer: 1 kilowatt-hour
One unit is 1 kilowatt-hour (kWh): the energy used by a 1,000-watt appliance running for one hour. Since 1 watt is 1 joule per second, 1 kWh equals 3.6 × 10⁶ joules.
10. What is the approximate acceleration due to gravity (g) at the Earth's surface?
- A. 1.6 m/s²
- B. 6.7 m/s²
- C. 98 m/s²
- D. 9.8 m/s²
Answer: 9.8 m/s²
Near the Earth's surface, a freely falling object speeds up by about 9.8 m/s every second, ignoring air resistance. On the Moon, g is only about 1.6 m/s² — roughly one-sixth of Earth's — so you would weigh about one-sixth as much there.