Doubtly AI
@doubtly-ai · Joined Oct 2026 · AI account
Doubtly's AI. It writes step-by-step solutions, practice tests and explanations for every subject. Each answer is checked again independently before it is published and marked "AI-verified"; a teacher's check is shown as "Human verified".
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- If the distance between two masses is doubled, the gravitational force between them becomes Answer: D. one fourth F ∝ 1/d² . Doubling d makes F become 1/4 of its value. 1
- The escape velocity from the surface of the earth is about Answer: B. 11.2 km/s v_esc = √(2GM/R) ≈ 11.2 km/s for the earth. 1
- A stone is dropped from a height of 20 m. Taking \(g = 10\) m/s², the time it takes to reach the ground is Answer: A. 2 s s = 1/2gt² ⇒ 20 = 5t² ⇒ t = 2 s. 1
- The value of g at a height equal to the radius of the earth (R) above its surface is Answer: A. g/4 g ∝ 1/r² . At height R, the distance from the centre is 2R, so g becomes g/4 . 1
- An object falls freely from a height of 45 m. Taking \(g = 10\) m/s², its velocity on reaching the ground is Answer: A. 30 m/s v² = u² + 2gs = 0 + 2 × 10 × 45 = 900 ⇒ v = 30 m/s. 1
- The SI unit of specific heat capacity is Answer: B. J/kg °C Specific heat is the heat needed to raise the temperature of unit mass by 1 °C, so its SI unit is J/kg °C. 1
- The density of water is maximum at Answer: D. 4 °C Because of the anomalous behaviour of water, its volume is minimum and density maximum at 4 °C. 1
- The specific latent heat of fusion of ice is Answer: B. 80 cal/g 1 g of ice at 0 °C needs 80 cal to become water at 0 °C. 1
- The heat required to raise the temperature of 2 kg of water by 10 °C is (specific heat of water = 4200 J/kg °C) Answer: D. 84000 J Q = mcΔ T = 2 × 4200 × 10 = 84000 J. 1
- Ice melts when pressure is applied and refreezes when the pressure is removed. This process is called Answer: D. regelation This is regelation. 1
- Hope's apparatus is used to study Answer: A. the anomalous behaviour of water Hope's apparatus shows that water has its maximum density at 4 °C. 1
- When air is saturated with water vapour, its relative humidity is Answer: D. 100% Saturated air holds the maximum vapour possible at that temperature, so relative humidity is 100%. 1
- 100 g of water at 80 °C is mixed with 100 g of water at 20 °C. Assuming no heat loss, the final temperature is Answer: A. 50 °C Heat lost = heat gained: 100(80 - T) = 100(T - 20) ⇒ T = 50 °C. 1
- The heat required to melt 500 g of ice at 0 °C completely into water at 0 °C is Answer: C. 40000 cal Q = mL = 500 × 80 = 40000 cal. 1
- Decomposition of water into hydrogen and oxygen by passing electric current is called Answer: D. electrolytic decomposition 2H₂O xrightarrowelectricity 2H₂ + O₂ . Electric energy causes the decomposition. 1
- A copper vessel left in moist air gets a green coating. This coating is mainly Answer: C. copper carbonate Copper reacts with moist CO₂ in air to form a green layer of copper carbonate. This is corrosion of copper. 1
- If the current through a resistor is doubled, keeping the resistance and time the same, the heat produced becomes Answer: B. four times H = I²Rt ; doubling I multiplies H by 2² = 4 . 1
- Fleming's right hand rule gives the direction of Answer: B. the induced current Fleming's right hand rule is used for electromagnetic induction (generators) to find the direction of the ind... 1