CBSE Class 7 Science Advanced Physics Our Universe MCQs Set 03

Answer: (a) DCBEA

Explanation:
1. The fuel burns in the combustion chamber and produces gases: (D).
2. The hot gases escape through a nozzle: (C).
3. They rush out with great force, which is the action: (B).
4. The gases push the rocket with an equal and opposite force, the reaction: (E).
5. So the rocket moves rapidly in the opposite direction: (A).
6. The order is D, C, B, E, A: option (a).

Teacher's Note:
1. This follows Newton's third law: action and reaction.
2. Fuel burns first, motion comes last.

Answer: (d) \(\frac{g}{6}\)

Explanation:
1. The moon is smaller and lighter than the earth.
2. Its surface gravity is one sixth of the earth's: gₘ = g/6.
3. The answer is g/6.

Teacher's Note:
1. Remember: moon g is about 1.6 m s⁻², earth g is about 9.8 m s⁻².
2. The ratio is about 1/6.

Answer: (c) Its velocity increases and acceleration remains constant.

Explanation:
1. A freely falling body is pulled by gravity with a constant acceleration g.
2. So its velocity increases steadily (v = gt) while the acceleration stays constant.
3. The answer is (c).

Teacher's Note:
1. Constant acceleration means velocity changes by equal amounts each second.
2. Air resistance is ignored.

Answer: (d) None of these

Explanation:
1. G is a universal constant with the same value everywhere (6.67 × 10⁻¹¹ N m² kg⁻²).
2. It does not depend on the masses or on the distance.
3. The answer is none of these.

Teacher's Note:
1. Force depends on masses and distance; the constant G does not.
2. G is not the same as g.

Answer: (c) \(\frac{F}{8}\)

Explanation:
1. On the earth F = G M m ÷ R².
2. On the planet F′ = G (M/2) m ÷ (2R)² = G M m ÷ (2 × 4R²) = F/8.
3. The answer is F/8.

Teacher's Note:
1. Half the mass halves the force; double the radius divides it by 4.
2. Combine: 1/2 × 1/4 = 1/8.

Answer: (b) 40

Explanation:
1. For a freely falling body, h = ½ g t², so h is proportional to t².
2. Time doubles from 1 s to 2 s, so the height becomes 2² = 4 times: 4 × 10 = 40 m.
3. The answer is 40 m.

Teacher's Note:
1. Distance fallen depends on the square of the time.
2. Same planet, same g.

Answer: (a) Directly proportional to the time.

Explanation:
1. Starting from rest, v = g t.
2. So the velocity is directly proportional to time: option (a).

Teacher's Note:
1. Velocity ~ t, distance ~ t².
2. Constant g means the velocity grows steadily.

Answer: (c) 2.5

Explanation:
1. Mass of y = 5 kg, so mass of x = 10 kg. The distance x–y is d and the distance y–z is d/2.
2. F(xy) = G × 10 × 5 ÷ d² = 50G/d².
3. F(yz) = G × 5 × m ÷ (d/2)² = 20 G m/d².
4. Equating: 20m = 50, so m = 2.5 kg: option (c).

Teacher's Note:
1. Write the two forces with Newton's law of gravitation and equate them.
2. A shorter distance gives a larger force, so z can be lighter.

Answer: (b) A → d; B → b; C → c; D → a

Explanation:
1. A barometer measures atmospheric pressure: A → d.
2. An anemometer measures wind speed: B → b.
3. As the book states, the spring tide on a full moon day is less strong: C → c.
4. The spring tide on a new moon day is the strong one: D → a.
5. This is option (b).

Teacher's Note:
1. Barometer: pressure; anemometer: wind.
2. The book treats the new moon tide as the stronger spring tide.

Answer: (d) CABD

Explanation:
1. Water vapour condenses into rain and releases heat, warming the air: (C).
2. The warm air rises and the pressure drops: (A).
3. Cool air rushes in towards the centre: (B).
4. A very low-pressure system forms with fast winds spinning around it: (D).
5. The order is C, A, B, D: option (d).

Teacher's Note:
1. Heat release by condensation starts the process.
2. Low pressure at the centre draws in air.

Answer: (d) Both (a) and (b)

Explanation:
1. Warming makes water expand and lowers its density.
2. Fresh water from rivers and rain dilutes salt water and lowers its density.
3. Both causes are correct: (d).

Teacher's Note:
1. Density depends on temperature and salinity.
2. Salty water is denser than fresh water.

Answer: (a) The Moon and the Sun on the Earth

Explanation:
1. Tides are the rise and fall of sea level.
2. They are caused mainly by the gravitational pull of the moon, and to a smaller extent the sun, on the earth's water.
3. The answer is (a).

Teacher's Note:
1. The moon is close, so its pull is strong on the oceans.
2. The other planets are too far to matter.

Answer: (a) F

Explanation:
1. Gravitational force acts in equal and opposite pairs (Newton's third law).
2. So the 3m body attracts the m body with the same force F.
3. The answer is F.

Teacher's Note:
1. Each body pulls the other equally.
2. The force formula is symmetric in the two masses.

Answer: (c) Air resistance is more on the feather and is not negligible as compared to its weight.

Explanation:
1. Acceleration due to gravity is the same for all bodies.
2. The feather is light, so air resistance is large compared with its weight and slows it down.
3. The answer is (c).

Teacher's Note:
1. In a vacuum they fall together.
2. Air resistance matters most for light, spread-out objects.

Answer: (b) Directly proportional to the square of time.

Explanation:
1. For a body starting from rest, s = ½ g t².
2. So the distance is proportional to the square of the time: option (b).

Teacher's Note:
1. Double the time, four times the distance.
2. Velocity is proportional to t; distance to t².

Answer: (d) 3 : 2

Explanation:
1. Weight on the earth = 3 × g = 3g.
2. Weight on the moon = 12 × g/6 = 2g.
3. Ratio = 3g : 2g = 3 : 2.
4. The answer is 3 : 2.

Teacher's Note:
1. Use g on the moon = g/6.
2. Mass times g gives weight.

Answer: (c) Altitude

Explanation:
1. A barometer measures air pressure, and air pressure falls as we go higher.
2. So from the pressure reading we can find the altitude of a place.
3. The answer is altitude.

Teacher's Note:
1. Pressure decreases with height.
2. A barometer is not used for temperature or wind.

Answer: (a) \(\frac{200}{\sqrt{2}+1}\)

Explanation:
1. Let the distance from the 5 kg mass be x, so the distance from the 10 kg mass is 200 − x.
2. Equal forces: G·5·2/x² = G·10·2/(200 − x)², so (200 − x)² = 2x².
3. Taking the square root, 200 − x = √2 x, so x = 200/(√2 + 1).
4. The answer is option (a).

Teacher's Note:
1. Cancel G and the 2 kg mass from both sides.
2. The heavier mass is farther away, so x is less than 100 m.

Answer: (d) Is quadrupled

Explanation:
1. Gravitational force varies inversely as the square of the distance.
2. Halving the distance makes the force (1 ÷ ½)² = 4 times as large.
3. The force is quadrupled.

Teacher's Note:
1. Inverse square law: half the distance, four times the force.
2. F = G m₁ m₂ / d².

Answer: (b) ACBED

Explanation:
1. Fuel burns and makes gases: (A).
2. The gases escape through the nozzle: (C).
3. They come out with great force, the action: (B).
4. The reaction force acts on the rocket: (E).
5. The rocket moves in the opposite direction: (D).
6. The order is A, C, B, E, D: option (b).

Teacher's Note:
1. Action first, then reaction, then motion.
2. Same steps as the earlier rocket question in a different order.

Answer: (c) Libra

Explanation:
1. Libra is the constellation shaped like a weighing balance (scales).
2. The answer is Libra.

Teacher's Note:
1. Libra means scales in Latin.
2. Aries is the ram and Leo the lion.

Answer: (c) 11.27 km s⁻¹

Explanation:
1. The escape velocity from the earth is about 11.2 km s⁻¹, which the book gives as 11.27 km s⁻¹.
2. Only option (c) has the right unit, km per second.
3. The answer is (c).

Teacher's Note:
1. Check the unit as well as the number.
2. Escape velocity is in the same units as speed.

Answer: (d) Double

Explanation:
1. g = G M ÷ R². For the planet Mₚ = M/2 and Rₚ = R/2.
2. gₚ ÷ gₑ = (1/2) ÷ (1/2)² = (1/2) × 4 = 2.
3. So g on the planet is double its value on the earth.

Teacher's Note:
1. Halving the radius multiplies g by 4; halving the mass halves it.
2. Combine: 4 × 1/2 = 2.

Answer: (c) 21 March to 19 April

Explanation:
1. Aries is the first sign of the zodiac and is seen from 21 March to 19 April.
2. The answer is 21 March to 19 April.

Teacher's Note:
1. Aries starts the zodiac year around the March equinox.
2. 23 Aug to 22 Sept is Virgo.

Answer: (c) \(\frac{3}{4}\)F

Explanation:
1. Doubling the distance makes the force 1/4 of the original: F/4.
2. Change in force = F − F/4 = 3F/4.
3. The answer is 3F/4.

Teacher's Note:
1. The question asks for the change, not the new force.
2. Inverse square law gives F/4.

Answer: (a) 2.66

Explanation:
1. 10 × (1.625 − gₚ) = 10.15, so 1.625 − gₚ = 1.015 and gₚ = 0.61 m s⁻².
2. Ratio g(moon) ÷ g(Pluto) = 1.625 ÷ 0.61 = 2.66.
3. The value 0.61 is g on Pluto, not the ratio. The answer is 2.66.

Teacher's Note:
1. Find g on Pluto first, then the ratio.
2. The question asks for a ratio, which has no unit.

Answer: (d) F

Explanation:
1. F is proportional to m₁m₂/d².
2. One mass becomes 1/4, which makes the force 1/4.
3. The distance halves, which makes the force 4 times: (1/½)² = 4.
4. New force = F × 1/4 × 4 = F: option (d).

Teacher's Note:
1. Treat each change separately and multiply.
2. 1/4 × 4 = 1.

Answer: (b) A → r; B → p; C → q

Explanation:
1. Neap tides occur at the first and last quarter phases of the moon: A → r.
2. The escape velocity on the earth is 11.27 km s⁻¹: B → p.
3. Spring tides occur on full moon and new moon days: C → q.
4. This is option (b).

Teacher's Note:
1. At spring tides the sun, moon and earth are in a line.
2. At neap tides the sun and moon pull at right angles.

Answer: (a) A pot

Explanation:
1. Aquarius is the water bearer and is shown as a pot (water pot).
2. The answer is a pot.

Teacher's Note:
1. Virgo is the virgin and Gemini the twins.
2. Aquarius means the water carrier.

Answer: (c) Escape velocity

Explanation:
1. The least speed needed to leave a planet's gravity is its escape velocity.
2. The answer is escape velocity.

Teacher's Note:
1. Orbital velocity keeps a satellite in orbit.
2. Escape velocity of the earth is 11.27 km s⁻¹.

Answer: (c) 2 : 1

Explanation:
1. g = GM/R². The radius is the same, so g is proportional to the mass.
2. The planet has half the mass, so gₚ = gₑ/2.
3. gₑ : gₚ = 2 : 1: option (c).

Teacher's Note:
1. Same radius, so only the mass matters.
2. The earth's g is double that of the planet.

Answer: (a) Capricorn

Explanation:
1. Capricorn is seen from 22 December to 19 January.
2. The answer is Capricorn.

Teacher's Note:
1. Aquarius follows in January and February.
2. Leo is in summer.

Answer: (b) \(\frac{2}{3}\)d

Explanation:
1. F is proportional to 1/d². For 9F, d² must become 1/9, so the new distance is d/3.
2. Decrease in distance = d − d/3 = 2d/3.
3. The answer is 2d/3.

Teacher's Note:
1. Ask for the decrease, not the new distance.
2. Nine times the force means one third the distance.

Answer: (d) \(\frac{4}{3}\) kg

Explanation:
1. g on the planet = 10 ÷ 4 = 2.5 m s⁻².
2. Difference in weights = m(10 − 2.5) = 7.5 m = 10 N.
3. m = 10 ÷ 7.5 = 4/3 kg: option (d).

Teacher's Note:
1. Weight = mass × g.
2. Subtract the two g values first.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: weight on the moon = 1/6 of the weight on the earth = 6 ÷ 6 = 1 kg weight.
2. R is true: g on the moon is one sixth of g on the earth.
3. The smaller g is the reason for the smaller weight: (a).

Teacher's Note:
1. Weight = mass × g.
2. Mass stays the same; only weight changes.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: the moon has no atmosphere.
2. R is true: the moon's escape velocity is small, and gas particles move faster than it, so they escape into space.
3. R is the reason for A: (a).

Teacher's Note:
1. Low gravity cannot hold gases.
2. Escape velocity on the moon is about 2.4 km s⁻¹.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. A is true: these float on water.
2. R is true: their densities are less than the density of water.
3. A body floats in a liquid when its density is less than that of the liquid, so R explains A: (a).

Teacher's Note:
1. Less dense floats; more dense sinks.
2. Petrol floats on water, which is why oil spills spread on top.