CBSE Class 7 Science Advanced Physics Our Universe MCQs Set 01

Answer: (a) the moon’s gravitational pull is small

Explanation:
1. Escape velocity depends on the mass and radius of the body: \( v = \sqrt{\frac{2GM}{R}} \).
2. The moon is much lighter than the earth, so its gravitational pull at the surface is weak and a small speed is enough to escape.
3. So the reason is that the moon’s gravitational pull is small.

Teacher's Note:
1. Weak gravity means a low escape speed.
2. The lack of atmosphere is a result of the low escape speed, not its cause.

Answer: (d) 7 mile s⁻¹

Explanation:
1. The escape velocity of the earth is about 11.2 km s⁻¹ (taken as 11.27 km s⁻¹ in the book).
2. 1 mile is about 1.6 km, so 11.2 km s⁻¹ ÷ 1.6 ≈ 7 mile s⁻¹.
3. So the value is 7 mile s⁻¹; the other options are far too small or too large.

Teacher's Note:
1. Remember the value in both units: about 11 km s⁻¹ and about 7 mile s⁻¹.
2. Check the unit first, since the numbers alone can mislead.

Answer: (b) the moon also moves round the earth in the same direction of the earth’s rotation

Explanation:
1. The earth turns once in 24 hours, so a place would face the moon again after 24 hours if the moon stood still.
2. The moon moves ahead in its orbit in the same direction as the earth turns, so the place needs about 26 extra minutes to face the moon again.
3. Hence the interval is 12 h 26 min, which is option (b).

Teacher's Note:
1. The moon does not stand still while the earth turns.
2. The extra 26 minutes are the time the earth needs to catch up with the moon.

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

Explanation:
1. Spring tides occur when the sun, the earth and the moon lie along a straight line.
2. This happens on a full-moon day and on a new-moon day.
3. The first and last quarter phases give neap tides, so the answer is both (a) and (b).

Teacher's Note:
1. Straight line gives spring tides; right angle gives neap tides.
2. Do not confuse the two kinds of tides.

Answer: (a) F

Explanation:
1. Gravitational force is mutual, and the two forces are an equal and opposite pair.
2. So the body of mass 2m attracts m with the same force, F.

Teacher's Note:
1. Think of Newton’s third law: action and reaction are equal.
2. The larger mass does not pull harder.

Answer: (d) None of the above

Explanation:
1. G is a universal constant. It has the same value everywhere and for all bodies.
2. It does not depend on the masses, the distance or the medium, so the answer is none of the above.

Teacher's Note:
1. The masses and distance appear separately in the formula F = GMm/d².
2. G itself stays fixed.

Answer: (b) \( F \propto \frac{1}{r^2} \)

Explanation:
1. For spheres in contact, the distance between their centres is d = 2r.
2. F = G m₁m₂ ÷ d² = G m₁m₂ ÷ (2r)², so F is proportional to 1/r² for given masses.
3. So \( F \propto \frac{1}{r^2} \).

Teacher's Note:
1. Use the distance between centres, which is 2r here.
2. The inverse-square law gives 1/r².

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

Explanation:
1. Tides are caused by the pull of the moon and the sun on the sea water.
2. The moon is near and the sun is massive, so both matter, while distant planets such as Jupiter have a negligible effect.
3. The answer is both (b) and (c).

Teacher's Note:
1. The moon’s effect is larger, the sun’s effect is smaller but real.
2. Other planets are too far to matter.

Answer: (d) 8 F

Explanation:
1. Force on the object = GMm ÷ R², so F ∝ M ÷ R².
2. For the planet, M is doubled and R is halved, so R² becomes ¼.
3. New force = 2 ÷ (¼) × F = 8 F.

Teacher's Note:
1. Treat each change as a factor and multiply the factors.
2. Halving the radius multiplies the force by 4.

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

Explanation:
1. A falling body has a constant acceleration g due to gravity.
2. Its velocity grows steadily, as v = gt.
3. So the velocity increases and the acceleration stays constant: option (c).

Teacher's Note:
1. Acceleration g is almost constant near the earth.
2. Velocity changes, acceleration does not.

Answer: (c) altitude

Explanation:
1. Atmospheric pressure falls as the height above sea level rises.
2. So the barometer reading tells the height of a place; used this way it is an altimeter.
3. The answer is altitude.

Teacher's Note:
1. Barometer: pressure. Altimeter: height. Same instrument, different use.
2. Temperature needs a thermometer.

Answer: (d) M_m = M_e

Explanation:
1. Mass is the amount of matter in a body and does not change with place.
2. So the mass on the moon equals the mass on the earth: M_m = M_e.
3. Only the weight changes, because g is different.

Teacher's Note:
1. Mass stays the same, weight changes.
2. Do not mix up 1/6 of weight with 1/6 of mass.

Answer: (a) mass of the earth.

Explanation:
1. g = GM ÷ R² depends on the mass M and radius R of the earth.
2. It does not depend on the mass of the falling body, so the answer is mass of the earth.

Teacher's Note:
1. Look at the formula: the body’s own mass cancels out.
2. This is why all bodies fall together in vacuum.

Answer: (d) All the above

Explanation:
1. (a) is wrong, since the mass of any body is never zero.
2. (b) is wrong, because the weight of a body at rest on the earth is mg, which is not zero.
3. (c) is wrong, since gravity on the body does not change merely because it moves.
4. All three are wrong, so the answer is all the above.

Teacher's Note:
1. Check every statement one by one.
2. Weight depends on m and g, not on whether the body moves.

Answer: (d) None of these

Explanation:
1. The spheres do attract each other, so (c) is wrong, and they never repel, so (b) is wrong.
2. But the force between two 1 kg bodies is extremely small, far less than the friction with the ground, so they do not visibly move towards each other either.
3. So (a) is not observed, and the answer is none of these.

Teacher's Note:
1. Tiny gravitational forces are hidden by friction.
2. Gravity between everyday objects is real but very weak.

Answer: (b) negligible.

Explanation:
1. The ball pulls the earth with an equal force of 9.8 N.
2. Acceleration = force ÷ mass, and the mass of the earth is huge.
3. So the earth’s acceleration is negligible.

Teacher's Note:
1. Same force, enormous mass, so tiny acceleration.
2. Use a = F/m.

Answer: (d) None of these

Explanation:
1. G is a universal constant with the same value everywhere.
2. It does not depend on size, mass or distance, so the answer is none of these.

Teacher's Note:
1. A constant does not change with the situation.
2. Its value is 6.67 × 10⁻¹¹ N m² kg⁻² in SI units.

Answer: (a) ACBED

Explanation:
1. First the fuel burns and makes gases (A).
2. The hot gases escape through the nozzle (C), coming out with great force, which is the action (B).
3. The gases push the rocket back as the reaction (E), so the rocket moves forward (D).
4. The order is ACBED.

Teacher's Note:
1. Follow the story: burn, escape, action, reaction, motion.
2. This is Newton’s third law at work.

Answer: (b) BDAC

Explanation:
1. Take the container with water (B).
2. Heat it and cap it firmly (D).
3. Cool it (A); the steam condenses and the inside pressure falls.
4. The container is crushed by the outside air (C). The order is BDAC.

Teacher's Note:
1. Set up first, then heat, then cool, then observe.
2. The crushing shows that outside air presses on things.

Answer: (d) the escape velocity on its surface is very small.

Explanation:
1. The escape velocity of the moon is small because its mass is small.
2. The speed of gas particles is almost equal to or more than this escape velocity, so the gases fly away into space.
3. So the reason is the very small escape velocity.

Teacher's Note:
1. Low escape speed lets gases leave.
2. Gravity on the moon is smaller, not greater, than on the earth.

Answer: (d) Its value does not depend upon the nature of the medium in which the object is placed.

Explanation:
1. G has units (N m² kg⁻² in SI), so (a) is wrong.
2. Its numerical value changes with the system of units (6.67 × 10⁻¹¹ in SI, 6.67 × 10⁻⁸ in CGS), so (b) is wrong.
3. It does not depend on mass, but it also does not depend on the medium, so (d) is true.

Teacher's Note:
1. A universal constant ignores the medium.
2. Remember both values of G.

Answer: (a) 10 N

Explanation:
1. Gravitational force is mutual: the ball and the earth pull on each other with equal and opposite forces.
2. So the ball attracts the earth with 10 N.

Teacher's Note:
1. Action and reaction are always equal.
2. The earth does not move visibly because of its huge mass.

Answer: (b) is quadrupled.

Explanation:
1. F is proportional to 1/d².
2. If d becomes d/2, then 1/d² becomes 4/d².
3. So the force becomes four times.

Teacher's Note:
1. Halve the distance, get four times the force.
2. Double the distance, get one-fourth of the force.

Answer: (d) zero

Explanation:
1. During a free fall the man and the load fall together with the same acceleration g.
2. The load does not press on his head, and he feels weightless.
3. So the weight experienced is zero.

Teacher's Note:
1. A freely falling body is weightless.
2. The felt weight is the contact force, which vanishes in free fall.

Answer: (d) 1.625

Explanation:
1. g on the moon is about 1/6 of 9.8 m s⁻².
2. The value is 1.625 m s⁻²; 274.1 belongs to the sun and 0.610 to Pluto.

Teacher's Note:
1. Learn the sun, moon and pluto values together.
2. About 1/6 of the earth’s value gives 1.6.

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

Explanation:
1. Acceleration due to gravity is the same for both, so (b) is wrong.
2. The paper is light and wide, so air resistance on it is comparable to its weight and slows it down.
3. The correct reason is option (c).

Teacher's Note:
1. In air, resistance matters for light objects.
2. In vacuum both would land together.

Answer: (a) directly proportional to the time of fall.

Explanation:
1. For free fall from rest, v = gt, so v is proportional to t.
2. It does not depend on the mass, so (c) is wrong, and it is not proportional to t², so (b) is wrong.
3. The answer is option (a).

Teacher's Note:
1. Velocity ∝ time, distance ∝ time².
2. Mass does not matter in free fall.

Answer: (d) All the above

Explanation:
1. Cyclones develop depending on wind speed, wind direction, temperature and humidity.
2. So all of the given factors contribute: all the above.

Teacher's Note:
1. Several weather factors act together.
2. Learn the four: wind speed, direction, temperature, humidity.

Answer: (b) 76 cm

Explanation:
1. At sea level the mercury column stands at 76 cm.
2. So the answer is 76 cm.

Teacher's Note:
1. 76 cm of mercury is the standard atmospheric pressure.
2. Check the unit: cm, not m.

Answer: (b) storms

Explanation:
1. A sudden fall in the column means a sudden fall in air pressure.
2. Low pressure systems bring storms, so it forecasts storms.

Teacher's Note:
1. Falling pressure warns of storms.
2. Storms and cyclones are low pressure systems.

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

Explanation:
1. Temperature changes the density of sea water.
2. Adding fresh water (from rivers and rain) also changes it.
3. So the answer is both (a) and (b).

Teacher's Note:
1. Ships carry load lines because of this change.
2. Fresh water is less dense than sea water.

Answer: (c) CADB

Explanation:
1. Take the tube (C).
2. Fill it with mercury (A).
3. Close the open end with the thumb (D).
4. Invert it in the trough and remove the thumb (B). The order is CADB.

Teacher's Note:
1. The tube must be full before the thumb is placed.
2. Inverting comes last.

Answer: (d) ACBD

Explanation:
1. Water vapour condenses and releases heat (A).
2. The warm air rises and the pressure drops (C).
3. Cool air rushes in (B).
4. This ends as a low pressure system with fast winds (D). The order is ACBD.

Teacher's Note:
1. Heat release comes first, then rising air, then inrush of cool air.
2. The cyclone is the final outcome.

Answer: (a) d_A > d_B > d_O

Explanation:
1. Both objects float, so each liquid is denser than the object: d_A > d_O and d_B > d_O.
2. In liquid A a larger part of the object is above the surface, so A is denser than B.
3. Hence d_A > d_B > d_O.

Teacher's Note:
1. The denser the liquid, the less the floating object sinks.
2. Compare the parts above the surface in the two beakers.

Answer: (b) On a new moon day, the sun and the moon lie on the same side of the earth and along the same straight line.

Explanation:
1. On a new moon day the sun and the moon are on the same side of the earth in a straight line.
2. Their pulls on the water add up at the same place, so the tide is stronger.
3. This is option (b); on a full moon day they are on opposite sides.

Teacher's Note:
1. Same side means the two pulls reinforce each other directly.
2. Right angles give neap tides.

Answer: (c) 1 : 3

Explanation:
1. Let the distances be x (from 10 kg) and y (from 90 kg).
2. Equal forces give G × 10 × 1 ÷ x² = G × 90 × 1 ÷ y², so y²/x² = 9.
3. Then y/x = 3, so x : y = 1 : 3.

Teacher's Note:
1. Set the two forces equal and cancel G and the 1 kg mass.
2. Take the square root at the end.

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

Explanation:
1. The gravitational forces are different (mg is larger for B), so (a) is wrong.
2. The acceleration is g for both, so (b) is true, and they hit the ground together, so (c) is true.
3. The answer is both (b) and (c).

Teacher's Note:
1. Force depends on mass, acceleration does not.
2. Free fall from the same height takes the same time.

Answer: (b) 5

Explanation:
1. m_A = 20 kg and m_B = 10 kg. The distances are d (A to B) and d/2 (B to C).
2. G × 20 × 10 ÷ d² = G × 10 × m_C ÷ (d/2)² = 40 G × m_C ÷ d².
3. So 200 = 40 m_C, giving m_C = 5 kg.

Teacher's Note:
1. Halving the distance multiplies the force by 4.
2. Write both forces with the same d² and compare.

Answer: (d) None of these

Explanation:
1. Gravitation is universal and is not affected by heat, pressure, or electric and magnetic fields.
2. So the answer is none of these.

Teacher's Note:
1. Only the masses and the distance matter.
2. This is why gravitation is called universal.

Answer: (b) 1 : 6

Explanation:
1. m_A g_e = m_B g_m and g_e = 6 g_m.
2. So 6 m_A = m_B, and m_A : m_B = 1 : 6.

Teacher's Note:
1. The body on the moon must be heavier in mass to weigh the same.
2. Write weight = mass × g for both.