CBSE Class 7 Science Advanced Physics Our Universe MCQs Set 02

Answer: (d) All the above

Explanation:
1. g = GM ÷ R² depends on the planet’s mass and size, and it varies with the position (latitude, height) of the body.
2. So the answer is all the above.

Teacher's Note:
1. Mass and radius of the planet both enter g.
2. Position matters, for example poles versus equator.

Answer: (b) the gravitational force experienced by the body.

Explanation:
1. Weight is the force of attraction of the earth on the body.
2. It changes from place to place and depends on mass, so the answer is the gravitational force experienced by the body.

Teacher's Note:
1. Weight is a force; mass is an amount of matter.
2. W = mg.

Answer: (a) they move towards each other.

Explanation:
1. Zero weight means the earth’s pull is absent, but the balls still attract each other by gravitation.
2. So they move slowly towards each other.

Teacher's Note:
1. Gravitation acts between all bodies, not only with the earth.
2. Weightless does not mean force-free.

Answer: (b) 1 : 3

Explanation:
1. Mass = density × volume. With the radius constant, doubling the density doubles the mass.
2. W_e = m g_e and W_m = 2m g_m = 2m g_e ÷ 6.
3. W_m/W_e = 2/6 = 1 : 3.

Teacher's Note:
1. Find what changes: the mass doubles and g becomes one-sixth.
2. Multiply the factors: 2 × 1/6.

Answer: (d) None of these

Explanation:
1. The law holds for all masses, temperatures and large distances.
2. It fails only for particles separated by extremely small distances such as 10⁻⁷ cm or less.
3. None of the given cases is like that, so the answer is none of these.

Teacher's Note:
1. The law works from small to astronomical scales except at atomic distances.
2. Temperature has no effect.

Answer: (d) \( \frac{1}{\sqrt{2}} \)

Explanation:
1. F ∝ 1/d². For twice the force, 1/x² = 2/1², so x² = 1/2.
2. x = \( \frac{1}{\sqrt{2}} \) m ≈ 0.71 m.

Teacher's Note:
1. Twice the force needs the distance to shrink by a factor √2, not by 2.
2. Halving the distance would give four times the force.

Answer: (a) Acceleration due to gravity is independent of the mass of the falling bodies.

Explanation:
1. In vacuum there is no air resistance, and g does not depend on the mass of the body.
2. So both balls fall with the same acceleration and land together: option (a).

Teacher's Note:
1. Iron and cork differ in mass but not in g.
2. Vacuum means no air resistance.

Answer: (a) 100

Explanation:
1. Let x be the distance from the 5 kg body and r from the 20 kg body.
2. 5/x² = 20/r², so r² = 4x² and r = 2x.
3. x + r = 300 gives 3x = 300, so x = 100 m.

Teacher's Note:
1. Square roots turn 4 into 2.
2. The lighter body is nearer.

Answer: (b) is greater than the volume of the liquid A.

Explanation:
1. In the figure the container dips into B, and the level of A inside is lower than the level of B outside.
2. The displaced volume is the whole submerged part of the container, which is larger than the volume of A in it.
3. So it is greater than the volume of the liquid A.

Teacher's Note:
1. Count the container’s own submerged part too.
2. Compare the two levels in the figure.

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

Explanation:
1. A vice grips a metal plate or rod firmly while it is cut or filed.
2. It is hard to hold such a rod steadily in the hand, and this is exactly why a vice is used.
3. So both are true and R explains A: option (a).

Teacher's Note:
1. A vice is a workshop clamp.
2. Look for the reason that actually causes the use.

Answer: (a) BAC

Explanation:
1. The equator gets the most heat, so its air rises (B).
2. Cooler air from the 0° to 30° belts moves in (A).
3. That air in turn warms and rises, and cold polar air moves in (C).
4. The order is BAC.

Teacher's Note:
1. Start from the hottest place, the equator.
2. Each rising air mass is replaced by cooler air.

Answer: (a) to and fro

Explanation:
1. The hand moves back and forth repeatedly along a line.
2. This repeated back and forth motion is a to and fro motion.

Teacher's Note:
1. To and fro motion repeats forwards and backwards.
2. Rectilinear only means along a straight line.

Answer: (a) A harrow.

Explanation:
1. A harrow breaks soil clods and uproots weeds.
2. A leveller only levels the soil, and a plough turns the soil over.
3. So the answer is a harrow.

Teacher's Note:
1. Match each tool with its job.
2. Seed drill sows seeds, harrow removes weeds.

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

Explanation:
1. Neap tides are low tides, so A → c.
2. Spring tides are high tides, so B → a.
3. The zodiac is the group of 12 constellations, so C → d.
4. Bhaskara is an Indian artificial satellite, so D → b. The answer is option (c).

Teacher's Note:
1. Match the sure ones first.
2. Spring means high, neap means low.

Answer: (d) A is false, but R is true.

Explanation:
1. F is proportional to the product of the masses, so doubling both masses makes the product 4 times and the force 4 times, not ¼.
2. So A is false, while R is true.
3. The answer is option (d).

Teacher's Note:
1. Check the assertion with the formula before looking at the reason.
2. Doubling both masses multiplies F by 2 × 2 = 4.

Answer: (a) Acceleration due to gravity is independent of the mass of the falling bodies

Explanation:
1. In vacuum there is no air resistance.
2. g is the same for every body whatever its mass, so both land together: option (a).

Teacher's Note:
1. Mass cancels out when finding g.
2. Vacuum removes air resistance.

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

Explanation:
1. g on the moon is 1/6 of g on the earth, so the weight on the moon is 36 ÷ 6 = 6 kgwt, which makes A true.
2. R is also true, and it is the reason for A.
3. The answer is option (a).

Teacher's Note:
1. Weight on the moon is one-sixth of the weight on the earth.
2. The reason is the smaller g.

Answer: (b) 5

Explanation:
1. m_A = 20 kg, m_B = 10 kg, distances d and d/2.
2. G × 20 × 10 ÷ d² = G × 10 × m_C ÷ (d/2)² = 40 G m_C ÷ d².
3. So m_C = 5 kg.

Teacher's Note:
1. A shorter distance gives a larger force, so C must be lighter.
2. Cancel G and d² from both sides.

Answer: (c) 1 : 3

Explanation:
1. 10/x² = 90/y², so y²/x² = 9 and y = 3x.
2. The ratio x : y = 1 : 3.

Teacher's Note:
1. The heavier body is farther away, by the square root of the mass ratio.
2. √9 = 3.

Answer: (b) 25

Explanation:
1. Area levelled = length of the strip × length travelled = 1 m × 25 m.
2. The area is 25 square metre.

Teacher's Note:
1. Area = width covered × distance moved.
2. The strip is the width.

Answer: (b) 1 : 3

Explanation:
1. Doubling the density at the same radius doubles the mass.
2. W_m/W_e = (2m × g_m) ÷ (m × 6 g_m) = 2/6 = 1 : 3.

Teacher's Note:
1. Mass doubles and g is one-sixth.
2. Multiply 2 by 1/6.

Answer: (a) 10 N

Explanation:
1. Gravitational forces between two bodies are equal and opposite.
2. So the ball pulls the earth with 10 N.

Teacher's Note:
1. Think of action and reaction.
2. Equal forces do not mean equal accelerations.

Answer: (b) 5

Explanation:
1. The plate is 3.5 cm = 35 mm thick, and the jaw must move 35 mm.
2. Number of rotations = 35 mm ÷ 7 mm = 5.

Teacher's Note:
1. Convert cm to mm before dividing.
2. Rotations = total distance ÷ distance per rotation.

Answer: (b) is greater than the weight of the liquid A.

Explanation:
1. A floating body displaces liquid whose weight equals the weight of the body.
2. The body here is the container plus the liquid A in it, so the displaced weight is the weight of the container plus the weight of A.
3. That is greater than the weight of the liquid A.

Teacher's Note:
1. Weight of displaced liquid = total weight of the floating thing.
2. The empty container also has weight.

Answer: (c) A is true, but R is false.

Explanation:
1. Oiling and greasing protect metal tools from rusting and reduce wear, so A is true.
2. Rusting is caused by moisture and oxygen of the air, not simply by continuous use, so R is false.
3. A is true but R is false: option (c).

Teacher's Note:
1. Rust needs air and water.
2. Oil forms a coating that keeps them away from the metal.

Answer: (c) CAB

Explanation:
1. High temperature in humid tropical regions makes strong rising winds (C).
2. These winds carry water droplets up (A).
3. The droplets freeze and fall, and with the rising air they cause lightning and sound (B).
4. The order is CAB.

Teacher's Note:
1. Cause first, carrying of droplets next, lightning last.
2. Rising warm air is the starting point.

Answer: (b) rotatory

Explanation:
1. The pulley turns about a fixed axis through its centre.
2. Turning about an axis is rotatory motion.

Teacher's Note:
1. Spinning about an axis is rotatory.
2. Oscillatory is swinging back and forth.

Answer: (a) Seed-drill

Explanation:
1. A seed drill drops seeds into the soil at regular spacing.
2. A plough turns the soil and a harrow breaks clods, so the answer is seed-drill.

Teacher's Note:
1. The name tells the job.
2. Seed drill sows, plough turns soil.

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

Explanation:
1. Spring tides occur on full moon day: A → d.
2. Hydra is the biggest constellation: B → c.
3. Chandrayan-1 is an artificial satellite: C → b.
4. Neap tides occur when the pulls of moon and sun are perpendicular: D → a.
5. The answer is option (a).

Teacher's Note:
1. Match the unique ones first, like Chandrayan.
2. Perpendicular pulls mean neap tides.

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

Explanation:
1. F ∝ 1/d², so halving d makes the force 4 times, and A is true.
2. R states exactly this law and so explains A.
3. The answer is option (a).

Teacher's Note:
1. The inverse square law explains the assertion.
2. Half the distance gives 2² = 4 times the force.

Answer: (d) both (b) and (c).

Explanation:
1. Forces differ (mg), so (a) is wrong.
2. Acceleration g is the same and they reach the ground together, so (b) and (c) are true: option (d).

Teacher's Note:
1. Different masses give different forces but equal accelerations.
2. The time of fall depends only on the height.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Jupiter is much more massive, so g is larger there and the weight is more; A is true.
2. R is also true, as it is the definition of weight on the earth.
3. But R does not explain why the weight is more on Jupiter, so option (b).

Teacher's Note:
1. True statements need not explain each other.
2. The explanation lies in the larger g of Jupiter.

Answer: (d) \( \frac{1}{\sqrt{2}} \)

Explanation:
1. F ∝ 1/d². For twice the force, x² = 1/2.
2. So x = \( \frac{1}{\sqrt{2}} \) m.

Teacher's Note:
1. Use the inverse square law.
2. Do not just halve the distance.

Answer: (a) 100

Explanation:
1. 5/x² = 20/r² gives r = 2x.
2. x + 2x = 300, so x = 100 m.

Teacher's Note:
1. The smaller mass is closer.
2. The distances are in the ratio 1 : 2.

Answer: (b) 30

Explanation:
1. Area = length of the strip × length of the field levelled.
2. 45 = 1.5 × length, so length = 45 ÷ 1.5 = 30 m.

Teacher's Note:
1. Divide area by the width of the strip.
2. Check: 1.5 × 30 = 45.

Answer: (b) 1 : 6

Explanation:
1. m_A g_e = m_B g_m, with g_e = 6 g_m.
2. So 6 m_A = m_B, and the ratio is 1 : 6.

Teacher's Note:
1. Equal weights with smaller g on the moon need a larger mass there.
2. Cancel g_m from both sides.

Answer: (b) negligible.

Explanation:
1. The force on the earth is also 9.8 N.
2. Acceleration = force ÷ mass, and the earth’s mass is enormous, so the acceleration is negligible.

Teacher's Note:
1. Use a = F/m.
2. A huge mass gives a tiny acceleration.

Answer: (c) 1 cm

Explanation:
1. 4 × 360° is 4 rotations, so the jaw moves 4 × 0.5 = 2 cm.
2. This distance equals the diameter of the rod, 2r = 2 cm.
3. The radius is 1 cm.

Teacher's Note:
1. 4 × 360° means four turns.
2. The jaws hold the diameter, not the radius.

Answer: (a) d_A > d_B > d_O

Explanation:
1. Both objects float, so d_O is less than both liquid densities.
2. The object is less immersed in A, so A is denser than B.
3. So d_A > d_B > d_O.

Teacher's Note:
1. Less immersion means a denser liquid.
2. A floating object is less dense than the liquid.