CBSE Class 7 Science Advanced Physics Measurements MCQs Set 05

Answer: (c) 3 kg

Explanation:
1. Newton, kgf and kgwt are units of weight (force).
2. Kilogram (kg) is the unit of mass, so 3 kg is a measurement of mass, not of weight.

Teacher's Note:
1. kg is for mass; kgf or kgwt is for weight.
2. A force unit always has an 'f' or 'wt' or is the newton.

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

Explanation:
1. Weight is a force, so it is measured in newtons: A → b.
2. Mass is measured in kg: B → a.
3. Force is measured in newtons: C → b.
4. Work is measured in joules: D → c.
5. This is option (a).

Teacher's Note:
1. Weight and force share the same unit family.
2. Work has the same unit as energy.

Answer: (a) Spring balance

Explanation:
1. A spring balance measures weight, which depends on the value of g. g changes from place to place, so the reading changes.
2. A physical balance compares masses, and a meter scale and density bottle do not depend on g.

Teacher's Note:
1. Instruments that measure weight depend on g.
2. A physical balance gives the same mass reading everywhere.

Answer: (a) BCA

Explanation:
1. First note down the given mass and volume (B).
2. Then write the formula (C).
3. Then substitute the values (A) and calculate.
4. The order is B, C, A: (a) BCA.

Teacher's Note:
1. Substitution always comes after the formula.
2. Option BAC puts substitution before the formula, which is not possible.

Answer: (b) 500.5 gf

Explanation:
1. The spring balance can show only multiples of 0.5 gf, such as 500, 500.5, 501.
2. 500.7 gf lies between 500.5 and 501, nearer to 500.5.
3. The reading shown is 500.5 gf.

Teacher's Note:
1. An instrument cannot give a reading finer than its least count.
2. Pick the division mark nearest to the true value.

Answer: (c) 0.4

Explanation:
1. Between the 0 mark and the 1 cm mark there are 25 equal divisions.
2. Least count = 1 cm ÷ 25 = 0.04 cm = 0.4 mm.

Teacher's Note:
1. Convert 0.04 cm to mm by multiplying by 10.
2. Count the divisions from the picture with care.

Answer: (d) g cm⁻²

Explanation:
1. Density = mass ÷ volume, so its unit is (unit of mass) ÷ (unit of volume).
2. kg m⁻³, g m⁻³ and g cm⁻³ all have this form.
3. g cm⁻² is mass per area, so it is not a unit of density.

Teacher's Note:
1. The exponent of length in a density unit is −3.
2. g cm⁻² would be a unit for mass per unit area.

Answer: (b) 10³ mg

Explanation:
1. 1 g = 1000 mg = 10³ mg.
2. 1 g = 10⁻³ kg and 1 g = 100 cg, so the other options are wrong.
3. The answer is (b).

Teacher's Note:
1. Remember 1 g = 1000 mg and 1 kg = 1000 g.
2. Check the sign of the exponent: going to a smaller unit, the number gets larger.

Answer: (a) \( \frac{15}{8} \) mm

Explanation:
1. The wire has 8 turns that stretch from 0 to 15 on the scale, that is 15 mm.
2. Thickness of the wire = diameter of one turn = \( \frac{15}{8} \) mm: (a).

Teacher's Note:
1. Divide the length covered by the number of turns.
2. Count the turns of the wire in the picture, not the divisions.

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

Explanation:
1. The mass stays the same, so density is inversely proportional to volume.
2. \( \frac{d_A}{d_B} = \frac{V_B}{V_A} = \frac{150}{100} = \frac{3}{2} \): (d).

Teacher's Note:
1. Heating makes the volume larger and the density smaller.
2. The ratio of densities is the reverse of the ratio of volumes.

Answer: (b) 2 kg

Explanation:
1. Volume of the liquid = 1 L = 1000 cm³.
2. Mass of the liquid = 1 g cm⁻³ × 1000 cm³ = 1000 g = 1 kg.
3. Mass of the filled container = 1 + 1 = 2 kg.

Teacher's Note:
1. 1 L = 1000 cm³.
2. Add the mass of the container to the mass of the liquid.

Answer: (a) 16 g

Explanation:
1. Volume = 2 × 2 × 2 = 8 cm³.
2. Mass = density × volume = 2 × 8 = 16 g.

Teacher's Note:
1. Volume of a cube = side³.
2. Mass = density × volume.

Answer: (b) 13.6 : 1

Explanation:
1. For equal volumes V, mass = density × V.
2. Mass of mercury : mass of water = 13.6 V : 1 V = 13.6 : 1.

Teacher's Note:
1. For equal volumes, masses are in the ratio of the densities.
2. Mercury is much heavier than water.

Answer: (c) 1 kg

Explanation:
1. Half of the body is immersed and it displaces 1000 cm³, so the total volume of the body = 2000 cm³.
2. Mass = density × volume = 0.5 × 2000 = 1000 g = 1 kg.

Teacher's Note:
1. Total volume is double the displaced volume here.
2. Change 1000 g to kg at the end.

Answer: (d) 8 cm

Explanation:
1. PQ = 6 cm on the diagram stands for 3 km, so 1 cm = 0.5 km, that is 1 km = 2 cm.
2. PR = 4 km, so PR on the diagram = 4 × 2 = 8 cm.

Teacher's Note:
1. Find how many centimetres stand for 1 km, then multiply by 4.
2. The drawing is only a sketch; use the numbers in the question.

Answer: (a) 10 kgf

Explanation:
1. kg, g and mg are units of mass.
2. kgf is the unit of weight (force), so 10 kgf is a weight.

Teacher's Note:
1. The letter f in kgf stands for force.
2. 10 kgf is the weight of a 10 kg mass at the earth's surface.

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

Explanation:
1. Kilo means 10³: A → b.
2. Nano means 10⁻⁹: B → d.
3. Mega means 10⁶: C → c.
4. Milli means 10⁻³: D → a.
5. This is option (c).

Teacher's Note:
1. Learn the prefixes in order: nano 10⁻⁹, milli 10⁻³, kilo 10³, mega 10⁶.
2. Negative powers go with the smaller units.

Answer: (a) kgf

Explanation:
1. kgf is a unit of weight, and weight depends on g, which changes from place to place.
2. kg (mass), m³ (volume) and joule (energy) do not change with place in this way.

Teacher's Note:
1. Weight changes with place; mass does not.
2. Think of a person weighing less on the moon but having the same mass.

Answer: (b) ABC

Explanation:
1. Note down the given values (A).
2. Write the formula mass = density × volume (B).
3. Substitute the values and calculate (C).
4. The order is A, B, C: (b) ABC.

Teacher's Note:
1. Data, formula, substitution: this order works for most numerical problems.
2. A formula must be written before values are put in.

Answer: (b) 203.6 gf

Explanation:
1. The balance can show only multiples of 0.2 gf: ..., 203.4, 203.6, 203.8, 204.0 ...
2. Of the options, only 203.6 gf and 203 gf are multiples of 0.2, and 203.6 is the closest to 203.78.
3. The pointer has passed the 203.6 gf mark, so the reading is 203.6 gf.

Teacher's Note:
1. The reading must be a multiple of the least count, 0.2 gf.
2. 203.7 and 203.9 cannot be read because they are not multiples of 0.2.