CBSE Class 7 Science Advanced Physics Measurements Long Answer Questions Set 01

Long Answer Questions

Question 1. Explain the steps through which science is applied.
Answer:
1. Aim: a phenomenon is chosen to determine its cause.
2. Observation: the phenomenon is noticed carefully in different conditions, using sense organs and instruments. An experiment creates the same phenomenon in the laboratory.
3. Measurement: the quantity is compared with a standard unit, in CGS or SI.
4. Systematization: the measurements are arranged in a specific way.
5. Inference: a conclusion is drawn from the arranged data.
In simple words: Pick a question, watch, measure, arrange the numbers and conclude.

Teacher's Note:
1. Write all five steps in order.
2. Add a one-line example to score full marks.

Question 2. Describe how the triangulation method is used to measure large distances, by taking a suitable example.
Answer:
To find the distance of a toy (C) from the corner of a table (A):
1. Place a white sheet of paper along the edges of the table at A. Fix two pins p₁ and p₂ on the paper so that the pins and the toy are in a straight line. Join p₁ and p₂.
2. Move to the other corner B, and with the same paper fix two pins p₃ and p₄ in line with the toy. Join p₃ and p₄.
3. Extend both lines. They meet at a point p, which forms a triangle.
4. If AB = 1 m and p₁p₃ measures 5 cm on the paper, then 1 cm = 20 cm in real life.
5. Measure p₁p and multiply by 20 cm to get the real distance of the toy.
In simple words: Look at the toy from two places, draw both lines, and use the scale of the drawing.

Teacher's Note:
1. Draw the triangle in your answer.
2. The scale is real length ÷ drawn length.

Question 3. With the help of a suitable example, explain how science is applied.
Answer:
Aim: to show that the volume of a liquid displaced by a solid, which is insoluble and completely immersed in the liquid, increases with the volume of the solid.
Observation: objects such as stone, glass and plastic of different volumes are immersed in liquids such as water, kerosene and alcohol, in overflowing jars, at different places and times.
Measurement: the displaced liquid is collected in a beaker and its volume measured with a measuring cylinder.
Systematization: the displaced volumes are arranged in increasing or decreasing order.
Inference: the liquid displaced by a solid increases with the volume of the solid, for all kinds of liquids.
In simple words: Drop bigger objects into water and see that more water spills out.

Teacher's Note:
1. Use all five headings in the answer.
2. Quote the aim exactly.

Question 4. When half of the total volume of an empty beaker of mass 100 g is filled with water, its mass is found to be 500 g and when the remaining volume of the beaker is filled with a liquid, its mass is found to be 1 kg. Calculate the density of the liquid.
Answer:
Mass of water = 500 − 100 = 400 g, so the volume of water = 400 cm³ (half of the beaker).
The other half of the beaker, also 400 cm³, is filled with the liquid.
Mass of liquid = 1000 − 500 = 500 g.
Density of the liquid = 500 ÷ 400 = 1.25 g cm⁻³ (1250 kg m⁻³).
In simple words: Half the beaker is 400 cm³, and the liquid in it weighs 500 g.

Teacher's Note:
1. Convert 1 kg to 1000 g first.
2. The liquid fills the same volume as the water.

Teacher's Note:
1. Draw a table; examiners like it.
2. Always include the SI units.

Question 6. What is a density bottle? Explain how it is used to determine the density of a liquid.
Answer:
A density bottle is a specially designed, long-necked glass bottle with a ground glass stopper that has a capillary tube. A 50 ml bottle is commonly used.
1. Wash and dry the bottle, and find its mass with the stopper (m₁).
2. Fill it with distilled water, insert the stopper, wipe off the overflow and find the mass (m₂).
3. Empty, dry and fill it with the liquid; insert the stopper, wipe the overflow and find the mass (m₃).
4. Mass of liquid = m₃ − m₁ and mass of water = m₂ − m₁. They have equal volume.
5. Relative density = \( \frac{m_3 - m_1}{m_2 - m_1} \). Since the density of water is 1 g cm⁻³, this is also the density of the liquid in g cm⁻³.
In simple words: The stopper makes the bottle hold exactly the same volume every time, so the masses can be compared.

Teacher's Note:
1. Draw the bottle with its stopper and capillary tube.
2. Name the three weighings clearly.

Question 7. Test tube A contains 10 ml of liquid X and test tube B contains a liquid Y of volume 50 ml. The ratio of the density of Y to X is 2 : 5. Compare the mass of liquids A and B.
Answer:
Let the density of X = 5d, so the density of Y = 2d.
Mass of X = 5d × 10 = 50d.
Mass of Y = 2d × 50 = 100d.
Mass of A : mass of B = 50d : 100d = 1 : 2.
In simple words: Mass = density × volume, and the two masses are in the ratio 1 to 2.

Teacher's Note:
1. Give the densities a common symbol d.
2. The d cancels at the end.

Teacher's Note:
1. Learn this table; it is asked often.
2. Weight is a force, so its units are force units.