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

Short Answer Questions

Question 1. The mass of a bottle filled with water is found to be 150 g. The mass of the empty bottle is 50 g and the external volume of the container is 125 cm³. Find out the density of the material of the bottle.
Answer:
Mass of water = 150 − 50 = 100 g.
Volume of water = 100 cm³, so the inside volume (capacity) of the bottle = 100 cm³.
Volume of the material = 125 − 100 = 25 cm³.
Density = 50 g ÷ 25 cm³ = 2 g cm⁻³ (2000 kg m⁻³).
In simple words: The glass takes up the space between the outside and the inside of the bottle.

Teacher's Note:
1. Find the inside volume from the water.
2. Subtract it from the outside volume to get the volume of the material.

Question 2. Ten identical metallic balls of density 5 g cm⁻³ when dropped into water, the volume of the water displaced is found to be 500 cm³. Determine the mass of each metallic ball.
Answer:
Volume of 10 balls = 500 cm³, so one ball = 50 cm³.
Mass of one ball = density × volume = 5 × 50 = 250 g.
In simple words: Divide the displaced volume by 10, then multiply by the density.

Teacher's Note:
1. Displaced water equals the volume of the solid.
2. Divide by the number of balls before using density.

Question 3. When 20 drops of water is added to a graduated cylindrical container filled with water, the level of the liquid rises from 10 ml to 20 ml. Calculate the mass of each water drop.
Answer:
Volume of 20 drops = 20 − 10 = 10 ml = 10 cm³.
Mass of 10 cm³ of water = 10 g.
Mass of one drop = 10 ÷ 20 = 0.5 g.
In simple words: Ten ml of water weighs 10 g, shared by 20 drops.

Teacher's Note:
1. 1 ml of water has a mass of 1 g.
2. Divide the total by the number of drops.

Question 4. The density of a substance is 5 kg per litre. Express it in g cm⁻³.
Answer:
5 kg = 5000 g and 1 litre = 1000 cm³.
Density = 5000 ÷ 1000 = 5 g cm⁻³.
In simple words: Kg per litre is the same number as g per cm³.

Teacher's Note:
1. 1 kg per litre = 1 g cm⁻³.
2. This is a handy shortcut.

Question 5. Give a few examples from day to day life, where physics is employed.
Answer:
1. Physics explains why lightning occurs during rainy days.
2. It explains why we wear light coloured cotton clothes in summer and woollen clothes in winter.
3. It explains the working of devices such as the bulb, fluorescent tube and electric motor.
4. X-rays used to detect a fracture in a bone are an application of physics.
5. Ventilation in a room works through convection, which physics explains.
In simple words: Physics is behind everyday things like lights, clothes and X-rays.

Teacher's Note:
1. Write at least three different examples.
2. Pick examples from your own home.

Question 6. How is the volume of an irregular shaped solid determined? Explain.
Answer:
1. Take a measuring cylinder with graduations in ml or cm³ and fill it with water to a level V₁.
2. Tie the solid with a thread and lower it slowly into the water.
3. The water level rises to V₂.
4. The volume of the solid = V₂ − V₁.
In simple words: The solid pushes water up, and the rise in water is the volume of the solid.

Teacher's Note:
1. Water must cover the solid completely.
2. Use a solid that does not dissolve or float.

Question 7. A density bottle weighs 100 g when filled with liquid and 80 g when filled with water. If the weight of an empty density bottle is 20 g, find the density of the liquid.
Answer:
Mass of liquid = 100 − 20 = 80 g.
Mass of water = 80 − 20 = 60 g, so volume = 60 cm³ = volume of liquid.
Density = 80 ÷ 60 = 1.33 g cm⁻³ (about 1333 kg m⁻³).
In simple words: The bottle holds the same volume of both, so compare their masses.

Teacher's Note:
1. Relative density = mass of liquid ÷ mass of water.
2. Multiply by 1 g cm⁻³ to get the density.

Question 8. The volume of mercury and water is 50 ml each. What is the ratio of their mass, if their densities are in the ratio 68 : 5?
Answer:
For the same volume, mass = density × volume, so mass ∝ density.
Mass of mercury : mass of water = 68 : 5.
In simple words: Same volume means the masses are in the same ratio as the densities.

Teacher's Note:
1. Cancel the equal volumes.
2. Mercury is about 13.6 times as dense as water.

Question 9. Find the density of a cuboid of dimensions 3 cm × 5 cm × 7 cm and having mass 1 kg in SI system.
Answer:
Volume = 3 × 5 × 7 = 105 cm³ = 105 × 10⁻⁶ m³.
Density = 1 kg ÷ (105 × 10⁻⁶ m³) = 9523.8 kg m⁻³ (about 9.5 × 10³ kg m⁻³).
In simple words: Change cm³ to m³ before dividing.

Teacher's Note:
1. 1 cm³ = 10⁻⁶ m³.
2. Check: 9523.8 kg m⁻³ is 9.52 g cm⁻³.

Question 10. Is it possible to find the density of a given liquid using a density bottle of unknown capacity and a physical balance? Explain.
Answer:
Yes. The same bottle holds the same volume of the liquid and of water.
So relative density = mass of liquid ÷ mass of water, and density of the liquid = relative density × 1 g cm⁻³.
In simple words: You do not need the bottle's size, because it cancels in the ratio.

Teacher's Note:
1. The capacity cancels out when you divide.
2. Only the three weighings are needed.

Question 11. What are the uses of the triangulation method?
Answer:
The triangulation method is used to measure large distances such as the distance between planets and stars, the width of a river and the height of hills, where survey chains or big tapes cannot be used.
In simple words: It measures things we cannot reach with a tape.

Teacher's Note:
1. It works by drawing a triangle to scale.
2. Surveyors still use it.

Question 12. Explain how the ventilation in a room is possible.
Answer:
Air in a room gets heated and becomes lighter, so it rises and goes out through the ventilators near the ceiling. Cooler, denser air enters through the windows or doors near the floor. This convection current keeps fresh air coming in.
In simple words: Hot air goes out at the top and cool air comes in at the bottom.

Teacher's Note:
1. Ventilators are placed near the roof for this reason.
2. This is a convection current in a gas.

Question 13. Explain why a flame points in the upward direction.
Answer:
The gases around a flame become hot and their density decreases, so they rise. Cooler air flows in from the sides to replace them. This convection current in air carries the flame upward.
In simple words: Hot gases are lighter, so they go up and take the flame with them.

Teacher's Note:
1. Convection explains many everyday things.
2. In a spacecraft without gravity the flame is round.

Question 14. Find the mass of 1 m³ of gold in kg when the density of gold is 19 g cm⁻³.
Answer:
19 g cm⁻³ = 19 × 1000 = 19000 kg m⁻³.
Mass of 1 m³ = density × volume = 19000 × 1 = 19000 kg.
In simple words: Change the density to SI, then multiply by 1 m³.

Teacher's Note:
1. Multiply by 1000 to go from g cm⁻³ to kg m⁻³.
2. Gold is a very dense metal.

Question 15. If 5 kg of glycerine occupies 0.004 m³, determine the density of glycerine in g cm⁻³ and kg m⁻³.
Answer:
Density = 5 kg ÷ 0.004 m³ = 1250 kg m⁻³.
In CGS: 1250 ÷ 1000 = 1.25 g cm⁻³.
In simple words: Divide mass by volume, then divide by 1000 for CGS.

Teacher's Note:
1. Do the calculation in SI first.
2. Check by multiplying back.

Question 16. Take two cubes each of side 4 cm made up of iron and aluminium. Why is there a difference in masses of the two cubes in spite of having the same volume?
Answer:
The two cubes have the same volume (4 × 4 × 4 = 64 cm³), but iron and aluminium have different densities. Iron atoms are packed more densely than aluminium atoms, so the iron cube has more mass.
In simple words: Same size does not mean same mass, because the atoms are packed differently.

Teacher's Note:
1. Mass = density × volume.
2. The question says 'spheres' in the book, but the cubes are meant.