CBSE Class 7 Science Advanced Physics Our Universe Long Answer Questions Set 01

Long Answer Questions

Question 1. What are spring tides? Why are they formed? Explain.
Answer:
Spring tides are high tides that occur on full moon and new moon days, when the sun, the earth and the moon lie along a straight line.
Full moon day: the sun and the moon are on opposite sides of the earth. The place facing the moon gets a primary tide due to the moon, which is reinforced by the secondary tide caused by the sun, and vice versa.

New moon day: the sun and the moon are on the same side of the earth, so both produce primary tides at the same place.

The spring tide on a new moon day is stronger than that on a full moon day, since the water is attracted jointly by the moon and the sun on the same side.
In simple words: When the sun, moon and earth are in a line their pulls add up and make very high tides.

Teacher's Note:
1. Draw both figures.
2. Say clearly that the new moon tide is stronger.

Question 2. Explain the working of a rocket.
Answer:
A rocket works on Newton’s third law of motion: for every action there is an equal and opposite reaction.
Fuel is burnt in the combustion chamber and hot gases are produced.
The hot gases escape through a nozzle at a great speed, with great force. This is the action.
As the reaction, the gases push the rocket with an equal force in the opposite direction, so the rocket moves forward with great speed.

In simple words: The rocket throws gas backwards fast, and the gas pushes the rocket forwards.

Teacher's Note:
1. Name the third law.
2. Mention the combustion chamber and the nozzle.

Question 3. A body dropped from a height of 5 m, reaches the ground in 1 s. If it takes 2 s to reach the ground, find the height from which it is dropped.
Answer:
The distance fallen is proportional to the square of the time, s ∝ t².
h₂/h₁ = (2)²/(1)² = 4, so h₂ = 4 × 5 = 20 m.
In simple words: Twice the time means four times the height.

Teacher's Note:
1. Use s ∝ t² and a ratio.
2. 20 m is also given by ½ × 10 × 2².

Question 4. A body is dropped from a certain height and it reaches the ground in 2 s. Another body which has twice the mass as that of the first body, when dropped reaches the ground in 4 s. Find the ratio of velocities acquired by them when they reach the ground.
Answer:
Velocity acquired in free fall is proportional to time, v = gt, and it does not depend on the mass.
v₁ : v₂ = 2 : 4 = 1 : 2.
In simple words: The body that falls for twice as long reaches twice the speed.

Teacher's Note:
1. Mass is a distraction.
2. Use v ∝ t.

Question 5. Explain the construction of a barometer.
Answer:
Take a glass tube closed at one end, about 1 m long, with a narrow uniform bore of diameter 1 cm.
Fill it carefully with pure and dry mercury, so that no air bubbles remain, and cover the open end with the thumb.
Turn the tube upside down into a trough containing mercury and remove the thumb.
Some mercury flows into the trough, leaving an empty space at the top called the Torricellian vacuum.
The mercury column that remains stands vertically and its pressure equals the atmospheric pressure. At sea level its height is 76 cm, so the pressure is 76 cm of mercury. It decreases with altitude.
In simple words: A mercury-filled tube is inverted in a trough, and the height of the column left shows the air pressure.

Teacher's Note:
1. Draw a neat labelled figure.
2. Mention 76 cm.

Question 6. State and explain the law of gravitation.
Answer:
Statement: In the universe, any two material bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The force acts along the line joining their centres.
Explanation: For masses M and m at a distance d, F ∝ Mm and F ∝ 1/d², so F = G Mm/d², where G is the universal gravitational constant.
G = 6.67 × 10⁻¹¹ N m² kg⁻² (SI) or 6.67 × 10⁻⁸ dyne cm² g⁻² (CGS). It does not depend on the masses, the medium, heat, pressure, or electric and magnetic fields.
The force is mutual: both bodies pull each other with equal force. If the distance is halved, the force becomes four times, and if it is doubled the force becomes one-fourth.

In simple words: Bigger masses pull harder, and greater distance makes the pull weaker by the square of the distance.

Teacher's Note:
1. Write the formula and define G.
2. Add the effect of changing the distance.

Question 7. What are tides? Discuss the causes of the occurrence of tides.
Answer:
Tides are the regular rise and fall of the water level in the oceans, about every 12 hours 26 minutes.
Causes: (1) The gravitational pull of the moon and the sun on the earth. The sun’s pull is about 46% of the moon’s. (2) The rotation of the earth about its axis gives rise to centrifugal force, which tends to throw the water outwards.
In the figure, the water at A, facing the moon, swells up as a direct (primary) tide. The water at B, on the far side, swells up as an antipodal (secondary) tide due to centrifugal force and a reaction to the pull at A. D and E have low tides.

The time between two successive tides is 24 h 52 min, because the moon moves ahead in its orbit while the earth rotates.
In simple words: The moon and sun pull the sea water and the earth’s spin throws it outwards, so the water rises and falls.

Teacher's Note:
1. Mention both gravitational and centrifugal causes.
2. Explain direct and antipodal tides with the figure.

Question 8. State the laws of freely falling bodies and describe Newton’s ‘guinea and feather’ experiment. What is the inference drawn from the experiment?
Answer:
Laws: (1) All bodies fall with equal rapidity in vacuum. (2) The velocity acquired by a falling body is directly proportional to the time of fall. (3) The distance travelled is directly proportional to the square of the time.
Experiment: A guinea (a coin) and a feather were dropped through a long glass tube, first with air in it, then with the air pumped out and the tube inverted.
With air, the guinea reached the bottom first. With the air removed, both reached the bottom at the same time.
Inference: In the absence of air resistance, gravity makes all bodies, light or heavy, fall together if they start from rest.
In simple words: Without air the feather and the coin fall together.

Teacher's Note:
1. List all three laws.
2. State the inference in one line.

Question 9. The height of the mercury column on a barometer at sea level is 76 cm and that at a particular height is 63.3 cm. Calculate the height from the sea level (For every 272.7 m of height from the sea level 2.54 centimetres of pressure decreases).
Answer:
Fall in the column = 76 − 63.3 = 12.7 cm.
Each 2.54 cm fall corresponds to 272.7 m, so the number of steps = 12.7 ÷ 2.54 = 5.
Height = 5 × 272.7 = 1363.5 m = 1.36 km.
In simple words: A 12.7 cm fall is five 2.54 cm steps of 272.7 m each.

Teacher's Note:
1. Find the number of steps first.
2. Convert 1363.5 m to about 1.36 km.

Question 10. Find the gravitational force between the sun and the Jupiter. Given that the mass of the sun = 2 × 10³⁰ kg, the mass of the Jupiter = 1.89 × 10²⁷ kg, the radius of the Jupiter’s orbit = 7.73 × 10¹¹ m and the value of universal gravitational constant G = 6.67 × 10⁻¹¹ N m² kg⁻².
Answer:
F = G m₁ m₂ / r²
= (6.67 × 10⁻¹¹ × 2 × 10³⁰ × 1.89 × 10²⁷) ÷ (7.73 × 10¹¹)²
= 2.52 × 10⁴⁷ ÷ 5.975 × 10²³
= 4.22 × 10²³ N.
In simple words: Put the numbers in F = Gm₁m₂/r² and work out the powers of ten.

Teacher's Note:
1. Square the orbital radius carefully.
2. Quote the unit, newton.

Question 11. We know that gravitational force is present between any two bodies in the universe. But when we stand near a rock, we are not pulled by the rock. Why?
Answer:
The gravitational force between a person and a rock is extremely small because their masses are tiny compared with the mass of the earth.
The pull of the earth on the person is far greater, so the pull of the rock is negligible and we do not notice it.
In simple words: The rock’s pull is far too weak to notice next to the earth’s pull.

Teacher's Note:
1. Force depends on the masses.
2. The earth’s mass is huge.

Question 12. Why do we use a common balance to measure the mass of an object instead of a spring balance? Can we determine the mass of an object by using a spring balance?
Answer:
A spring balance measures weight, W = mg, and g changes from place to place, so its reading changes with place.
A common balance compares masses, so it gives the same result everywhere.
Yes, the mass can be found with a spring balance as m = W/g if the value of g at that place is known.
In simple words: A common balance compares masses and is not affected by g, while a spring balance shows weight.

Teacher's Note:
1. Weight changes with g, mass does not.
2. m = W/g.

Question 13. When does a person experience more weight in a lift?
Answer:
A person feels more weight when the lift starts moving upwards (accelerating upwards).
The lift’s velocity changes from zero, so it is accelerated, and the floor presses the person with a larger force.
In simple words: When the lift speeds up while going up, the floor pushes harder on our feet.

Teacher's Note:
1. Upward acceleration increases the felt weight.
2. Downward acceleration reduces it.

Question 14. Why do planets revolve around the sun?
Answer:
The mass of the sun is about 99.9% of the mass of the solar system, so its gravitational attraction on the planets is far greater than that between planets.
This gravitational pull between the sun and the planets keeps the planets revolving around the sun.
In simple words: The sun’s huge pull holds the planets in orbit.

Teacher's Note:
1. Name the force: gravitation.
2. The sun is by far the heaviest body.

Question 15. Explain why the number of meteorites striking the surface is quite large whereas very few reach the surface of the earth even though the gravitational force of attraction of moon is less than that of the earth.
Answer:
The moon has no atmosphere, because the escape velocity there is small and gas particles escape.
With no air, meteorites reach the surface without burning, so many strike the moon.
The earth has an atmosphere. Falling meteorites get heated by friction with air and most burn up before reaching the ground.
In simple words: The earth’s air burns up meteorites, but the airless moon has no such shield.

Teacher's Note:
1. Link air with friction heating.
2. Meteors are ‘shooting stars’.

Question 16. How does a perfume sprayer work?
Answer:
When the piston is pushed, air moves quickly over the upper end of a tube whose lower end is dipped in the liquid.
The fast air reduces the pressure at the top of the tube.
The atmospheric pressure on the liquid in the container is higher, so it pushes the liquid up the tube, and the stream of air carries it out as a spray.
In simple words: Fast moving air lowers the pressure, and the outside air pressure pushes the perfume up.

Teacher's Note:
1. Fast air means low pressure.
2. Name the pressure difference.

Question 17. An object is made to float in two immiscible liquids as shown in figure.
Compare the densities of two liquids A and B and the object.
Answer:
Liquid A is at the bottom, so it is denser than liquid B, which floats above it.
The object sinks in B, so it is denser than B, and it floats on A, so it is less dense than A.
Therefore density of A > density of the object > density of B.
In simple words: The object sits on the denser liquid and under the lighter one.

Teacher's Note:
1. Heavier liquid settles at the bottom.
2. Use the position of the object.

Question 18. The gravitational attraction of the earth on any object is proportional to its mass. Then, why do heavy objects not fall faster than light objects?
Answer:
The force on a heavy object is larger, but its mass is also larger by the same factor.
Acceleration = force ÷ mass, so the mass cancels and g is the same for all bodies.
Hence all bodies fall with equal rapidity, irrespective of their masses (in the absence of air resistance).
In simple words: Heavier bodies are pulled harder but are also harder to move, so the two effects cancel.

Teacher's Note:
1. Use a = F/m.
2. State that air resistance is ignored.

Question 19. A lighted candle is placed inside a rectangular box having two holes at its top, and a wheel made up of paper is placed inside it as shown in figure.
In which direction will the wheel rotate? Why?
Answer:
The air above the candle gets heated, expands and becomes lighter, so it rises out through hole B.
Fresh air enters through hole A. This produces an air current (wind) from A to B.
The wheel, which lies in this current, rotates in the clockwise direction.
In simple words: Warm air goes out at B and cool air comes in at A, and the moving air turns the wheel.

Teacher's Note:
1. Convection current: hot air rises, cool air replaces it.
2. Follow the direction of the air flow.