CBSE Class 7 Science Advanced Physics Our Universe Very Short Answer Questions Set 02

Very Short Answer Questions

Question 1. What is the time interval between two successive tides at a given place on the earth?
Answer:
The interval between two successive primary (or secondary) tides at a place is 24 hours 52 minutes, that is, 2 × (12 hours 26 minutes).
In simple words: A given place has a high tide about every 12 hours 26 minutes.

Teacher's Note:
1. The 26 extra minutes come from the moon’s motion in its orbit.
2. The gap between a primary and the next secondary tide is 12 h 26 min.

Question 2. What is the value of escape velocity on the earth?
Answer:
The escape velocity on the earth is about 11.2 km s⁻¹ (the book takes it as 11.27 km s⁻¹).
In simple words: A body must be thrown at about 11 km every second to leave the earth for good.

Teacher's Note:
1. It is also about 7 mile s⁻¹.
2. Quote it with the unit km s⁻¹.

Question 3. When does a solid float in a given liquid? When does it sink?
Answer:
A solid floats when its density is less than that of the liquid.
It sinks when its density is greater than that of the liquid.
In simple words: Lighter-for-its-size things float and heavier-for-their-size things sink.

Teacher's Note:
1. Compare densities, not weights.
2. Water has density 1 g cm⁻³.

Question 4. Till now, how many constellations have been discovered? Name the biggest of the constellations.
Answer:
88 constellations have been discovered.
The biggest is Hydra.
In simple words: There are 88 star patterns, and Hydra is the largest.

Teacher's Note:
1. Hydra has at least 78 very big stars.
2. The twelve zodiac signs are only a part of the 88.

Question 5. What is the difference between ‘air’ and ‘wind’?
Answer:
Air is the mixture of gases that surrounds the earth.
Wind is air in motion, moving from a region of high pressure to a region of low pressure.
In simple words: Air is the stuff, wind is that stuff moving.

Teacher's Note:
1. Wind needs a pressure difference.
2. The greater the difference, the faster the wind.

Question 6. What is gravity? Does it change from place to place?
Answer:
Gravity is the force of attraction between the earth and any other body on or near it.
Yes. The value of g changes from place to place: it is maximum at the poles (9.831 m s⁻²) and minimum at the equator (9.781 m s⁻²). It also decreases with height.
In simple words: Gravity is gravitation when one of the bodies is the earth, and it is a little different in different places.

Teacher's Note:
1. Gravity is a special case of gravitation.
2. Give the pole and equator values.

Question 7. Define escape velocity.
Answer:
Escape velocity is the minimum velocity that a body must have to escape the gravity of the earth (or any planet) and not fall back.
In simple words: It is the least launch speed that lets something get away from the earth for ever.

Teacher's Note:
1. Quote the earth’s value too.
2. The moon’s escape velocity is much smaller.

Question 8. What is the ratio of acceleration due to gravity on earth to moon?
Answer:
g_e : g_m = 6 : 1, since g on the moon is 1/6 of g on the earth.
In simple words: Gravity on the earth is six times that on the moon.

Teacher's Note:
1. A body weighing 6 kgwt on the earth weighs 1 kgwt on the moon.
2. Read the order of the ratio carefully: earth to moon.

Question 9. What are constellations?
Answer:
Constellations are small groups of stars in the night sky, named by people after persons, animals or objects according to their shapes.
In simple words: They are star patterns used to find our way around the sky.

Teacher's Note:
1. The twelve constellations on the sun’s path form the zodiac.
2. In all 88 constellations are known.

Question 10. Name the device used to find the atmospheric pressure.
Answer:
The barometer (first designed by Torricelli).
In simple words: A barometer reads how hard the air presses.

Teacher's Note:
1. The mercury barometer is called Torricelli’s barometer.
2. Do not confuse with the anemometer.

Question 11. What is torricellian vacuum?
Answer:
The empty space above the mercury column in a barometer tube, left when the mercury falls into the trough, is called the Torricellian vacuum.
In simple words: It is the empty gap at the closed top of the barometer tube.

Teacher's Note:
1. There is no air in this space.
2. It appears when the mercury level settles at about 76 cm.

Question 12. (a) Two bodies A and B fall freely from heights h₁ and h₂, respectively, where, h₁ > h₂. Which body will take more time to reach the ground? Explain.
(b) What is the weight of a body on the moon whose weight is 12 kgwt on the earth?
Answer:
(a) Body A takes more time. The distance fallen is proportional to the square of the time (s ∝ t²), so a larger height needs a longer time, whatever the masses.
(b) Weight on the moon = 12 ÷ 6 = 2 kgwt.
In simple words: Falling from higher takes longer, and the moon’s pull is one-sixth of the earth’s.

Teacher's Note:
1. Mass does not affect the time of fall.
2. For (b) divide by 6.

Question 13. When a solid body is placed over the surface of a liquid A, half of its volume gets immersed in it. When the same body is made to float on liquid B, one-fourth of its volume is seen above the surface of the liquid. Explain.
Answer:
In liquid A the body is half immersed, so the density of the body is ½ of the density of A.
In liquid B, ¼ is above the surface, so ¾ is immersed and the density of the body is ¾ of the density of B.
Equating: d_A/2 = 3d_B/4, so d_A : d_B = 3 : 2. Liquid A is denser than liquid B, and the body is more immersed in the less dense liquid B.
In simple words: The body sinks less in the denser liquid, so A is denser than B.

Teacher's Note:
1. Fraction immersed = density of body ÷ density of liquid.
2. Use it for both liquids and compare.

Question 14. What are the precautions to be taken during thunderstorm accompanied by lightning?
Answer:
1. Do not take shelter under tall isolated trees.
2. A vehicle with closed windows is a safe place to take shelter.
3. Avoid sitting near a window.
4. An umbrella with a metallic end, open garages and metal sheds are not safe places.
5. Do not stay in water during lightning; come out of water and stay inside a building.
In simple words: Stay indoors or in a closed vehicle and keep away from trees, metal and water.

Teacher's Note:
1. Learn all five points.
2. Metal and water conduct electricity.

Question 15. What are monsoon winds?
Answer:
Monsoon winds are seasonal winds that blow for several months.
In summer the land is warmer than the ocean, so winds loaded with water vapour blow from the ocean to the land and bring rain.
In winter the direction reverses, from land to ocean, and these winds bring little rain.
In simple words: These are winds that change direction with the season and bring the rains.

Teacher's Note:
1. Heating difference between land and sea is the cause.
2. Summer monsoon brings rain, winter monsoon is dry.

Question 16. State true or false: There exists an attractive force between any two bodies on the earth.
Answer:
True. Any two material bodies attract each other.
In simple words: Every pair of objects pulls on each other, even if the pull is tiny.

Teacher's Note:
1. Gravitation is universal.
2. Give the reason when you can.

Question 17. State true or false: For a freely falling body the distance covered is directly proportional to the square of time.
Answer:
True. s ∝ t².
In simple words: Fall twice as long and you cover four times the distance.

Teacher's Note:
1. This is Galileo’s third law.
2. Give the reason when you can.

Question 18. State true or false: Gravitational force is affected by the temperature and pressure.
Answer:
False. Gravitational force is not affected by temperature, pressure, or electric and magnetic fields.
In simple words: Heat and pressure do not change gravity.

Teacher's Note:
1. This is why gravitation is universal.
2. Give the reason when you can.