Class 10 Science Lesson 12

Exercise

1. Choose the correct option for the following questions.

(a) Due to the effect of which force, are the celestial bodies in the universe situated at their position?

(i) nuclear force of attraction
(ii) gravitational force
(iii) magnetic force
(iv) electromagnetic force

Answer: (ii) gravitational force

(b) Which theory is considered the most realistic theory about the origin of the universe?

(i) Geocentric theory
(ii) Heliocentric theory
(iii) Big Bang Theory
(iv) Newton’s Law of Gravitation

Answer: (iii) Big Bang Theory

(c) According to the Big Bang theory, how did the universe originate?

(i) from the explosion of an atom
(ii) from the collision of celestial bodies
(iii) from the explosion of a star
(iv) from the adjustment of nebulae

Answer: (i) from the explosion of an atom

(d) According to Hubble’s theory, which one of the following facts is correct?

(i) The farther away the galaxies are the more speed they will have.
(ii) The farther away the galaxies are the lesser the speed they will have.
(iii) There is no relation between the speed and distance of galaxies.
(iv) The gravitational force of the galaxies does not affect their speeds.

Answer: (i) The farther away the galaxies are the more speed they will have.

(e) Under what condition can an open universe be hypothesized?

(i) When the average density is greater than the critical density.
(ii) When the average density is less than the critical density.
(iii) When average density and critical density are equal.
(iv) When average density and critical density are not comparable.

Answer: (ii) When the average density is less than the critical density.

(f) What is the meaning of H in Hubble’s law; v = Hd?

(i) height
(ii) gravitational constant
(iii) constant of proportionality
(iv) distance

Answer: (iii) constant of proportionality

2. Differentiate between:

(a) Closed Universe vs Open Universe

(b) Flat Universe vs Closed Universe

3. Give reasons:

(a) As the distance between celestial bodies increases, their separating speed increases.

Ans: The reason behind this observation is explained by Hubble’s Law, which states that galaxies are moving away from each other at speeds proportional to their distances. This implies that the farther two celestial bodies are from each other, the faster they seem to be receding. This happens because the fabric of space itself is expanding, stretching the distance between all objects in the universe. So, it is not that the galaxies themselves are moving through space faster, but rather the space between them is increasing.

(b) Celestial bodies continue to remain in their own place.

Ans: Despite gravitational forces between celestial bodies, they do not collide due to the balance between their velocity and gravitational attraction. For example, Earth orbits the Sun because its forward velocity is balanced by the Sun’s gravitational pull. This balance causes bodies to stay in stable orbits rather than falling into each other. Additionally, the vast distances and relative motions in space prevent random collisions. Hence, gravitational forces ensure that bodies remain in predictable paths, maintaining order in the universe.

4. Answer the following questions:

(a) What is the universe?

Answer: The universe is the totality of all space, time, matter, and energy that exists. It includes everything from the smallest particles to the largest galaxies, encompassing all celestial bodies, cosmic radiation, and the laws of physics that govern them. Essentially, the universe is the vast, all-encompassing environment in which all physical existence occurs.

(b) State the Big Bang theory.

Answer: The Big Bang theory is the prevailing cosmological model explaining the origin of the universe. It suggests that the universe began approximately 13.8 billion years ago from an extremely hot, dense point known as a singularity. From this initial explosion or expansion, the universe has been continuously expanding and cooling, leading to the formation of galaxies, stars, planets, and eventually life.

(c) Write the conclusion of Hubble’s study.

Answer: Hubble’s study concluded that the universe is expanding. By observing distant galaxies, he found that their light was redshifted, indicating they were moving away from us. Moreover, he discovered that the speed at which a galaxy recedes is proportional to its distance from Earth, now known as Hubble’s Law. This discovery was critical in establishing that the universe is not static but dynamic and growing over time.

(d) According to the Big Bang theory, the universe is continuously expanding. Is there no limitation to the expansion of the universe? Explain your arguments.

Answer: According to the Big Bang theory, the universe has been expanding since its beginning, but whether this expansion will continue forever or eventually stop depends on the universe’s density and the effects of gravity and dark energy. If the density of matter is high enough (closed universe), gravitational forces could slow and reverse expansion, causing a “Big Crunch.” If the density is low (open universe), expansion will continue indefinitely. A flat universe represents the critical density where expansion slows but never completely stops. Additionally, dark energy, a mysterious force, is believed to accelerate expansion, suggesting there might be no limit to how much the universe will expand in the future.

(e) Write the importance of gravitational force in the position of celestial bodies in the universe.

Answer: Gravitational force is fundamental in determining the motion and position of celestial bodies. It holds planets in orbit around stars, like Earth around the Sun, and moons around planets, like the Moon around Earth. Gravity also governs the formation and stability of galaxies, clusters, and larger cosmic structures by pulling matter together. Without gravity, the universe would be a chaotic collection of objects moving randomly. It provides the necessary centripetal force for orbital motion, preventing collisions and maintaining cosmic order.

(f) Earth revolves around the Sun; Moon revolves around the Earth. They have their force of gravity as well as the gravitational force between them, but they never collide with each other. Clarify this with reasons.

Answer: The Earth and Moon remain in stable orbits because the gravitational attraction between them is balanced by their forward motion (velocity). The Moon is constantly falling towards the Earth due to gravity but has enough sideways velocity to keep missing it, resulting in an orbit rather than a collision. Similarly, Earth’s velocity around the Sun balances the Sun’s gravity. This delicate balance between gravity and motion ensures that celestial bodies revolve around each other without colliding.

(g) Explain the concept of a flat universe.

Answer: A flat universe is one in which the overall geometry of space is flat or Euclidean, meaning parallel lines remain parallel, and the angles of a triangle add up to 180 degrees. This implies the universe has zero curvature and its density is exactly equal to the critical density. In such a universe, expansion continues forever but slows down over time, never collapsing or expanding too rapidly. Observations from cosmic microwave background radiation suggest our universe is very close to flat.

(h) In your opinion, what might be the future of the universe: open, flat or closed? Explain your answer with suitable arguments.

Answer: In my opinion, the future of the universe is likely to be flat or open. Current astronomical observations, including measurements of cosmic microwave background radiation and galaxy distributions, indicate that the universe’s density is very close to the critical density, which supports a flat universe scenario. Additionally, the presence of dark energy suggests that the expansion of the universe is accelerating, making a closed universe less likely. If dark energy dominates, the universe may expand forever, thinning out galaxies and matter over time. Therefore, it seems probable that the universe will keep expanding indefinitely in a flat or open state.