Samacheer Class 9 Science - Universe

• (a) Tycho Brahe
• (b) Nicolaus Copernicus
• (c) Ptolemy
• (d) Archimedes

Answer:
(b) Nicolaus Copernicus

• (a) Mercury
• (b) Saturn
• (c) Uranus
• (d) Neptune

Answer:
(a) Mercury

• (a) meteor
• (b) star
• (c) planet
• (d) asteroid

Answer:
(d) asteroid

Ceres is also classified as a dwarf planet.

• (a) 4
• (b) 5
• (c) 2
• (d) 3

Answer:
(a) 4

Using Kepler’s Third Law:

Answer: True

This statement is false. Halley's comet does not appear after nearly 67 hours. The correct statement is that Halley's comet appears approximately every 76 years. This periodic comet is one of the most famous comets visible from Earth and follows a regular orbital path around the Sun. It was last visible in 1986 and is expected to return in 2061. The comet is named after English astronomer Edmond Halley, who predicted its periodic return based on observations of comets in previous centuries.

This statement is false. Satellites nearer to Earth require greater orbital velocity, not lesser. The orbital velocity of a satellite depends on its distance from Earth's center. Satellites in lower orbits, closer to Earth, must travel faster to maintain their circular orbit and counteract Earth's stronger gravitational pull at that distance. Conversely, satellites in higher orbits, farther from Earth, experience weaker gravitational force and therefore require lower orbital velocities. This relationship is described by the formula for orbital velocity, which shows that velocity is inversely proportional to the square root of the orbital radius.

This statement is true. Mars is called the Red Planet because its surface appears reddish in color. This reddish appearance is due to the presence of iron oxide, commonly known as rust, on the Martian surface. The iron oxide gives Mars its distinctive red hue, which is visible even from Earth when observed through telescopes. This characteristic color is one of the most recognizable features of Mars and is the reason it has earned the nickname the Red Planet.

Answer:

The Sun and all celestial bodies revolving around it together form the Solar System.

It includes:

• planets
• satellites
• asteroids
• comets
• meteors

Orbital velocity is defined as the horizontal velocity required for a satellite to revolve around Earth in a stable circular orbit. It is the minimum velocity at which a satellite must travel tangentially to the Earth's surface so that it falls toward Earth at the same rate as Earth's surface curves away from it. At this velocity, the gravitational force provides exactly the centripetal force needed to keep the satellite in circular motion. The orbital velocity depends on the mass of Earth and the orbital radius of the satellite. For a satellite orbiting close to Earth's surface, the orbital velocity is approximately 7.9 km/s or 28,400 km/h.

Answer:

Time taken by satellite to complete one revolution around Earth is called orbital period.

Answer:

A body revolving around a planet is called satellite.

Types:

• Natural satellites
• Artificial (man-made) satellites

Answer:

Inner planets are planets close to Sun.

They are:

• Mercury
• Venus
• Earth
• Mars

Features:

• Rocky planets
• High density
• Smaller size

They are also called:

Terrestrial Planets

Answer:

• Comets are made of dust, rock and ice.
• They revolve around Sun in highly elliptical orbits.
• On approaching Sun, they form glowing head and tail.
• Tail always points away from Sun.

Example:

Halley’s Comet

Answer:

• Suitable distance from Sun
• Suitable temperature
• Presence of water
• Atmosphere
• Ozone layer

ISS = International Space Station

Benefits:

• Water purification technology
• Medical advancements
• Robotic surgery techniques
• Vaccine development
• Cancer research

Answer:

Colour depends on temperature.

• Hot stars → blue/white
• Cooler stars → orange/red

A satellite is maintained in a circular orbit around Earth through the balance between its horizontal velocity and Earth's gravitational force. When a satellite is launched, it is given a high horizontal velocity in a direction tangent to Earth's surface. Earth's gravity continuously pulls the satellite toward Earth's center, providing the centripetal force necessary for circular motion. The satellite falls toward Earth, but because of its high horizontal velocity, it keeps missing Earth and instead follows a curved path around it. This balance between the satellite's inertia, which tends to make it move in a straight line, and Earth's gravitational pull, which pulls it toward the center, results in a stable circular orbit. If the velocity were too low, the satellite would fall back to Earth; if too high, it would escape Earth's gravitational field.

Satellites are called geostationary because they revolve around Earth in the equatorial plane in the same direction as Earth's rotation, with an orbital period of approximately 24 hours, which matches Earth's rotational period. This synchronization causes the satellite to remain fixed above the same point on Earth's surface at all times. A geostationary orbit is located at a height of about 36,000 km above Earth's surface. These satellites are extremely useful for telecommunications, weather monitoring, and broadcasting because their stationary position relative to Earth allows continuous coverage of a fixed geographical area without the need for tracking equipment.

Given:

Using orbital velocity formula:

Answer:

or