Questions On Gravitation For Class 11

Gravitation is one of the fundamental forces of nature that governs the motion of celestial bodies and objects on Earth. It plays a crucial role in physics, helping us understand planetary motion, tides, satellite orbits, and even weight variations at different locations on Earth.

For Class 11 physics students, mastering the concepts of gravitation is essential for competitive exams like NEET, JEE, and other entrance tests. This topic presents important questions on gravitation, covering basic concepts, numerical problems, applications, and misconceptions.

Key Concepts of Gravitation

Before diving into questions, let’s revise the fundamental concepts:

1. Newton’s Law of Universal Gravitation
   • Every mass in the universe attracts every other mass with a force given by:
     F = G frac{m_1 m_2}{r^2}

     where:

     • F = gravitational force
     • G = universal gravitational constant ( $6.674 times 10^{-11} , Nm^2/kg2$ )
     • m_1, m_2 = masses of two objects
     • r = distance between the centers of two masses
2. Acceleration Due to Gravity (g)
   • The acceleration experienced by an object due to Earth’s gravity is ** g = 9.8 m/s^2 **.
   • The formula for gravity at a height h is:
     g’ = g left( frac{R}{R+h} right)^2
3. Kepler’s Laws of Planetary Motion
   • Law of Orbits: Planets move in elliptical orbits with the Sun at one focus.
   • Law of Areas: The line joining a planet to the Sun sweeps equal areas in equal time intervals.
   • Law of Periods: The square of a planet’s orbital period is proportional to the cube of its semi-major axis.
4. Gravitational Potential Energy
   • The potential energy of an object at a distance r from a planet’s center is:
     U = – G frac{mM}{r}
5. Escape Velocity
   • The minimum velocity required to escape Earth’s gravity is:
     v_e = sqrt{frac{2GM}{R}}

     For Earth, v_e = 11.2 km/s.

6. Orbital Velocity
   • The velocity needed to keep a satellite in orbit around Earth is:
     v_o = sqrt{frac{GM}{R}}

Conceptual Questions on Gravitation

Basic Understanding Questions

1. What is Newton’s Universal Law of Gravitation?
2. How does the gravitational force between two objects change if the distance between them is doubled?
3. What happens to the weight of an object as it moves from the equator to the poles?
4. Why do astronauts feel weightless in space?
5. What is the value of the universal gravitational constant G ?
6. What are Kepler’s three laws of planetary motion?
7. How is gravitational potential energy different from elastic potential energy?
8. Why does the Moon not fall into the Earth even though Earth’s gravity pulls it?
9. What is the difference between mass and weight?
10. How does gravitational acceleration change with altitude?

Application-Based Questions

1. Why is the weight of an object less on the Moon than on Earth?
2. How does Earth’s gravity affect the motion of artificial satellites?
3. Why does an object experience more gravitational force on Jupiter than on Earth?
4. Why is escape velocity independent of the mass of the escaping object?
5. Why do geostationary satellites remain fixed over one location on Earth?
6. If a body is taken to the center of the Earth, what happens to its weight?
7. How does the gravitational force between two bodies change if one mass is tripled?
8. Why does the force of gravity decrease with height?
9. How does the value of g change on the Moon compared to Earth?
10. What happens to Earth’s orbital speed if its distance from the Sun increases?

Numerical Problems on Gravitation

Question 1: Gravitational Force Between Two Masses

Problem: Two masses of 10 kg and 20 kg are placed 1 meter apart. Find the gravitational force between them.

Solution:

Using Newton’s gravitational formula:

So, the force is ** $1.33 times 10^{-8} N$ **.

Question 2: Weight Variation on the Moon

Problem: The mass of an astronaut is 80 kg. If the acceleration due to gravity on the Moon is ** frac{1}{6} **th of that on Earth, what is his weight on the Moon?

Solution:

Weight is given by:

On Earth:

On the Moon:

So, the astronaut’s weight on the Moon is 130.4 N.

Question 3: Escape Velocity Calculation

Problem: Find the escape velocity of a body on Mars. Given:

• **Mass of Mars M = 6.42 times 10^{23} kg **
• **Radius of Mars R = 3.39 times 10^6 m **
• ** G = 6.674 times 10^{-11} Nm^2/kg^2 **

Solution:

Escape velocity formula:

So, the escape velocity on Mars is 5.03 km/s.

Common Misconceptions in Gravitation

1. Gravitational force is only present on Earth.
   • False! Gravity exists everywhere in the universe.
2. Gravity and acceleration due to gravity are the same.
   • No, gravity is a force, while acceleration due to gravity is the effect of that force.
3. There is no gravity in space.
   • Wrong! Gravity exists in space, but astronauts feel weightless because they are in free fall.
4. Mass changes when an object moves to another planet.
   • No, only weight changes because gravity is different on each planet.
5. All objects fall at the same speed regardless of mass.
   • This is only true in a vacuum where there is no air resistance.

Gravitation is an essential topic in Class 11 physics with vast applications in astronomy, space exploration, and daily life. By practicing both conceptual and numerical problems, students can strengthen their understanding of the subject.

Mastering gravitation helps in solving real-world physics problems and preparing for competitive exams effectively. Keep practicing, and you’ll gain confidence in solving gravitation-related questions with ease!