Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Mechanics → Gravitational force ↓


Free fall and weightlessness


In this article, we're going to explore two fascinating concepts in physics: free fall and weightlessness. These ideas not only govern the way we understand motion on Earth, but also explain the behavior of objects in outer space. Let's dive into these concepts with simple explanations, textual examples, and visual illustrations.

What is free fall?

Free fall is a special type of motion that an object experiences when it is only under the influence of gravity. In other words, when an object is falling only due to the force of gravity, without any resistance from the air or any other force, it is said to be in free fall.

Mathematically, the gravitational force acting on an object is given as:

F = m * g

Here:

  • F is the gravitational force,
  • m is the mass of the object, and
  • g is the acceleration due to gravity, which is about 9.8 m/s² on Earth.

During free fall, an object will accelerate downward at this rate if there is no air resistance to slow it down. This acceleration due to gravity is constant, which means that the object's speed increases uniformly as it falls.

Example 1: Dropping the ball

Imagine that you drop a ball from a height. At the beginning, the ball is at rest, so its initial velocity is 0 m/s. As it falls, it accelerates due to the Earth's gravity. If we ignore air resistance, after 1 second the ball's velocity is about 9.8 m/s, after 2 seconds it is about 19.6 m/s, and so on.

Free fall fantasy

0 sec 1 second 2 sec

The diagram above shows a ball falling freely under gravity. Notice the distance it travels each second, which shows constant acceleration.

Where does free fall occur?

Free fall can be observed in a variety of scenarios. While it may be challenging to observe pure free fall on Earth due to air resistance, there are environments where free fall occurs naturally:

  • Space: Satellites and other bodies in orbit around Earth experience a continuous free fall toward the planet due to gravity, while also accelerating in a tangential direction, which keeps them in orbit.
  • Vacuum chamber: On Earth, scientists use vacuum chambers to study free fall by eliminating air resistance.

The concept of weightlessness

Now, let's discuss the idea of weightlessness. As cool as it may sound, weightlessness doesn't mean that an object has no weight. Instead, it refers to the sensation experienced by objects or people when they are in free fall or in other situations where they experience no supporting forces.

Why do we feel weight?

Here on Earth, we feel weight because of the normal force exerted by the ground or any surface that supports us. This force counteracts our weight due to gravity, keeping us stationary. The sensation of weight is essentially the normal force pushing against us.

Example 2: Standing on the scales

When you stand on a weighing scale, the scale measures the normal force exerted on you. If you are stationary, this force is equal to your weight. Now, imagine you are standing on the scale inside an elevator that is in free fall. The scale will show zero because both you and the scale are moving downward at the same rate, resulting in no normal force acting between you and the scale.

Experiencing weightlessness

Weightlessness can be experienced in many scenarios, especially in space missions:

  • Orbiting astronauts: Astronauts on the International Space Station experience weightlessness because they are in constant free fall around Earth. Their spacecraft moves so fast that, as they fall, they never reach Earth's surface.
  • Parabolic flights: Airplanes can simulate weightlessness by flying in a parabolic flight path. During the free-fall portion of the parabola, passengers experience weightlessness.

Physics of weightlessness

Mathematically, weight is given by this equation:

Weight = m * g

Where:

  • Weight is the force due to gravity,
  • m is the mass of the object, and
  • g is the acceleration due to gravity.

In free fall, although the force of gravity still acts on the object, any supporting forces that normally counteract gravity are absent. This lack of supporting forces is what creates the sensation of weightlessness.

Visualization of weight and weightlessness

Earth weight

The diagram depicts a person standing on Earth (left) experiencing weight due to gravity and feeling a normal force pushing upward. On the other hand, in space (right), although gravity acts on them, there is no upward force to give them a sensation of weight, giving them a feeling of weightlessness.

Misconceptions about weightlessness

It is a common misconception that weightlessness is caused by the absence of gravity. However, as we discussed, gravity still exists. It is the absence of a normal force or supporting force that leads to weightlessness. In other words, an object in weightlessness is in free fall, but it falls freely at a uniform speed along its orbit or trajectory, giving the feeling of being weightless.

Conclusion

Understanding free fall and weightlessness helps us understand the fundamental nature of gravity and its effect on objects on Earth and in space. These concepts explain why astronauts float in space and why weight is measured differently depending on the forces acting upon them. Whether you're dropping a ball or observing an object in space, remembering these principles provides valuable insight into how our universe works.


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Free fall and weightlessness

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