Students understand free fall as motion under gravity alone, distinguish motion in air vs in vacuum, and relate constant acceleration g to g = GM/R² near Earth’s surface. They apply uniformly accelerated motion equations to vertical motion, and interpret simple demonstrations (feather vs ball; projectile under gravity). Safety and scientific thinking are emphasised.
Students will be able to:
Reference: NCERT Book Alignment
The lesson is aligned with the NCERT Grade 9 Science Textbook, Chapter 9: Gravitation, Section 9.2: Free Fall
By the end of the lesson, students will be able to:
In this session, students will compare falling in air (where drag matters) versus falling in a vacuum (where there is no drag). They will connect F = ma with gravitational attraction to derive g = GM/R², and practice kinematics with real examples and the VR projectile activity.
| Title | Approximate Duration | Procedure | Reference Material |
|---|---|---|---|
| Engage | 5 | Ask: “Why does a feather fall slower than a ball in air, but together in a vacuum?” Introduce free fall and gravity. | Slides |
| Explore | 10 | Explore free fall with two cases:
| Slides |
| Explain | 19 | Explain:
Virtual Lab: Students visualise feather vs ball, apple drop, set initial height/velocity, and observe motion. | Slides and Virtual Lab |
| Evaluate | 10 | Students attempt the Self-Evaluation task on LMS. | Virtual Lab |
| Extend | 5 | Think–Pair–Share: “Why do all objects fall the same in vacuum? What makes g ≈ constant near Earth?” | Slides |
In this lesson, you will explore the concept of gravitational free fall, understanding how objects move under the sole influence of gravity, and learn to apply equations of motion to solve problems related to free fall.
1. Introduction
Free fall is a type of motion in which an object falls towards the Earth only under the influence of gravitational force. This means no other force, such as air resistance or push/pull, acts on the object. In free fall, gravity is the dominant force causing the object to accelerate towards the Earth.
2. Gravitational Force and Acceleration
When an object is released from a height, it experiences the Earth’s gravitational pull. This acceleration due to gravity is denoted by g.
The value of g = 9.8 m/s² (approximately 10 m/s²).
It is constant near the Earth’s surface.
The value of g does not depend on the mass of the falling object.
Hence, in the absence of air resistance, a stone and a feather would fall at the same rate.
3. Motion During Free Fall
Free fall is considered uniformly accelerated motion because the acceleration (g) remains constant.
Equations of motion for free fall
Using downward direction as positive:
v=u+g
s=ut+ gt²/2
v²=u²+2gs
Where:
u = initial velocity
v = final velocity
s = distance fallen
t = time taken
g = acceleration due to gravity
If an object is simply dropped from rest,
u = 0
It starts accelerating at g from the very beginning.
4. Nature of Free Fall
Free fall is not the same as “falling freely through the air” in real life.
In the real world, objects experience air resistance, which slows them down.
But in a vacuum, air resistance is zero, so free fall becomes ideal.
In vacuum:
All objects fall at the same rate
Their acceleration is exactly g
5. Weight and Free Fall
Weight of an object is the force with which the Earth attracts it:
W=mg
During free fall, even though gravity acts on the object, it experiences apparent weightlessness because no support force acts on it. Astronauts in orbit feel weightless due to this condition.
6. Examples of Free Fall
A stone dropped from a height (ignoring air resistance)
Objects falling in a vacuum chamber
Motion of satellites and astronauts in space (apparent free fall)
7. Key Points
Free fall occurs under the sole influence of gravity.
Acceleration due to gravity (g) is nearly constant.
Free fall is an example of uniformly accelerated motion.
In a vacuum, all objects fall with the same acceleration.
This is the list of vocabulary terms used throughout the lesson.
In this Virtual Lab, you will compare falling in air versus vacuum, discover why all objects accelerate equally at g ≈ 9.8 m s⁻² in free fall, see how g = GM/R² is obtained, and explore projectile motion with an archer on a cliff (no air resistance).
Features:
Step-by-step Procedure for VR Experience
1. Access the Virtual Lab using the provided link and press Green Flag.
2. Introduction Scene — Apple Fall:
3. Feather vs Ball in Air:
4. Feather vs Ball in Vacuum:
5. Constant Gravitational Acceleration:
6. Take the Quiz — Test Your Understanding