1. What is Free Fall?
Free fall is the motion of an object when gravity is the only force acting on it. In this condition, the object accelerates toward the Earth with a constant acceleration \( g \approx 9.8\,\text{m/s}^2 \).
Air resistance, friction, or any other forces are neglected in free fall. This makes the motion simple and predictable.
1.1. Key Idea
In free fall, all objects fall with the same acceleration regardless of their mass, shape, or size (when air resistance is absent).
1.1.1. Famous Demonstration
The Apollo astronauts performed an experiment on the Moon by dropping a hammer and a feather together. With no air resistance, both hit the ground at the same time—perfectly demonstrating free fall.
2. Characteristics of Free Fall
Free fall has several important features that distinguish it from general motion under gravity.
2.1. Constant Acceleration
The body accelerates downward with constant acceleration \( g \), meaning its speed increases at a rate of 9.8 m/s every second.
2.2. Mass Does Not Matter
Whether it is a heavy stone or a light ball, both fall with the same acceleration in free fall.
2.3. Path of Motion
The motion is always vertical and directed toward the centre of the Earth.
3. Equations of Motion in Free Fall
Free fall uses the same kinematic equations as motion under gravity, but the initial velocity may often be zero (if the object is simply dropped).
Taking downward direction as positive:
\( v = u + gt \)
\( s = ut + \dfrac{1}{2}gt^2 \)
\( v^2 = u^2 + 2gs \)
3.1. Special Case: Dropped Object
If an object is released from rest, \( u = 0 \), so the equations become:
\( v = gt \)
\( s = \dfrac{1}{2}gt^2 \)
\( v^2 = 2gs \)
4. Free Fall vs Normal Falling
In the real world, objects often fall through air, which provides resistance. This slows down lighter objects more than heavier ones.
4.1. Role of Air Resistance
With air resistance, falling objects experience drag:
- Feathers fall slower than stones.
- Flat objects fall slower than compact ones.
- Objects eventually reach terminal velocity, where gravity equals air resistance.
4.2. Why Free Fall is Ideal
Free fall assumes no air resistance. This helps us study motion cleanly and understand the pure effect of gravity.
5. Free Fall in Space
Astronauts in space seem to float, but they are actually in continuous free fall around Earth. Because they fall along with their spacecraft, they feel weightless even though gravity is still acting on them.
5.1. Orbiting is Free Fall
Satellites move sideways fast enough that as they fall towards Earth, the planet's surface curves away beneath them. This continuous free fall keeps them in orbit.
6. Why Free Fall is Important
Understanding free fall helps explain:
- Why objects fall at the same rate in a vacuum
- How motion under gravity works
- Why astronauts appear weightless
- How projectiles move
- How satellites orbit planets
Next, we will explore a closely related idea: escape velocity, the minimum speed needed to completely overcome Earth’s gravitational pull.