Q.A stone dropped from the top of a tower of height $300\ \text{m}$ splashes into the water of a pond near the base of the tower. When is the splash heard at the top given that the speed of sound in air is $340\ \text{m s}^{-1}$? ($g = 9.8\ \text{m s}^{-2}$)

Free Fall: The Intuition

Imagine you're holding a ball in your hand. The moment you let go, it drops. That's free fall — but only the simplest version. The real idea is more interesting.

Think about what happens when you drop a feather and a hammer on Earth. The feather flutters down slowly; the hammer crashes straight down. Most people say the hammer falls faster because it's heavier. That's wrong. The feather is slowed by air resistance — the air pushes up against its large surface area. The hammer, being dense and compact, cuts through air easily.

Now imagine doing the same experiment on the Moon. There's no air. When Apollo 15 astronaut David Scott dropped a hammer and a feather on the Moon, they hit the ground at the exact same time. That's free fall: falling under the influence of gravity alone, with no other forces acting.

The Precise Statement

In free fall, every object — regardless of mass, shape, or size — accelerates downward at the same rate. On Earth, that acceleration is approximately $g = 9.8 \, \text{m/s}^2$ (often taken as $10 \, \text{m/s}^2$ for quick calculations).

What This Means Mathematically

If you drop an object from rest, its motion is described by three simple equations (assuming downward is positive):

• Velocity after time $t$: $v = g t$
• Distance fallen after time $t$: $s = \frac{1}{2} g t^2$
• Relation between velocity and distance: $v^2 = 2 g s$

These come directly from the equations of motion with constant acceleration $a = g$.

The Key Insight That Confuses Most Students

Free fall does NOT mean "falling downward." An object thrown upward is also in free fall from the moment it leaves your hand until it lands. Why? Because the only force acting on it during that entire journey is gravity (ignoring air). It slows down going up, stops at the top, then speeds up coming down — all with the same constant acceleration $g$ downward.

Real-World vs. Ideal Free Fall

On Earth, true free fall is rare because air resistance is almost always present. A skydiver is in free fall only for the first few seconds — until air resistance builds up and balances gravity, at which point they reach terminal velocity and are no longer accelerating. That's not free fall anymore.

In exam problems, unless stated otherwise, you always assume free fall — meaning you ignore air resistance. The only force is gravity, and the acceleration is constant $g$.

One More Thing: The Direction Convention

You can choose upward as positive or downward as positive — just be consistent. If upward is positive, then $g = -9.8 \, \text{m/s}^2$ because gravity pulls downward. If downward is positive, $g = +9.8 \, \text{m/s}^2$. Both work; pick one and stick with it.

Summary

Free fall is motion under gravity alone. All objects in free fall accelerate at $g$, regardless of mass. The equations are the same as constant-acceleration motion with $a = g$. And remember: an object moving upward is in free fall too — gravity doesn't take a break.

Looking up "Free Fall: Definition, Formula & Real-World Examples" or "Free Fall important questions 11" is a common way students land here, and rightly so — free fall is a core part of the Class 11 Physics NCERT/CBSE curriculum. Expect it to reappear, often in a slightly disguised form, across JEE Main, NEET and state engineering/medical entrance exams.