Free fall is defined as the movement of an object or body solely under the influence of gravity. In this specific type of motion, the only force acting on the object is Earth's gravitational pull, assuming that other forces like air resistance are negligible or absent.
Understanding Free Fall
When an object is in free fall, its motion is always accelerated. This acceleration is caused by the external force of gravity acting on the object. Because of this constant acceleration, free-fall motion is also widely known as acceleration due to gravity.
Here are the key characteristics of free fall:
- Sole Influence of Gravity: The defining feature of free fall is that gravity is the only force significantly affecting the object's motion.
- Constant Acceleration: Objects in free fall accelerate downwards at a constant rate, assuming they are close to the Earth's surface. This acceleration is denoted by the letter 'g'.
- Value of 'g': The approximate value of acceleration due to gravity (g) near the Earth's surface is 9.8 meters per second squared (m/s²). This means that for every second an object is in free fall, its downward velocity increases by 9.8 m/s.
- Independence of Mass: A common misconception is that heavier objects fall faster. In free fall, the acceleration due to gravity is the same for all objects, regardless of their mass, provided air resistance is ignored. This means a feather and a hammer dropped simultaneously in a vacuum would hit the ground at the same time.
- Idealized Condition: True free fall usually refers to motion in a vacuum (where there is no air) to eliminate the effect of air resistance. In real-world scenarios on Earth, air resistance plays a role, causing lighter or flatter objects to fall slower than denser or more streamlined ones.
Examples of Free Fall
While perfect free fall (without any air resistance) is rare outside a vacuum, we can observe approximate free fall in everyday situations:
- A dropped stone: When you drop a stone from a height, it accelerates towards the ground. Although air resistance is present, for a dense object like a stone, the effect of gravity is predominant.
- An object falling from a building: An object released from the top of a tall building demonstrates accelerated motion primarily due to gravity.
- Skydiving (before parachute deployment): A skydiver initially experiences significant air resistance, but if they were in a vacuum, they would accelerate continuously at 'g'. In reality, they reach a "terminal velocity" where air resistance balances gravity. However, the initial phase of their fall is largely governed by gravity.
Important Considerations for Class 9 Physics
In Class 9, when studying free fall, you will typically learn:
- Equations of Motion: The standard equations of motion (kinematic equations) can be adapted for free fall.
v = u + gts = ut + ½gt²v² = u² + 2gs- Where:
u= initial velocityv= final velocityt= times= displacement (height fallen)g= acceleration due to gravity (usually taken as +9.8 m/s² for downward motion, or -9.8 m/s² for upward motion if upward is positive)
- Vertical Motion: Free fall is a specific case of vertical motion under gravity. When an object is thrown upwards, it decelerates as it moves against gravity, momentarily stops at its highest point, and then accelerates downwards, exhibiting free fall on its way down.
Free Fall vs. Other Types of Motion
To better understand free fall, let's compare it to other motions:
| Feature | Free Fall | Motion with Air Resistance | Projectile Motion (without air resistance) |
|---|---|---|---|
| Dominant Force(s) | Only gravity | Gravity and air resistance | Only gravity (horizontal velocity is constant) |
| Acceleration | Constant 'g' (downwards) | Varies (depends on air resistance) | Constant 'g' vertically, zero horizontally |
| Dependent on Mass? | No (acceleration is independent of mass) | Yes (heavier objects less affected by air resistance) | No (vertical acceleration independent of mass) |
| Idealized Condition | Yes (requires a vacuum) | No (real-world scenario) | Yes (often assumes vacuum for simplicity) |
| Example | Object falling in a vacuum chamber | Feather falling in air | A ball thrown horizontally off a cliff |
Free fall is a fundamental concept in physics, providing the basis for understanding how gravity influences the motion of objects around us.
