Introduction to Free Fall

By Pratik Choudgury

(ACT, IGCSE/MYP Math & Physics Instructor at The Edge Learning Center)

The topic of free fall is integral to all higher secondary physics curriculum. At the very least, the basic fundamentals associated with free fall are introduced to the students at junior higher secondary and more complicated scenarios involving free fall are usually discussed at a higher secondary level. Free fall is also known as one-dimensional motion.

free-fall-accerelation

An object is said to be experiencing free fall when it falls solely under the influence of gravity. There are a few conceptual characteristics of a free falling object:

  1. Free falling objects are not subjected to air resistance.
  2. A free falling object experiences an acceleration of 9-8msec2. The acceleration of  9-8msec2is known as acceleration due to gravity. Whether explicitly stated or not in a particular free fall related question, the value of acceleration due to gravity is set to 9-8msec2 for any objects experiencing free fall.
  3. If an object is dropped from a particular height, then the initial velocity of the object is always considered to be 0.
  4. From experience we know that when an object is thrown vertically upwards, then it will slow down as it rises upwards, but what many of us do not know that the object will slow down at the same rate of deceleration of 9-8msec2 that it would otherwise accelerate with when dropped from a height.

To elucidate further on the aforementioned characteristic, when an object is thrown vertically upward, the object is still subject to gravity. As a result, the velocity of the object will decrease at a rate of 9-8msec2. When the highest point is reached and the velocity will decrease to 0, it will then start going down and the velocity will subsequently increase at a rate of 9-8msec2.

These four characteristics can be combined together to solve problems involving free falling objects.

4-characteristics-of-free-falling-objects

The Big Misconception

One big misconception is whether acceleration due to gravity is same for all the objects. In other words, “Doesn’t a more massive object accelerate at a greater rate than a less massive object?” “Wouldn’t an elephant fall faster than a mouse?” This misconception leads from our personal observation of the rate of fall of a single piece of paper and a textbook. The two objects clearly travel at a different speed towards the ground. The factor that actually plays a role in eluding us is somewhat invisible. As a result, we find it difficult to associate with the role it plays in the motion of a free falling object. The factor is the air resistance that an object encounters when free falling. If the object is very small (e.g a small metal ball) then the air resistance is almost zero. However, if we recollect the five characteristics above then a free falling object cannot be termed as free falling if it is subjected to air resistance.

The actual explanation of why all objects accelerate at the same rate (if there is negligible air resistance) involves the concept of force and mass, pretty much to do with Newton’s First Law of Motion.

Newton’s First Law of Motion states that the Force on an object is directly proportional to mass of the object and the acceleration that it undergoes. In other words, we can say that the acceleration of an object is directly proportional to force that it is subjected to and inversely proportional to its mass. Thus, greater force on a massive object is offset by the inverse influence of greater mass. Subsequently, all objects free fall at the same rate of acceleration, regardless of their mass.  

free-fall-explaination

Skydiving is considered free falling, as the air resistance is considered negligible, but not entirely zero. Since, the air resistance is not entirely zero, it plays a vital role in regulating the downward velocity of the object. After a certain period of free falling, the upward force resulting from the air resistance becomes equal to the downward force due to gravity and the mass of the object. At that point in time, the skydiver is said to have reached terminal velocity: a concept that I will be discussing further in my upcoming blog.

The formulas associated with free fall is tabulated as below:

free-fall-formulas

A typical free fall-IB examination question is demonstrated below:

free-fall-problem-throw-up