What is Free Fall?

To a physicist, the term "free fall" has a different meaning than it does to a skydiver. In physics, free fall is the (one-dimensional) motion of any object under the influence of gravity only - no air resistance or friction effects of any kind, whereas it is air resistance that makes skydiving a hobby rather than a suicide attempt!

You might think that since just about everything we observe falling is falling through the air, that "physics free fall" must be a pretty useless idea in practice. Not so! Any falling object's motion is at least approximately free fall as long as:

• ... it is relatively heavy compared to its size. (Dropping a ball, as in the picture at right, or jumping off a chair, is a free-fall motion, but dropping an unfolded piece of paper, or the motion of a dust particle floating in the air, is not. If you crumble the paper into a "paper wad", however, its motion is approximately free fall.
• ... it falls for a relatively short time. (If you jump off a chair, you are in free fall. After you have jumped out of an airplane and fallen for several seconds, you are not in free fall, since air resistance is now a factor in your motion.)
• ... it is moving relatively slowly. (If you drop a ball or throw it down its motion will be free fall. If you shoot it out of a cannon, its motion won't be free fall.)

You should also note that an object doesn't have to be falling to be in free fall - if you throw a ball upward its motion is still considered to be free fall, since it is moving under the influence of gravity.

According to...

. . . Aristotle a great philosopher said that heavier objects fell faster than lighter ones.

. . . To legend, Galileo an Italian physicist and Astronomer, discovered the law of falling objects by observing that two different weights dropped simultaneously from the Leaning Tower of Pisa hit the ground at approximately the same time.

What is Freely Falling Object?

A freely falling object is any object moving freely under the influence of gravity alone, regardless of its initial motion. This expression doesn't necessarily refer to an object dropped from rest. Any object thrown upward or downward and those released from rest are all considered freely falling.

An object that moves because of the action of gravity alone is said to be free falling. If the object falls through the atmosphere, there is an additional drag force acting on the object and the physics involved with the motion of the object is more complex.

Free Falling

Kinematic Equations for Free Fall

       Legend:
 V_{f= final velocity}    
 V_i= initial velocity     
 t = time                 
 a = acceleration     
Δx =displacement along the x-axis 

Elephant and a Man dropped in the same time. Is there any difference?

Gravity is equal to 9.8 m/s.

Examples:

a. Rocket-powered sleds are used to test the human response to acceleration. If a rocket-powered sled is accelerated to a speed of 444 m/s in 1.8 seconds, then what is the acceleration and what is the distance that the sled travels?

Solution:

Given: vi = 0 m/s vf = 44 m/s t = 1.80 s

Find: a = ?? d = ??

a = (Delta v)/t

a = (444 m/s - 0 m/s)/(1.80 s)

a = 247 m/s²

d = v_i*t + 0.5*a*t²

d = (0 m/s)*(1.80 s)+ 0.5*(247 m/s²)*(1.80 s)²

d = 0 m + 400 m

d = 400 m

(Note: the d can also be calculated using the equation v_f² = v_i² + 2*a*d)

b. An airplane accelerates down a runway at 3.20 m/s² for 32.8 s until is finally lifts off the ground. Determine the distance traveled before takeoff.

Solution:

Given: a = +3.2 m/s2 t = 32.8 s vi = 0 m/s

Find: d = ??

d = v_i*t + 0.5*a*t²

d = (0 m/s)*(32.8 s)+ 0.5*(3.20 m/s²)*(32.8 s)²

d = 1720 m

c. Upton Chuck is riding the Giant Drop at Great America. If Upton free falls for 2.6 seconds, what will be his final velocity and how far will he fall?

Solution:

Given: a = -9.8 m t = 2.6 s vi = 0 m/s

Find: d = ?? vf = ??

d = v_i*t + 0.5*a*t²

d = (0 m/s)*(2.6 s)+ 0.5*(-9.8 m/s²)*(2.6 s)²

d = -33 m (- indicates direction)

v_f = v_i + a*t

v_f = 0 + (-9.8 m/s²)*(2.6 s)

v_f = -25.5 m/s (- indicates direction)