Freefall and Aerodynamic Drag

This science project investigates gravity, freefall, air resistance and terminal velocity.bust of Aristotle with beard and wearing a togaAristotle (Ἀριστοτέλης) was a wise and learned ancient Greek polymath. He lived from 384 BC to 322 BC.He believed that if two objects of different weights were dropped at exactly the same time from the same height, the heavier object would fall faster than the lighter object.

So by Aristotle’s reasoning if two cannon balls of different weights were dropped at the same time from the same height, the large cannon ball would reach the ground faster than the small cannon ball.

  • Isn’t this what happens?!

two ancient cannon balls of different sizes

Aristotle’s theory about how objects move (the motion of objects) was believed to be correct for nearly 2000 years until the arrival of Italian scientist Galileo Galilei in the sixteenth century.Portrait of Galileo Galilei looking studious, wearing 16th century clothes with a long beard

Galileo Galilei was said to have carried out an experiment which tested Aristotle’s theory about the motion objects. He decided to conduct his experiment from the top of the Learning Tower of Pisa in his home town of Pisa, Northern Italy.

Leaning Tower of Pisa

Click for slide show

Galileo climbed the 296 steps to the top of The Leaning Tower carrying two cannon balls. Both cannon balls were identical in every way except for the fact that the larger cannon ball had twice the mass of the smaller cannon ball. (for an explanation of ‘mass’ read Gravity, Mass and Weight)

Galileo leaning over the side of the Leaning Tower holding two balls

He leaned over the side of the tower and dropped both balls at the same time from exactly the same height.

  • Which ball landed on the ground first?

two ancient cannon balls of different sizes

Strange but true!….. both cannon balls landed on the ground at the same time.

  • Why did this happen?

The answer lies in the fact that both cannon balls were falling to the ground in a state of ‘freefall’.

  • So what is ‘freefall’?

Basket ball accelerating downwards over a period of half a second in a state of freefallAn object in ‘freefall’ falls only under the influence of the force of gravity. No other forces act on the object to slow it down. In a state of ‘freefall’ objects with both large and small masses accelerate to the ground at the same rate.

Characteristics of objects in ‘freefall’

There are two main characteristics of objects in ‘freefall’. We consider each in turn.

1)No air resistance

Objects in ‘freefall’ do not encounter any air resistance.

So when an object is in ‘freefall’ few, if any, particles of air collide with the object’s ‘leading surface’.(see image on left for a visual explanation of ‘leading surface’.)

If air particles do collide with the ‘leading surface’ slowing down the rate of acceleration, then that object would not technically be in a state of ‘free fall’.

We will assume that the basketball in the image is in a state of ‘freefall’. There is no air resistance interfering with its descent and few particles of gases collide with its ‘leading surface’ as it is accelerating.

We also assume that when Galileo carried out his experiment his cannon balls fell to the ground in a state of ‘freefall’. This means that the only force acting on the cannon balls is the force of gravity. The cannon balls’ vertical acceleration is not slowed down by any air resistance.

On Earth it is not easy to demonstrate ‘freefall’. Whenever an object falls to the Earth’s surface, the dense gases in the Earth’s atmosphere create air resistance, preventing true ‘freefall’ taking place.

The only way to demonstrate true ‘freefall’ on our planet is under laboratory conditions in a vacuum chamber after all the particles of air have been pumped out.

An impressive example of a vacuum chamber is the one built by NASA at the Johnson Space Center in Houston, Texas, USA.

Johnson Space Center vacuum chamber with door open

This vacuum chamber can simulate conditions in deep space where there are no atmospheric gases present to create air resistance. Space telescopes and components used to build rockets for space exploration are all tested here.

vacuum chamber door closed

2) Rate of acceleration stays the same regardless of mass or shape.

All objects in freefall accelerate downwards at the same rate regardless of their mass or shape. It is amazing to think that objects with large mass such as airplanes and objects with small mass such as beach balls both accelerate downwards at the same rate.

On planet Earth that rate of acceleration is 9.8 meters per second every second. (9.8m/s2) We call this figure of 9.8 m/s2 ‘acceleration due to gravity’.

The table below summarizes the motion of an airplane and beach ball as they are being pulled downwards by the force of gravity. There is assumed to be no air resistance acting on the objects to slow them down; both objects are in a state of freefall.

As you can see from the table the velocities of both objects continue to increase at the same rate. The distance both objects fall is also identical.

table showing acceleration in Earth's atmosphere in state of freefall at 9.8 metres per second squared

While it is difficult to demonstrate true free fall in Earth’s atmosphere, a free fall experiment was conducted in the lunar atmosphere in August 1971.

During Apollo 15′ s mission to the Moon in 1971 astronaut David Scott dropped a falcon feather and a hammer at exactly the same time from the same height and waited to see what would happen.

feather and hammer experiment on the MoonImage credit:NASA

In the absence of any gases in the lunar atmosphere there was no air resistance acting to resist any downwards motion. Both objects, with different mass, landed on the lunar surface at exactly the same time. You can see the experiment in ‘real time’ below:

While the feather and hammer land at exactly the same time, both objects fall slower on the Moon than they would on Earth. This is because on the Moon acceleration due to gravity is only 1.6 meters per second squared, or 83.3% less than acceleration due to gravity on Earth.

The Moon’s force of gravity is substantially weaker than the Earth’s force of gravity.

Skydivers in ‘freefall’

The ‘freefall’ we have discussed above should not be confused with ‘freefall’ as used to describe the motion of skydivers.

‘Freefall’ is also the word we use to describe when skydrivers jump out of airplanes and fall through the atmosphere before opening their parachutes to land safely on Earth.

Since skydivers normally skydive in Earth’s lower atmosphere their descent is always slowed by air resistance.

eleven skydivers in freefall holding hands in a circleImage credit:Nasa

There is one skydiver, however, who has experienced skydiving from the edge of space- where there is little air resistance and where the Earth’s atmosphere is so thin that there are few molecules of air to hit the skydiver and slow his rate of descent.

air density decreases with height                                                             

His name is Felix Baumgartner and he completing this amazing feat on 14th October 2012. From the incredible height of  38,969 meters (127,852 feet) he jumped out of a capsule attached to an air balloon.

Felix Baumgartner skydives from stratosphere

Baumgartner reduced aerodynamic drag by adopting a streamlined ‘head first’ position with his arms tucked into his sides. Adopting this position meant that Baumgartner accelerated so fast that he broke the speed of sound within a minute of jumping. He ended up falling at the incredible speed of 1,357.64 km/h (843.6 mph)

Felix Baumgartner acclelerating unbalanced forces net force downwards

In the lower atmosphere Baumgartner’s descent was slowed down by increasing aerodynamic drag; the denser atmosphere slowed down his rate of descent. Of course he wanted to slow his descent any way so he could reach a safe speed at which to deploy his parachute in order to land safely.

As he began to slow down the force of aerodynamic drag became greater than the force of gravity; this resulted in a ‘net’ upwards force. Baumgartner adopted a ‘spread eagle’ position which increased his ‘cross sectional’ area. His body position increased the drag as more particles of air collided with his head and body.

felix baumgartner force of air resistance greater than force of gravity

In the lower atmosphere Baumgartner’ s rate of descent slowed so much that he reached ‘terminal velocity’. Reaching ‘terminal velocity’ meant that the force of air resistance became as large as the force of gravity.

Once the force of air resistance was as large as the force of gravity Baumgartner’s speed became constant. Reaching a constant speed meant that the forces of gravity and air resistance were in balance and now equal; Baumgartner neither accelerated nor slowed down.

baumgartner's gravitational force equals the force of air resistance

Baumgartner now needed to slow down still further from a terminal velocity of 122 mph (195 kmh) so he could reach a safe landing speed.

Once Baumgartner had opened his parachute the force of air resistance became greater than the force of gravity, resulting once again in an imbalance of forces. The net force (the difference between the forces of air resistance and gravity) was now upwards.

baumgartner forces are unbalanced gravity greater than air resistance

As Baumgartner’s speed decreased, the amount of upwards air resistance also decreased until once more Baumgartner reaches a terminal velocity when his speed is constant and no longer increasing or decreasing. The forces are once again in balance.

baumgartner about to land forces of air resistance and gravity in balance

Home at last! What a relief!

Felix Baumgartner lands safely

You can find out further information about Baumgartner’s feat by visiting the Red Bull Stratos website.

Summary of Baumgartner's incredible jump

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