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• All flying objects have some things in common. For some things, it is easy to decide if they are flying or not – a bird, a plane, Superman…

But for some things it is not as easy to decide – a stone being thrown through the air, a hang-glider, a hot-air balloon…

Douglas Adams once defined flight as the ability to throw yourself at the ground and miss. So how can we do this? And which things are truly flying?

The traditional definition of flight is that it needs thrust and lift. Some would use this to say that a hang-glider is not flying because it doesn’t have thrust, and it certainly excludes a stone being thrown through the air, because it has neither thrust nor lift.

A lot of people disagree with the definition of flight as ‘anything moving through the air’, such as a stone being thrown.

A hang-glider pilot was recently questioned about whether he was really flying. He responded: “Come up on my hang-glider and I’ll drop you off and see if you think I’m flying!!”

Most people would agree that the person falling from the hang-glider is not flying but that the person still on the hang-glider is. (Don’t worry, the imaginary falling person in this example has a parachute!)

This leads into a useful definition of flight that includes hang-gliders as flying, but certainly does not include the person falling.

To be classed as flying, an object needs two things:

• Lift to balance the force of gravity – without lift, an object will fall to the ground.
• Thrust to counter the force of drag (air resistance) – without thrust, an object will lose its forward momentum, which will also result in it falling to the ground.

## Falling

The person falling from the hang-glider has no lift to counter gravity, so they fall towards the ground, and they also have no thrust to counter air resistance. Because they have neither lift nor thrust, this is definitely not an example of flying – it is an example of falling.

What about a stone being thrown through the air? The stone gets its forward motion only while it is in contact with the person’s hand. While it is moving through the air, it doesn’t have any forward thrust to counter air resistance – only its momentum keeps it moving forwards, and its forward motion will gradually decrease.

Also, it doesn’t have any lift to counter gravity. For the whole time the stone is in the air, the force of gravity is pulling it downwards. The stone is falling for the whole time that it is in the air.

## Floating

Is a hot-air balloon flying? A hot-air balloon has lift to counter gravity. In this case, the lift isn’t created by air moving over wings, but is created by buoyancy. The hot air inside the balloon is less dense, which means that the air particles are not as closely packed together. The result is that the air inside the balloon is lighter than the air outside the balloon so the balloon will rise until the total weight force downwards is equal to the weight of the air that it is displacing.

However, the hot-air balloon has no thrust force to counter air resistance. In fact, the hot-air balloon will change speed until it is moving at the same speed as the air around it. It cannot counter drag. It will change speed until there is no drag. Because the hot-air balloon has no thrust, we cannot say that it is flying. It is only floating.

There is an art, it says, or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss.

The Hitchhiker’s Guide to the Galaxy, Douglas Adams.

## Flying

Is the hang-glider really flying? The hang-glider pilot has both lift and thrust, even though there aren’t any propellers. The lift is created as air flows over the wings. The thrust to counter air resistance to give it the force to keep it moving forwards comes from sacrificing some height. The hang-glider pilot will angle the hang-glider downwards slightly, and this downwards angle results in a forward force, so the hang-glider can keep moving forwards at a constant speed even though there is air resistance. A hang-glider pilot will often fly over areas of warmer land, hills or mountain ranges to make use of updrafts of air so that they can gain some height again.

So can a kiwi fly? Even if it falls off a cliff or learns how to hop, the answer is no. Like a stone being thrown, it can only fall. A hot-air balloon can only float, but a hang-glider can definitely fly.

## Related content

Four main forces affect the flight abilities of birds and planes, these forces are weight, lift, thrust and drag. Explore these key science ideas and concepts further in Principles of flight.

Building Science Concepts: The air around us is a suite of resources for the early years to upper primary. Use the information from these resources to aid explanation of why things fall or float.

In the activity What flies? students discuss what things can fly and how this happens and work towards identifying some key characteristics of flight.

Published 12 September 2011 Referencing Hub articles
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