AIR RESISTANCE, GRAVITY, & AIRPLANE FORCES - REVIEW

REVIEW:

REVIEW THE FOLLOWING ACTIVITIES & YOUR NOTES FOR QUIZ ON AIR RESISTANCE & FORCE ON AIRPLANE
QUIZ:  Tuesday, 12/17

TYPES OF FORCE #1

TYPES OF FORCE #2

TYPES OF FORCE #3

An airplane flying straight and level at a constant speed has four forces acting on it: lift, drag, weight and thrust. Weight and mass are not the same thing. 
Mass is a measure of the amount of matter in an object, while weight factors in the downward pull of gravity on an object.

In light aircraft, piston engines drive propellers. Propellers deflect air backward, and this air pushes back, creating thrust. The same principle applies to jet engines, which blast hot, expanding gases to the rear of the plane and, in turn, get pushed back by those same gases.

The forces acting on a plane work in opposing pairs. Weight opposes lift, drag opposes thrust. During steady, level flight, the pilot adjusts the engine power and various control surfaces to keep the opposing forces in balance.

Birds figured it out long before humans: You gotta have wings if you're going to fly. Wings create lift, the upward-acting force that gets your feet off the ground.

Wing sounds so simple, but airfoil soars with sophistication. Technically speaking, an airfoil is the shape of the wing -- a curved surface with a rounded leading edge and a sharp trailing edge.

Daniel Bernoulli and his famous principle get a lot of attention when it comes to lift. A lot of aviation enthusiasts would argue, however, that Bernoulli is only part of the lift story though.

Move over Bernoulli, Newton wants to fly this plane. According to authors Anderson and Eberhardt, Newton's third law of motion is perfectly capable of explaining how a wing works: Grossly simplified, it says that the wing pushes the air down, so the air pushes the wing up.

On an airfoil, the amount of curvature is determined by the camber line. Airfoils with positive camber -- the upper surface curves more than the lower surface -- generate better lift.

For an airfoil to work, the leading edge of the wing must be inclined upward. The more it's inclined, the greater the angle of attack. Put another way, the angle of attack is the angle between the chord, or midline, of an airfoil and the direction of the surrounding undisturbed flow of gas or liquid.

The angle of attack is related to the amount of lift. Lift will increase as the angle of attack is increased -- up to a point (called the critical angle of attack). For most aircraft, lift will be maximized if the angle of attack remains below 17 degrees.

While it's true that increasing the angle of attack increases lift, it's also true that you can have too much of a good thing. When the angle of attack becomes too steep, the wing can't generate lift, and the aircraft stalls.

Elevators are hinged flaps located on the tail of the plane. Raising the elevators deflects air downward, which pushes the tail down (and the nose up). Lowering the elevators pushes the tail up (and the nose down).


AIR RESISTANCE

GRAVITY


Every object in the Universe attracts every other object in the universe.  This invisible force for masses to move toward each other is called Gravity.

When you weight yourself, your weight may be around 30kg to maybe 50kg because of Gravity. 

Your weight is the result from the product of the force of gravity and the mass of you.

Why two masses separated in space have a gravitational attraction to one another remains unknown, despite much research and various theories.

Here are some important facts about gravity:

Using scientific language	Which translate to...
Gravity is the experience of two particles mutually attracting each other along the line joining them.	Imagine yourself deep in space and you are standing next to a brick.When you are running in a marathon, you are running in a particular direction, but gravity has no particular direction, but along the path joining you and that brick.
Spherically symmetric objects interact gravitationally as if their mass were located at their centers.	An example of a spherically symmetric object is the Earth.  Earth attracts as if it's mass were located at the centre of Earth.
It is gravity which causes the centripetal acceleration when a satellite moves in a circular orbit.	Gravity is what allows a satellite to move in a circular orbit around earth.
For a particular radius of circular orbit there is only one possible speed for a stable satellite orbit.	If a satellite wants to orbit 2,000km above Earth, there is only one speed at which it can stably orbit.

HOW MUCH WOULD WEIGHT ELSEWHERE?  (click on the link)

Elephant and Feather - Air Resistance

Suppose that an elephant and a feather are dropped off a very tall building from the same height at the same time. 

We will assume the realistic situation that both feather and elephant encounter air resistance. 

Which object - the elephant or the feather - will hit the ground first? 

Further, the acceleration of each object is represented by a vector arrow.
Most people are not surprised by the fact that the elephant strikes the ground before the feather. But why does the elephant fall faster? This question is the source of much confusion (as well as a variety of misconceptions). 

Test your understanding by making an effort to identify the following statements as being either true or false.  Review any vocab terms to help you with your thinking process; weight, force of gravity, acceleration of gravity, air resistance and terminal velocity.
(write questions in your notebook)
TRUE or FALSE:

1. The elephant encounters a smaller force of air resistance than the feather and therefore falls faster.
2. The elephant has a greater acceleration of gravity than the feather and therefore falls faster.
3. Both elephant and feather have the same force of gravity, yet the acceleration of gravity is greatest for the elephant.
4. Both elephant and feather have the same force of gravity, yet the feather experiences a greater air resistance.
5. Each object experiences the same amount of air resistance, yet the elephant experiences the greatest force of gravity.
6. Each object experiences the same amount of air resistance, yet the feather experiences the greatest force of gravity.
7. The feather weighs more than the elephant, and therefore will not accelerate as rapidly as the elephant.
8. Both elephant and feather weigh the same amount, yet the greater mass of the feather leads to a smaller acceleration.
9. The elephant experiences less air resistance and than the feather and thus reaches a larger terminal velocity.
10. The feather experiences more air resistance than the elephant and thus reaches a smaller terminal velocity.
11. The elephant and the feather encounter the same amount of air resistance, yet the elephant has a greater terminal velocity.

  If you answered TRUE to any of the above questions, then perhaps you have some confusion about either the concepts of weight, force of gravity, acceleration of gravity, air resistance and terminal velocity. 

The elephant and the feather are each being pulled downward due to the force of gravity. When initially dropped, this force of gravity is an unbalanced force. Thus, both elephant and feather begin to accelerate (i.e., gain speed). 

As the elephant and the feather begin to gain speed, they encounter the upward force of air resistance. Air resistance is the result of an object plowing through a layer of air and colliding with air molecules. The more air molecules which an object collides with, the greater the air resistance force. Subsequently, the amount of air resistance is dependent upon the speed of the falling object and the surface area of the falling object. Based on surface area alone, it is safe to assume that (for the same speed) the elephant would encounter more air resistance than the feather.

But why then does the elephant, which encounters more air resistance than the feather, fall faster? After all doesn't air resistance act to slow an object down? Wouldn't the object with greater air resistance fall slower?

Answering these questions demands an understanding of Newton's first and second law and the concept of terminal velocity. 

According to Newton's laws, an object will accelerate if the forces acting upon it are unbalanced; and further, the amount of acceleration is directly proportional to the amount of net force (unbalanced force) acting upon it. 

• Falling objects initially accelerate (gain speed) because there is no force big enough to balance the downward force of gravity. 
• Yet as an object gains speed, it encounters an increasing amount of upward air resistance force. 
• In fact, objects will continue to accelerate (gain speed) until the air resistance force increases to a large enough value to balance the downward force of gravity. 
• Since the elephant has more mass, it weighs more and experiences a greater downward force of gravity. The elephant will have to accelerate (gain speed) for a longer period of time before there is sufficient upward air resistance to balance the large downward force of gravity.

Once the upward force of air resistance upon an object is large enough to balance the downward force of gravity, the object is said to have reached a terminal velocity. 

The terminal velocity is the final velocity of the object; the object will continue to fall to the ground with this terminal velocity. 
In the case of the elephant and the feather, the elephant has a much greater terminal velocity than the feather. As mentioned above, the elephant would have to accelerate for a longer period of time. The elephant requires a greater speed to accumulate sufficient upward air resistance force to balance the downward force of gravity. In fact, the elephant never does reach a terminal velocity; the animation above shows that there is still an acceleration on the elephant the moment before striking the ground. If we were to depict the relative magnitude of the two forces acting upon the elephant and the feather at various times in their fall, perhaps it would appear as shown below. (NOTE: The magnitude of the force vector is indicated by the relative size of the arrow.)