NEWTON'S LAWS
 Click on the link & Answer the following questions in your notes

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.)

ELEPHANT & FEATHER WITHOUT AIR RESISTANCE

ELEPHANT & FEATHER WITHOUT AIR RESISTANCE
Suppose that an elephant and a feather are dropped off a very tall building from the same height at the same time. 
Suppose also that air resistance could somehow be eliminated such that neither the elephant nor the feather would experience any air drag during the course of their fall. 
Which object - the elephant or the feather - will hit the ground first? 

The animation at the right accurately depicts this situation. 

• The motion of the elephant and the feather in the absence of air resistance is shown. Further, the acceleration of each object is represented by a vector arrow.

Many people are surprised by the fact that in the absence of air resistance, the elephant and the feather strike the ground at the same time. Why is this so? 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. 
Before answering the following questions review the falling terms:  force, weight, gravity, mass, matter, weight, & acceleration
(add these questions & answers in your notes)
TRUE or FALSE:

  
1. The elephant and the feather each have the same force of gravity.
2. The elephant has more mass, yet both elephant and feather experience the same force of gravity.
3. The elephant experiences a greater force of gravity, yet both the elephant and the feather have the same mass.
4. On earth, all objects (whether an elephant or a feather) have the same force of gravity.
5. The elephant weighs more than the feather, yet they each have the same mass.
6. The elephant clearly has more mass than the feather, yet they each weigh the same.
7. The elephant clearly has more mass than the feather, yet the amount of gravity (force) is the same for each.
8. The elephant has the greatest acceleration, yet the amount of gravity is the same for each.

If you answered TRUE to any of the above, then perhaps you have some level of confusion concerning either the concepts or the words force, weight, gravity, mass, and acceleration. 

In the absence of air resistance, both the elephant and the feather are in a state of free-fall. 

• That is to say, the only force acting upon the two objects is the force of gravity. This force of gravity is what causes both the elephant and the feather to accelerate downwards. 
• The force of gravity experienced by an object is dependent upon the mass of that object. 
• Mass refers to the amount of matter in an object. 

Clearly, the elephant has more mass than the feather. Due to its greater mass, the elephant also experiences a greater force of gravity. That is, the Earth is pulling downwards upon the elephant with more force than it pulls downward upon the feather. Since weight is a measure of gravity's pull upon an object, it would also be appropriate to say that the elephant weighs more than the feather. For these reasons, all of the eight statements are false; there is an erroneous part to each statement due to the confusion of weight, mass, and force of gravity.
 
But if the elephant weighs more and experiences a greater downwards pull of gravity compared to the feather, why then does it hit the ground at the same time as the feather? 

Great question!! To answer this question, we must recall Newton's second law - the law of acceleration. 
Newton's second law states that the acts in the opposite direction of the object, pushing against it. 

When figuring the acceleration of object, there are two factors to consider - force and mass. 

Applied to the elephant-feather scenario, we can say that the elephant experiences a much greater force (which tends to produce large accelerations. Yet, the mass of an object resists acceleration. Thus, the greater mass of the elephant (which tends to produce small accelerations) offsets the influence of the greater force. 

It is the force/mass ratio which determines the acceleration. 
Even though a baby elephant may experience 100,000 times the force of a feather, it has 100,000 times the mass. The force/mass ratio is the same for each. 

The greater mass of the elephant requires the greater force just to maintain the same acceleration as the feather.

A simple rule to bear in mind is that all objects (regardless of their mass) experience the same acceleration when in a state of free fall. 

When the only force is gravity, the acceleration is the same value for all objects. On Earth, this acceleration value is 9.8 m/s/s. This is such an important value in physics that it is given a special name - the acceleration of gravity - and a special symbol - g.
But what about air resistance? Isn't it nonrealistic to ignore the influence of air resistance upon the two object? In the presence of air resistance, the elephant is sure to fall faster.    Right?  mmmmmm!!!

PARACHUTE EXPERIMENT

PARACHUTE EXPERIMENT

Background information:

Air resistance force is the force that opposes an object's motion. Air resistance acts in the opposite direction of the object, pushing against it. The cause of such resistance boils down to molecules and atoms colliding with one another. As an object moves, its atoms and molecules collide with the atoms and molecules in the air; these tiny collisions add up to one large collision. However, air resistance force is dependent upon velocity. 

Air Resistance and Velocity - Even when you walk, air resistance force works against you, albeit very gently. Because you are not moving very fast, you won't even notice it, unless it is a very windy day. When you run, however, even on a calm day, you notice the effects on your face as the air pushes against you and rushes by you. The reason is that as you speed up, the collision of atoms and molecules happens at a faster rate and with more force. This translates to the air resistance force gets bigger as the speed or velocity of the object increases.

Air Resistance and Mass - Only taking into account body size -- not athletic ability -- smaller people of the same age can usually run faster than larger people. The reason is the air resistance force. The faster an object moves, the more air resistance; similarly, the larger an object, the more air that object must displace to push through it. This means for a larger person running that he must not only overcome air resistance that the accelerated velocity causes, but he must also overcome additional air resistance due to his size and shape.

Air Resistance and Shape - Cars, buses, trains, airplanes and even boats must overcome air resistance in order to get people from point A to point B. Transport vehicles for obvious reasons are much larger than the average human and must move much faster to make it worth the time for companies to build them. Therefore transport vehicles must overcome much larger air resistance forces due to faster velocities and larger masses. To help accomplish this, manufacturers streamline vehicles in an attempt to reduce drag. This process, however, is never straightforward and often designs that seem worthy turn out to produce more drag. Usually such designs are modified continuously and subjected to multiple wind tunnel tests to improve efficiency.

Objects in the Air - For objects moving in the air, another force impacts air resistance: gravity. Gravity is constantly pushing things back toward the ground, while air resistance pushes up on the object.

In most cases, the gravity force is much stronger than air resistance, and eventually the object hits the ground. For an airplane to counteract gravity, the plane must produce lift to keep it in the air and level. Planes accomplish this through the aerodynamic design of its wings, thereby making air resistance work for the plane.

BERNOULLI'S PRINCIPLE:  Bernoulli’s principle helps explain that an aircraft can achieve lift because of the shape of its wings. They are shaped so that that air flows faster over the top of the wing and slower underneath.

·         Fast moving air equals low air pressure while slow moving air equals high air pressure.

·         The high air pressure underneath the wings will therefore push the aircraft up through the lower air pressure.

REVIEW:

Falling objects increase their speed as they fall, because their weight (the force of gravity) pulls them to Earth. They also experience an upward force called air resistance (drag), which slows them down.

Objects fall faster until they reach their terminal speed, which is reached when the upward (air resistance) and downward (weight) forces are equal.

Falling objects

When an object is dropped it gets faster and faster as it falls. This happens because their weight (the force of gravity) pulls them down towards the center of the Earth.

As they fall through the air, they also experience an upward force called air resistance (drag).

·         Objects with large surface areas, such as parachutes or shuttlecocks fall more slowly because they experience more air resistance.

Frictional forces such as air resistance, friction and drag act against the direction of motion, so tend to slow the object down.  This fact is put to good use in the design of the parachute and shuttlecock.
The size of frictional forces can be reduced by streamlining the object or lubricating any moving parts.

Reducing frictional force

Examples of reducing frictional force by streamlining include:

• A sports car is wedge-shaped to reduce air resistance and so increase top speed.
• Lorries and caravans have deflectors to reduce both air resistance and fuel consumption.
• A car with a roof box has increased air resistance and fuel consumption and a lower top speed.
• A downhill skier puts wax on the skis to reduce friction and so increase the top speed.

Terminal speed

When objects fall through the Earth's atmosphere they get faster and faster until they reach a speed where the upwards force (air resistance) and downwards force (weight) equal each other. At this point the object travels at its fastest speed called terminal speed.

What happens when you drop a coin and small feather at the same time?

·         They both have a similar surface area but the feather weighs less so has a smaller force of gravity pulling it down.

·         As the feather falls its upwards air resistance increases and soon equals its downwards weight, so it then travels at terminal speed. The coin is heavier and has to be traveling a lot faster before its air resistance is large enough to equal its weight.

Terminal speed on the moon

The astronaut, David Scott carried out a famous experiment on the Moon. He dropped a hammer and a feather at the same time and found they landed together. As there is no air on the Moon there is no air resistance. The only force on both the hammer and feather was the Moon’s gravity which made them both fall with the same acceleration.

http://youtu.be/KDp1tiUsZw8

Balanced forces - The motion of a skydiver

Graph showing the motion of a skydiver

Before the parachute opens

1. When the skydiver jumps out of the plane he accelerates due to the force of gravity pulling him down.
2. As he speeds up the upwards air resistance force increases. He carries on accelerating as long as the air resistance is less than his weight.
3. Eventually, he reaches his terminal speed when the air resistance and weight become equal. They're said to be balanced.

After the parachute opens

4. When the canopy opens it has a large surface area which increases the air resistance. This unbalances the forces and causes the parachutist to slow down.
5. As the parachutist slows down, his air resistance gets less until eventually it equals the downward force of gravity on him (his weight). Once again the two forces balance and he falls at terminal speed. This time it's a much slower terminal speed than before.