What happens in the absence of air resistance?

When an object falls with air resistance, both its acceleration and speed change during its motion. When an object falls in a vacuum, there is no air resistance because there is no air in a vacuum.

1. A rock and a feather are released from rest from the same height with air resistance on.
   Do they hit at the same time? Yes/No (answer shown in blue).
2. A rock and a feather are released from rest from the same height with air resistance off.
   Do they hit at the same time? Yes/No (answer shown in blue).
3. A rock and a feather are released from rest from the same height with air resistance on. Determine the time and the speed that each object hits the ground.
   Result:ObjectTime to impactSpeed at impactRock2.5 seconds7 m/secFeather25 seconds0.6 m/sec
   
4. Repeat the previous exercise with air resistance off.
   Result:ObjectTime to impactSpeed at impactRock1.6 seconds15.7 m/secFeather1.6 seconds15.7 m/sec
   
5. A rock falls fastest when there is no air resistance
   True/False (answer shown in blue).
6. A feather falls fastest when there is no air resistance
   True/False (answer shown in blue).
7. Without air resistance, a rock and a feather fall at the same speed
   True/False (answer shown in blue).
8. With air resistance, a rock and a feather fall at the same speed
   True/False (answer shown in blue).

Complete step by step answer:
When air resistance is absent the horizontal velocity of the body is unaffected by any other external force, as the only force which acts on the ball is gravity. And as we know gravity acts downwards or in the direction towards the earth, but the horizontal velocity is perpendicular to the gravitational force and thus tends to remain unaffected.
However, when air resistance is present the situation changes. Air resistance is the force opposing the motion of a body in the atmosphere similar to viscosity experienced by a moving body in a fluid. The opposing force is non conservative in nature.
The air resistance is proportional to velocity of the body, and thus the horizontal velocity would experience a force in the direction opposite to its direction. And this force would create an acceleration in the direction opposite to the motion and reduce the velocity.
The same drag would be experienced by the vertical velocity too, though initially the vertical velocity and air resistance is zero, as time increases the velocity increases and so does air resistance which would reduce the effect of gravity.
Thus, the total time taken by the abject would increase when air resistance is present, also the horizontal velocity would decrease.

So, the correct answer is “Option A”.

Note:
The air resistance depends on velocity of the body and the surface area of the object.
As the surface increases the air resistance experienced would also increase.Streamlined shape helps in reducing the effect of air resistance.
Air resistance is the force opposing the motion of a body in the atmosphere similar to viscosity experienced by a moving body in a fluid.

A velocity vector can only change if there is acceleration (acceleration is the rate of change of velocity). In order to accelerate a resultant force is required (according to Newton's Second Law,#vecF=mveca#).

In the absence of air resistance the only force acting on a projectile in flight is the weight of the object. Weight by definition acts vertically downwards, hence no horizontal component.

In a previous unit, it was stated that all objects (regardless of their mass) free fall with the same acceleration - 9.8 m/s/s. This particular acceleration value is so important in physics that it has its own peculiar name - the acceleration of gravity - and its own peculiar symbol - g. But why do all objects free fall at the same rate of acceleration regardless of their mass? Is it because they all weigh the same? ... because they all have the same gravity? ... because the air resistance is the same for each? Why? These questions will be explored in this section of Lesson 3.

In addition to an exploration of free fall, the motion of objects that encounter air resistance will also be analyzed. In particular, two questions will be explored:

• Why do objects that encounter air resistance ultimately reach a terminal velocity?
• In situations in which there is air resistance, why do more massive objects fall faster than less massive objects?

To answer the above questions, Newton's second law of motion (Fnet = m•a) will be applied to analyze the motion of objects that are falling under the sole influence of gravity (free fall) and under the dual influence of gravity and air resistance.

Free Fall Motion

As learned in an earlier unit, free fall is a special type of motion in which the only force acting upon an object is gravity. Objects that are said to be undergoing free fall, are not encountering a significant force of air resistance; they are falling under the sole influence of gravity. Under such conditions, all objects will fall with the same rate of acceleration, regardless of their mass. But why? Consider the free-falling motion of a 1000-kg baby elephant and a 1-kg overgrown mouse.

This ratio (Fnet/m) is sometimes called the gravitational field strength and is expressed as 9.8 N/kg (for a location upon Earth's surface). The gravitational field strength is a property of the location within Earth's gravitational field and not a property of the baby elephant nor the mouse. All objects placed upon Earth's surface will experience this amount of force (9.8 N) upon every 1 kilogram of mass within the object. Being a property of the location within Earth's gravitational field and not a property of the free falling object itself, all objects on Earth's surface will experience this amount of force per mass. As such, all objects free fall at the same rate regardless of their mass. Because the 9.8 N/kg gravitational field at Earth's surface causes a 9.8 m/s/s acceleration of any object placed there, we often call this ratio the acceleration of gravity. (Gravitational forces will be discussed in greater detail in a later unit of The Physics Classroom tutorial.)

  Even on the surface of the Earth, there are local variations in the value of g. These variations are due to latitude (the Earth isn't a perfect sphere; it buldges in the middle), altitude and the local geological structure of the region. Use the Gravitational Fields widget below to investigate how location affects the value of g.

As an object falls through air, it usually encounters some degree of air resistance. Air resistance is the result of collisions of the object's leading surface with air molecules. The actual amount of air resistance encountered by the object is dependent upon a variety of factors. To keep the topic simple, it can be said that the two most common factors that have a direct effect upon the amount of air resistance are the speed of the object and the cross-sectional area of the object. Increased speeds result in an increased amount of air resistance. Increased cross-sectional areas result in an increased amount of air resistance.

In the diagrams below, free-body diagrams showing the forces acting upon an 85-kg skydiver (equipment included) are shown. For each case, use the diagrams to determine the net force and acceleration of the skydiver at each instant in time. Then use the button to view the answers.

The diagrams above illustrate a key principle. As an object falls, it picks up speed. The increase in speed leads to an increase in the amount of air resistance. Eventually, the force of air resistance becomes large enough to balances the force of gravity. At this instant in time, the net force is 0 Newton; the object will stop accelerating. The object is said to have reached a terminal velocity. The change in velocity terminates as a result of the balance of forces. The velocity at which this happens is called the terminal velocity.

In situations in which there is air resistance, more massive objects fall faster than less massive objects. But why? To answer the why question, it is necessary to consider the free-body diagrams for objects of different mass. Consider the falling motion of two skydivers: one with a mass of 100 kg (skydiver plus parachute) and the other with a mass of 150 kg (skydiver plus parachute). The free-body diagrams are shown below for the instant in time in which they have reached terminal velocity.

As learned above, the amount of air resistance depends upon the speed of the object. A falling object will continue to accelerate to higher speeds until they encounter an amount of air resistance that is equal to their weight. Since the 150-kg skydiver weighs more (experiences a greater force of gravity), it will accelerate to higher speeds before reaching a terminal velocity. Thus, more massive objects fall faster than less massive objects because they are acted upon by a larger force of gravity; for this reason, they accelerate to higher speeds until the air resistance force equals the gravity force.

What would result if air resistance is absent to a falling object?

What happens to acceleration when air resistance is neglected?