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# Gravity and Motion

This lesson aligns with Next Generation Science Standards (NGSS) PS2.B

Introduction
When we drop a ball from the top of the building, it will hit the ground straight. This type of motion has occurred only under the force of gravity. It is known as free fall motion.  When the wind blows, the leaf is broken from the tree and flutters slowly to the ground. In the meantime, an apple falls to the ground steadily. The difference between their motion is due to some kind of friction that both are experiencing through the air. The friction experienced by the air is known as air resistance. In this article, we will learn about free fall motion, the influence of gravity, and how air resistance affects the motion of falling objects.

Free Fall Motion
Free fall represents a motion in which the only force experienced by an object is gravity. This type of motion generally occurs in a vacuum. During free fall motion, air resistance and all other external forces are negligible. Gravity is acting alone on the object. This is the reason the objects accelerate toward the earth at the rate of $9.8 m//s^2$.

Because all other external forces are not encountering falling objects, all objects will reach the ground at the same time despite their sizes and masses. Suppose a football and a baseball are dropped from a certain height, both will hit the ground at the same time.

During free fall motion, for every second, the velocity of the object increases by $9.8 m//s$. For instance, as mentioned above, a ball is dropped from the top of the building. The initial velocity of the ball is zero. After one second, its velocity will be $9.8 m//s$. For the next two seconds, its velocity will be $19.6 m//s$. The velocity of the ball keeps increasing by $9.8 m//s$ until it hits the ground.

Air Resistance
The objects often travel under conditions in which a vacuum is not present. Thus; they face various conditions that affect the speed at which they travel. One of them is air resistance. Air resistance refers to the type of fluid friction experienced by an object as it moves through the air.

As we are familiar with the fact that friction always acts in the opposite direction of the motion. For falling objects, air resistance is encountered in the upward direction. For example, when a tree sheds its leaves, they tend to float in the air for a while before falling to the ground. This is because of air resistance which acts on the leaves in an upward direction and helps them to stay in the air for a while. Air resistance depends on two main factors.

Surface Area
The first factor is the surface area of the object that is directly experiencing air resistance. When objects with greater surface area fall, they experience more air resistance.

For example, two persons jump from an airplane with parachutes. Parachute A has more surface area than parachute B as shown in the figure. More air pushes parachute A, and more air resistance acts on parachute A. Therefore, the person with parachute A falls more slowly. Similarly, less air passes through parachute B, so there is less air resistance. That is why the person with parachute B falls more quickly.

Speed
The second factor is the speed at which the object travels, also termed velocity. Objects that are moving fast experience more air resistance as compared to objects that move slowly. For example, if you run fast, you will feel air friction against your body. The faster you move, the more air resistance you feel against your body.

Summary
• The motion in which an object falls under the action of gravity is known as free fall motion.
• During free fall motion, the objects accelerate in the downward direction at the rate of $9.8 m//s^2$regardless of their masses.
• Air resistance acts on the falling object as it moves through the air.
• Objects with a greater surface area will experience more air resistance and vice versa. Faster the object, the more encountered air resistance.

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