Freely falling bodies

Concept Notes 1

• All objects in free fall experience same acceleration
  
• When there is no air, the acceleration of an object under free fall is independent of its mass
  
• When we let a coin and feather fall in an airless tube, both reach the bottom at same time.
  

Freely falling bodies

In physics, acceleration is the rate at which the velocity of a body changes with time. When an object falls towards the earth under gravity in the absence of air resistance, it is called a freely falling body. All objects in free fall near the earth have the same acceleration called the acceleration due to gravity (g). The gravitational force acting on an object of mass (m) near the earth is its weight W = mg. The acceleration of an object in free fall is independent of its mass. As air resistance normally acts on a falling body and opposes its motion, the object falls at a slower rate than in free fall. When we drop a coin and a feather simultaneously in a tube fill with air and evacuated tube, we get the following observations.

When the tube has air, coin which is heavier than the feather reaches the bottom of the tube more rapidly while the feather flutters down slowly.

When there is no air in the tube, coin and the feather to fall together.

From this experiment we understand that air resistance affects the motion of a falling body. The air resistance on a falling body depends on its shape, size and speed.

Concept Notes 2    

• A skydiver with an unopened parachute falls quite rapidly and when the chute opens due to the shape and size of the body the air resistance increases and the descent is slowed.
  
• Automobiles are streamlined in shape to reduce air resistance and improve fuel consumption
  

Terminal Velocity

When a body falls, it accelerates due to gravity and the retarding force of air resistance increases with speed. This continues till the force of air resistance equals the weight of the object. Now the object no longer accelerates but falls with a constant speed called the terminal velocity. The terminal velocity is about 200 km/hr for a skydiver with an unopened parachute. While falling, the skydivers use a “spread-eagle” position to increase the air resistance and prolong the time of fall. When the parachute is opened, the fall is slowed by the additional resistive force.