What is time of flight of a projectile motion?

CONCEPT: 

• Projectile motion: A kind of motion that is experienced by an object when it is projected near the Earth's surface and it moves along a curved path under the action of gravitational force.
• The time of flight ​of a projection is the time from when the object is projected to the time it reaches the ground.
• In other words, The time of flight is the time for which the ball will be in the air before hitting the ground.

It is given by the formula:

T = 

where u is initial velocity, θ is an angle of projection with horizontal, and g is the gravitational acceleration.

EXPLANATION:

• Option 1: The time after which the ball reaches the highest point: in this time the ball has not completed its projection.
  • It is still in the air. It has not hit the ground. (as shown in figure 1)
  • So this is not the time of flight.

• Option 2: The time of projection or the time a ball takes to get projected: This is the time before the projection of ball.
  • The ball has not projected yet.
  • So it can't be a time of flight.

• Option 3: The time for which a ball will be in the air after projection and before hitting the ground: 
  • In this time the ball has completed its time in the air.
  • The ball has hit the ground.
  • So this time from projection (point O) to hit the ground (point B) is the time of projection.
  • So this is the correct option.

• So the correct answer is option 3.