When throwing or firing a projectile up at an angle, the force put into it is working against earth's gravity. As the projectile reaches its highest point, the force of gravity overcomes the force of its motion. This slows the projectile down before it again begins to gain momentum as it travels toward the ground. Without gravity, the projectile would just keep going in the direction it was sent and at the same speed.
Gravity does not affect the horizontal movement of a projectile thrown in a straight line parallel to the ground. Throw a ball directly up from a moving vehicle and it will land on the vehicle, as the ball will keep the same horizontal movement in the air that it had while in the vehicle. This is because the projectile's horizontal movement has not changed, only the vertical movement of the projectile. Horizontal and vertical movements act independently of each other. Similarly, if you drop one ball, and fire another horizontally from the same height, both balls will land at the same time.
Throwing a projectile straight downward means its highest point is at the beginning of its trajectory, so that is the slowest point. As the projectile moves toward the ground, it will gain momentum and speed. Gravitational acceleration is 32.2 feet per second squared, and all objects accelerate downward at the same rate. If you dropped an object from an airplane while going a consistent speed, the package would land directly below the airplane. However, air resistance can affect this.
As horizontal and vertical motions are independent of each other, launching projectiles to the same height will have them stay in the air for the same amount of time. If you throw a ball three times at the same height with different horizontal distances, every throw will take the same amount of time to hit the ground. However, if you threw a ball while on the moon, which has a gravitational acceleration of 5.34 feet per second squared, your projectile would be able to travel farther horizontally, but objects thrown at the same height would still fall at the same time.