Falling Objects
The vertical force acting upon a projectile in motion is gravity. All falling bodies accelerate as they fall. Neglecting air resistance, this acceleration is the same for any mass and any horizontal speed the object may have. Demonstrate gravity by pushing three balls from a level table at the same time but with different speeds. The faster balls travel a longer horizontal distance before hitting the ground. But in all three cases, their vertical speed is zero at the moment they move off the table. Therefore, they are subject to the same vertical force and all hit the ground at the same time.
Cannon Range
A projectile is fired from a cannon four times. When the projectile is fired at 90 degrees, perpendicular to the horizon, the horizontal distance covered is zero and it returns back to the place from which it was fired. Fire the projectile with the cannon set at 30 degrees, 45 degrees and 60 degrees to the horizon. It covers the greatest horizontal distance when the cannon is at 45 degrees to the horizon, and covers equal horizontal distances when the cannon is at both the 30- and 60-degree positions.
Skateboarding
A skateboarder rides along a flat surface at a constant speed. He jumps from the skateboard and propels himself vertically with his legs. The skateboard's movement provides the horizontal thrust for the skateboarder's jump. As he moves through the air, he is the projectile and his trajectory is a parabola. The skateboard continues its horizontal motion while the skateboarder is in the air until he lands on it some distance from his take-off point.
Long Jump
A long jump athlete has to cover a long horizontal distance against the vertical force of gravity. Like the cannon increasing its firing range by lowering its angle to the ground, the long jump athlete needs to sprint to his take-off point. This run increases the horizontal force and hurls the jumper at a low angle across the sand pit to finish the jump. His trajectory is parabolic.