Projectile Motion

Constant Velocity

What is a Projectile?

A projectile is any object which experiences only the force of gravity. Projectiles don’t experience any normal forces, friction forces, applied forces, or any other force. A true projectile is hard to find, because everything on earth experiences air resistance when it moves. However, objects that experience a very small force from air resistance compared to a much larger force of gravity are often treated as projectiles by Physicists. If we ignore the small amount of air resistance and just treat the object as a projectile, the math becomes much easier.

A projectile will only experience a force in the downward direction. An object’s acceleration is always in the direction of the object’s net force. Since the only force is gravity and gravity pulls things toward the center of the earth, the acceleration is also always downward.

The horizontal motion of projectiles is not affected by gravity. A true projectile does not have any forces to the left or to the right.

Read over the following examples to make sure you understand what is an is not a projectile:

-A golf ball that has been hit is a projectile starting at time it leaves the golf club until the time it hits the ground.
-A rock that is dropped off of a cliff can be considered a projectile
-A person jumping out of a plane can be considered a projectile until they open a parachute. The parachute causes a fiction force that is too large to ignore
-An airplane or a bird would not be considered projectiles. Their wings cause large forces of air resistance that keeps them up.
-A baseball that is thrown can be considered a projectile starting from when it leaves the person’s hand until the ball collides with something else.
-A rock that is thrown up in the air can be considered a projectile even when it is moving upward.

How do Projectiles Move?

All projectiles have one thing in common. They always accelerate downward. The acceleration of an projectile on earth is about -10 m/s/s (-9.81 m/s/s if you need to be more exact). A projectile has no horizontal acceleration, because it has no horizontal forces acting on it. A projectile has only a vertical force. A vertical force will never speed up or slow down the horizontal velocity. If a projectile starts out moving horizontally, it will keep a the same constant horizontal velocity. If the projectile does not start moving horizontally, it will not start moving horizontally. For instance, if you throw a baseball against a wall, it will hit the wall with the same horizontal velocity you threw it with. Gravity does not slow it down or speed it up horizontally.