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Projectile Motion: Definition and Examples

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  • 0:01 Water Balloon Physics
  • 0:35 Projectile Motion
  • 3:15 Tackling Projectile…
  • 4:18 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

A projectile is any object that is given an initial velocity and then follows a path determined entirely by gravity. In this lesson, we will introduce projectile motion and touch on a few key facts to keep in mind when working through these problems.

Water Balloon Physics

Have you ever seen one of those water-balloon launchers? They sort of look like a giant rubber band with handles on each end and a water balloon-sized cup in the middle. Two people hold each end, while one in the middle holds a water balloon and stretches the elastic back. When that person lets go, the elastic snaps the balloon forward, shooting it a pretty good distance.

I always had fun trying to peg my friends from across the yard. I didn't know it at the time, but with a little knowledge of physics, I might have been a better shot. It's too late for me, but maybe I can help you soak a person or two.

Projectile Motion

Believe it or not, those water balloons are considered projectiles. Simply put, a projectile is any object that is given an initial velocity and then follows a path determined entirely by gravitational acceleration. Regardless of whether you're launching a balloon, a baseball, or an arrow, all projectiles follow a very predictable path, making them a great tool for studying kinematics.

Projectile motion is a predictable path traveled by an object that is influenced only by the initial launch speed, launch angle, and the acceleration due to gravity. You can try it out from where you're sitting. Pick up an object, and gently toss it up and away from you. It will rise as it flies away from you, reach a maximum height, and then start falling down to the floor. Toss a few more objects while you're at it. As long as you're not tossing pieces of paper or feathers, the projectile paths should be similar. We'll touch on this concept a bit later.

Let's quickly touch on a couple key points involved in projectile motion. First, projectiles follow a predictable parabolic path, like this:

Path of projective motion
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Depending on how hard or how high you throw the projectile, the path could be stretched out or high and steep, but it will always be a parabolic shape.

One thing to keep in mind, though: if you run into a question about an object thrown straight upward, it is not considered a projectile in parabolic motion. Those questions are best tackled as free fall problems instead of projectile motion problems, but either approach will get you to the correct answer. Free fall motion is covered in another set of lessons.

Second, once forced into motion, the projectile is only acted on by gravity, ignoring any impacts of air resistance. Projectile motion starts as soon as the balloon is released from the launcher. The projectile is given all the velocity it is going to get in the horizontal direction by the launcher. It won't continue to accelerate horizontally for the rest of its motion. If the projectile is moving to the right at 5 m/s after it is launched, it will still be moving 5 m/s to the right when it lands.

In the vertical direction, gravity will influence the balloon by acting downward on it, pulling it back to the ground. This is why the projectile will slow down as it rises and speed up as it falls.

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