Motion: Linear, Simple Harmonic, Circular & Projectile

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

While you may be able to define 'motion' as it relates to science, do you know how simple harmonic, circular, linear and projectile motion differ from one another? Continue reading to find out more about these different types of motion, and then see if you can answer the questions on the follow-up quiz.

What is Motion?

Motion in physics refers to the changes in position of an object over time. We usually describe motion using various numbers and terms, like displacement or distance, velocity or speed, acceleration and time.

To describe the motion of an object or person, you generally have to know how that motion relates to an object or person's location. Why? Well, right now, you are moving. It doesn't matter who you are, or where you're located, I can guarantee that you're moving.

If you're on the Earth, you're moving around the planet as it rotates on its axis; you're also moving around the sun with the Earth's motion. The sun is orbiting around the Milky Way Galaxy, and the galaxy is moving within the local group of galaxies. So, right now, you're actually moving at about million of miles per hour. When we talk about motion, we generally measure it in relation to something: usually the Earth's surface.

There are many types of motion that we study in physics, including linear, projectile, circular and simple harmonic motion. Today, we're going to briefly summarize these four kinds of motion and what they mean.

Types of Motion

Linear motion is motion in a straight line. It is also called 1-D motion, because you're moving in one dimension. Linear motion is generally the easiest to describe: you just have one graph for displacement over time, one graph for velocity over time and one graph for acceleration over time. A car driving on a perfectly straight road is a good example of linear motion.

1-D Motion on a Straight Road
1D Motion on a Long, Straight Road

Projectile motion is motion that involves two dimensions, such as that which occurs in a water fountain. When you release a projectile, like a cannonball, a bullet from a gun or a baseball coming off a bat, it moves in an up-and-down dimension and a forwards-and-back dimension.

Projectile Motion Can Be Seen in Water Fountains
Projectile Motion in Water Fountains

Describing projectile motion is similar to describing linear motion, except you graph it twice: once on the x axis and once on the y axis. As a result, you'll have two displacement-time graphs, two velocity-time graphs and two acceleration-time graphs. In projectile motion, the acceleration graph in the y-direction always looks the same. That's because any object affected only by gravity has a constant acceleration of -9.8 meters per second squared on Earth, which includes balls falling and those that have previously been thrown upwards.

Graph of Projectile Motion in Two Dimensions
Projectile Motion is in 2D

Circular motion is, unsurprisingly, motion in a circle. For example, imagine a ball being whirled above your head on a string or a satellite orbiting the Earth. When an object is moving in a perfect circle at a constant velocity, we have different equations we can use to describe the motion. One of the things that's special about circular motion is that it represents a case where an object is moving at a constant speed, but its velocity is changing.

Velocity refers to speed with a direction, and the direction of an object moving in a circle is constantly changing. So circular motion is accelerated motion: the velocity is changing.

Circular Motion
Circular Motion

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