State of Motion and Velocity

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  • 0:03 The Best and the Brightest
  • 0:35 Speed is Distance Over Time
  • 1:38 Velocity Has Direction
  • 2:59 Motion is Relative
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

An object's state of motion describes how it is moving. But there are many ways we can describe motion, such as speed and velocity. This motion is relative to other objects around it, such as the earth, the sun, and even other stars in our galaxy.

State of Motion

Galileo Galilei was a pretty smart guy. For example, he correctly described the concept of inertia, which is the tendency of objects to stay at rest or in motion. Isaac Newton (another really smart guy) took this one step further, refining it to the point of a scientific law. Newton's first law, usually called the law of inertia states that: an object in a state of uniform motion tends to remain in that state of motion unless acted on by an outside force. By state of motion, we simply mean how it's moving (or not moving).

Speed Is Distance Over Time

This is important because things aren't just mobile or stationary. There's all sorts of ways that objects can move - slowly, fast, in circles, in a straight line, up, down…well, you get the point. Galileo also understood this (remember - smart guy!), but at the time people were just describing things in vague terms of slow and fast. Realizing that there was so much more to movement, he decided to find a way to describe movement better.

In doing this, Galileo became the first to measure speed: the distance an object covers in the amount of time it takes to travel. In other words, how long does it take that object to go how far?

Speed is a straightforward calculation - just divide the distance by the time, and you'll know how fast you're going. The best part is that you can use any combination of distance and time for speed: kilometers per hour, meters per second, centimeters per day…the possibilities are endless! Though you may want to think hard about using that last one, unless maybe you're a snail.

Velocity Has Direction

Speed is a very useful thing, but what's even more helpful in understanding an object's state of motion is its direction. When an object has both speed and direction, this is the velocity of the object. Speed tells us how fast that object is going. Velocity tells us how fast and where.

For example, if a car is moving at 70 mph, that is its speed. But if it's traveling 70 mph to the north, then that's its velocity.

Something that has a constant speed is not going slower or faster - for example, driving exactly 70 mph the whole trip. But, something can have constant speed without constant direction. Think of a car driving in a circle. If it travels at the same speed the entire way around the circle, it has constant speed. But because it is constantly changing direction as it goes around, it definitely does not have a constant velocity. In order for an object's velocity to be constant, it has to move in a straight line at the same speed.

Remember inertia? Since we now know that inertia is the tendency of an object to resist changes in motion, and motion is described by velocity, we can think of inertia in a new way: the tendency of an object to resist changes in velocity.

Motion Is Relative

Believe it or not, everything is always moving! You think you're just sitting there on the couch, but actually, you're flying through space. Relative to things like the sun and stars, you're moving pretty fast: at about 100,000 kilometers per hour! You don't feel like you're moving though, because relative to Earth, you are at rest.

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