# Understanding Relative Velocity in Kinematics

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.

After completing this lesson, you will be able to explain what relative velocity is in kinematics and calculate relative velocities in basic situations. A short quiz will follow the lesson summary.

## What is Relative Motion?

Motion is the change in position of an object as time passes. How fast are you moving, right now? If you're sat still, in front of a computer, you probably answered, 'not at all'. But it turns out you're wrong.

Right now, if you're living in the United States, you're moving at approximately 750 miles per hour east. That's because the Earth is rotating at that speed. But that's not all. You're also moving at 66,000 miles per hour, because that's how fast the Earth is moving around the sun. Pretty fast, right? But that's nothing.

Did you know you're also moving at 483,000 miles per hour, because the whole solar system is orbiting around the center of the Milky Way Galaxy. Let's take our calculations to the next level: you're moving at 1.3 million miles per hour: that's the speed that the Milky Way galaxy is moving towards Andromeda.

So how can you be going at about 750 miles per hour, 66,000 miles per hour, 483,000 miles per hour and 1.3 million miles per hour at the same time? The answer to these different calculations of movement can be found in the theory of relative motion. When you measure motion, it has to be relative to something. When you're driving at 55 miles per hour down the highway, what you're really doing is moving at 55 miles per hour relative to the ground; but relative to the car seat, you're not moving at all.

## Calculating Relative Velocity

Relative velocity is important in physics, such as the study of kinematics, a branch of classical mechanics that examines motion without taking into consideration of the forces or masses involved in the process. Let's say that a bullet train is traveling at 300 miles per hour, but then you start running inside the train. If you run in the opposite direction to the train at 7 miles per hour, your velocity relative to the ground is now 293 miles per hour (300 - 7 = 293). Or if you turn around and run in the same direction as the train, now your velocity relative to the ground is 307 miles per hour (300 + 7 = 307).

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