what is relative motion?

Hi

An object is in motion when it is continuously changing its position relative to a reference point and as observed by a person or detection device. For example, you can see that an automobile is moving with respect to the ground. The distance the object goes in a period of time is its speed. If the speed of an object is in a specific direction, it is called velocity. The change in velocity over a period of time is the acceleration of the object.

Motion is relative

All motion is relative to the observer or to some fixed object. When you see a car drive by, it is moving with respect to you. If you are in a car that is going at the same speed, the other car will not by moving with respect to you. But both cars are moving with respect to the ground.

Point of reference

In talking about motion, it is important to indicate your point of reference. In the case of moving automobiles, it is usually assumed the speed is with respect to the ground. But there are situations where the speed or velocity may be with respect to another object or an observer.

For example, suppose a car was traveling at 60 miles per hour (mph) and hit another car, but there was hardly a dent. The reason is that the second car was traveling in the same direction at 59 mph, so the car was going only 1 mph with respect to the second car when it hit it.

Sun looks like it is moving in the sky

Another example of relative motion is how the sun appears to move across the sky, when the earth is actually spinning and causing that apparent motion.

Usually, we consider motion with respect to the ground or the Earth. Within the Universe there is no real fixed point. The basis for Einstein's Theory of Relativity is that all motion is relative to what you define as a fixed point.