Frame of reference

In phy

Inertial frame of reference

Figure 1: Two cars are moving at different velocities (v1 and v2) down the road. Their velocities remain constant and they are moving in the same direction. S and S' represent different inertial references frames.

An inertial frame of reference is a frame that is either not moving or moving at a constant velocity. In other words, an inertial frame of reference has no net force acting upon it, and therefore Newton's laws of motion are observed.

Inertial reference frames are often used in general relativity and special relativity to describe relative motion of observers or objects and the effects they will endure.

Inertial frames of reference are useful in everyday life as well. For example, consider a situation where two cars are traveling along a road. See Figure 1. If they are separated by a distance of 100 meters, the car in the back is moving at 10 meters per second and the car in front is moving at 5 meters per second, both relative to the ground.

There are three obvious inertial reference frames that can be used to determine when the cars will be even with each other: one where the back car is not moving but the front car is moving backwards at 5 m/s, one where the front car is stationary and the back car is moving forward at 5 m/s, or one where the ground is not moving and the cars are moving at their given velocities. These three perspectives all describe the situation accurately and are physically identical.

Related pages

• General relativity
• Special relativity
• Cartesian coordinate system
• Lorentz transformation
• Principle of relativity