Reference Frames

To describe motion, we must specify a reference frame.

Inertial Frame

An inertial frame is one in which:

• Newton’s laws are valid
• Objects move with constant velocity unless acted upon by a force

Examples:

• A stationary observer on Earth (approximately)
• A spacecraft moving at constant velocity

Non-Inertial Frame

A non-inertial frame is accelerating or rotating.

In such frames:

• Newton’s laws do not directly apply
• Additional fictitious forces must be introduced

Example:

• A rotating merry-go-round
• An accelerating car