Motion is a change in position over time. It is one of the most basic concepts in physics, and one of the most difficult to define precisely. To say something is moving, you need a reference point. A passenger on a train is at rest relative to the train but moving relative to the ground. That is why motion is relative. There is no absolute rest. Everything in the universe is moving relative to something else. The Earth orbits the sun at 30 kilometers per second. The sun orbits the Milky Way at 220 kilometers per second. The Milky Way moves through the local group of galaxies.

Motion is described by several quantities. Distance is the total path length. Displacement is the change in position, a vector. Speed is distance divided by time. Velocity is displacement divided by time, a vector. Acceleration is the rate of change of velocity. Newton's three laws describe how forces affect motion. The first law says an object at rest stays at rest and an object in motion stays in motion unless a force acts on it. The second law says force equals mass times acceleration. The third law says every action has an equal and opposite reaction. Motion can be linear, circular, rotational, or oscillatory. Each type has its own equations and concepts.

Types of motion

• Linear. Straight-line motion.
• Circular. Motion in a circle.
• Rotational. Spinning around an axis.
• Oscillatory. Back-and-forth motion.
• Projectile. Motion under gravity.

Motion is not the same as speed. Speed is how fast something moves. Motion is the change in position, which includes direction.